Text Book Solutions All Class

Ques 1: What are plastids? Name the different types of plastids found in plant cell.
Ans: Plastids are organelles found only in plants. They are:
(a) Chloroplast-Containing chlorophyll
(b) Chromoplast-Containing carotenoids and xanthophyll (coloured plastids)
(c) Leucoplast-White or colourless plastids

Ques 2: What is plasma membrane made up of?
Ans: Plasma membrane is made up of proteins and lipids.

Ques 3: What did Robert Hooke observed first in cork cell?
Ans: Robert Hooke observed that cork consists of box like compartments which formed a honeycomb structure.

Ques 4: Name the autonomous organelles in the cell.
Ans: Chloroplasts and mitochondria are the autonomous organelles in the cells.

Question.5. What does protoplasm refer to?
Ans: Protoplasm refer to cytoplasm and nucleus.

Ques 6: Name two cells which keep changing their shape.
Answer: Amoeba and white blood cells.

Ques 7: Name the smallest cell and the longest cell in human body.
Ans: The smallest cell is the red blood cell or sperm cell in male. Longest cell is the nerve cell.

Ques 8: Name 3 features seen/present in almost every cell.
Ans: Plasma membrane, nucleus and cytoplasm.

Ques 9: What is diffusion?
Ans: When gases like C0₂, 0₂, move across the cell membrane, this process is called diffusion.

Ques 10: What is osmosis? This takes place from high water concentration to low water concentration.
Ans: The movement of water molecules through a selectively permeable membrane is called osmosis. This takes place from high water concentration to low water concentration.

Ques 11: What is the full form of DNA?
Ans: DNA —> Deoxyribo Nucleic Acid.

Ques 12: What is the function of chromosome?
Answer: Chromosomes contain information for the inheritance of features from parents to next generation in the form of DNA molecules.

Ques 13: Name the organelles present in liver of animals for detoxifying many poisons and drugs. 
Ans: In the liver of animal cells smooth endoplasmic reticulum helps in detoxifying many poisons and drugs.

Ques 14: What is the energy currency of the cell?
Ans: ATP—Adenosine Triphosphate.

Ques 15: What is the function of ribosome?
Ans: Ribosomes help in protein synthesis.

Ques 16. Where are genes located in the cell?
Ans: Genes are located in the chromosomes in the nucleus of the cell.

Ques 17: Name the cell organelles that helps in packaging?
Ans: Golgi apparatus.

Ques 18: Name the cell organelle which helps in the transportation of material.
Ans: Endoplasmic reticulum.

Ques 19: Name the cell organelle due to which leaves, flowers and fruits get their colour.
Ans: Chromoplast.

Ques 20: Name the cell organelle which helps in the formation of lysosome.
Ans: Golgi apparatus.

Ques 21: Name the cleansing organelle in the cell.
Ans: Lysosomes.

Ques 22: Name two cells with cell wall.
Ans: Onion cell (plant cell) and fungi.

Ques 23: Why does mitochondria have largely folded inner membrane?
Ans: Mitochondria is the site for cellular respiration and provides energy to the cell. The largely folded inner membrane provides the increased surface area for ATP-generating chemical reactions.

Ques 24: Which organelle makes the digestive enzyme of lysosome?
Ans: Rough endoplasmic reticulum makes the digestive enzyme of lysosomes.

Ques 25: What are cisterns?
Ans: The golgi bodies consist Of a system of membrane-bound vesicles arranged in stacks called cisterns.

Short Answer Type Questions

Ques 1: State two conditions required for osmosis.
Ans: (i) The difference in the concentration of water, one should have higher concentration than the other.
(ii) Semi-permeable membrane is also required through which water will flow.

Ques 2: What is plasmolysis?
Ans: When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.

Ques 3: How does fungi and bacteria can withstand much greater changes in the surrounding medium than animal cells?
Ans: The cell wall present in fungi and bacteria permits these cells to withstand very dilute external medium without bursting.
The cells take up water by osmosis, swells, and builds the pressure against the cell wall. The wall exerts an equal pressure against the swollen cell. It is because of the cell wall, such cells can withstand much greater changes in the surrounding medium than animal cells.

Ques 4: Give the function of nuclear membrane.
Ans: The nuclear membrane present as outer covering in the nucleus allows the transfer of material inside and out of the nucleus to cytoplasm.

Ques 5: Name the cell-organelles that have their own DNA and ribosomes.
Ans: The cell organelles with their own DNA and ribosomes are mitochondria and plastids.

Ques 6: State the difference between smooth endoplasmic reticulum and rough endoplasmic reticulum.
Ans:

Ques 7: What is endocytosis?
Ans: The cell membranes flexibility allows the cell engulf in food and other material from its external environment. This process is known as endocytosis. E.g., Amoeba acquires its food through such processes.

Ques 8: What is the function of vacuoles?
Ans: Vacuoles are storage sacs for solid or liquid content. In plant cells it provides turgidity and rigidity to the cell. In single-celled organisms vacuoles store food, e.g., Amoeba.

Ques 9: When we put raisins in water, why do they swell?
Ans: Raisins are dry with less water inside, when they are kept in water, osmosis takes place, water flows through the cell wall, cell membrane of the raisins and therefore it swells.

Ques 10: Why are lysosomes called suicidal bags?
Ans: Lysosomes contain digestive enzymes in it and helps in the cleaning of cell by digesting any foreign materials entering the cell, such as bacteria, food and old cell organelles.
When the lysosomes burst, the digestive enzyme digest its own cell. Hence it is called as suicidal bag.

Ques 11: What is nucleoid? 
Ans: The nuclear region in some cells are poorly defined due to the absence of a nuclear membrane, it contains only nucleic acid. This undefined nuclear region with nucleic acid in it is called nucleoid.

Ques 12: What is the role, of cell organelles in the cell?
Ans: Each kind of cell organelles performs a specific function such as making new material, clearing of the waste, transporting material, etc.

Ques 13: Label the figure and answer the questions:
(i) A – It is the packaging organelle
(ii) B – Provides energy
(iii) C – helps in the transport of material
(iv) D – Carries the information.

Ans: (i) A – Golgi body
(ii) B – Mitochondria
(iii) C – Endoplasmic reticulum
(iv) D – Nucleus

Ques 14: What is the function of nucleus in a cell?
Ans: The nucleus plays a very important role in the reproduction of cells. It also helps the single cell to divide and form two new daughter cells.
It plays an important role in determining how the cell will develop and what form it will exhibit at maturity, by directing the chemical activities of the cell.

Ques 15: What is the Junction of plastids?
Ans: Plastids are present only in plant cells. There are two types of plastids chromoplasts (coloured plastids) and leucoplasts (white or colourless)
Chromoplast—Consists of coloured pigments and gives different colours to flowers, fruits and leaves. The green colour pigment present in leaf is called chlorophyll which helps in the photosynthesis and a plastid with chlorophyll is called chloroplast.
Leucoplast—It stores starch, oil and protein granules in it.

Ques 16: Do vacuoles store some material? If yes, name them.
Ans: Yes, vacuoles also store some important substances required in life of the plant cell. These are amino acids, sugars, various organic acids and some proteins. In some unicellular organisms, e.g. Amoeba, vacuoles also store food.

Ques 17: Explain the structure and function of Golgi bodies.
Ans: Structures: Golgi bodies consist of a system of membrane-bound vesicles arranged in stacks parallel to each other called cisterns. These membranes have connections with the membrane of endoplasmic reticulum (ER).
Functions:
(1) The material synthesised near the ER is packaged and dispatched to various target inside and outside the cell through Golgi apparatus.
(2) It also stores, modifies and helps in the packaging of products in vesicles.
(3) In some cases, complex sugars may be made from simple sugars in it.
(4) It also helps in the formation of lysosomes.

Ques 18: What are ribosomes? Where are they located in the cell? What is their function?
Ans: Ribosomes are spherical organelles present in the cell which are either freely distributed in the cytoplasm or may be attached to the endoplasmic reticulum.
It consists of ribosomal RNA (Ribonucleic acid) and proteins.
Functions of Ribosomes: It helps in the synthesis of proteins.

Ques 19: What is the difference in chromatin, chromosomes and gene?
Ans: (1) Chromatin: It is a fine network of thread-like structure made up of DNA or RNA. It gets condense to form chromosomes.
(2) Chromosome: The chromosomes are made from chromatin material and are located iri the cell.
(3) Genes are found in chromosomes.

Ques 20: Why do plant cells have more in number and big-sized vacuoles as compared to the animal cells?
Ans: Plant cells attain turgidity and rigidity due to the more number of vacuoles as well as large-sized vacuoles help the plant cells to withstand the wear and tear, external environmental conditions.
They also help in the storage of essential material required by plants for their growth like amino acids, sugar and various organic substances.

Ques 21: Explain the following terms:
(a) Plasma membrane
(b) Cytoplasm
(c) Nucleus.
Ans: (a) Plasma membrane: It is a thin membrane which controls the passage of materials in and out of the cell. It is also called as selectively permeable membrane. It makes the outer boundary of the cell and is made up of lipo-protein,
(b) Cytoplasm: It is transparent jelly-like thick substance present in the cell. It makes the ground of the cell in which all the cell organelles are suspended.
(c) Nucleus: It is a double-layered membrane structure which contains chromosomes required for the inheritance of characteristics from one generation to the other.

Ques 22: What is membrane biogenesis?
Ans: The endoplasmic reticulum helps in the manufacture of proteins and fat molecules or lipids which are important for the cell function. These proteins and lipids help in the building of the cell membrane. This process is known as membrane biogenesis.

Ques 23: Which organelle is known as powerhouse of the cell?
Ans: Mitochondria is known as powerhouse of the cell because they store energy in the form of ATP. [Adenosine Triphosphate]

Ques 24: What are genes?
Ans: Gene is a segment of DNA. They are located on chromosomes in linear fashions. One gene may perform one or more function. Genes are carrier of genetic codes.

Question 25. Draw various cells of human body.
Answer:

Long Answer Type Questions

Ques 1: Give five points of differences between plant cell and animal cell.
Ans:

Ques 2: Give five points of differences between prokaryotic cell and eukaryotic cell.
Ans:

Ques 3: Draw a neat labelled diagram of plant cell and label its parts.
Ans:

Ques 4: Draw a neat labelled diagram of animal cell.
Ans:

Ques 5: Name the cell organelle for the following:
(a) Present only in plant cell, provides strength and rigidity to the cell.
(b) It is the site for lipids synthesis and helps in detoxification of drugs.
(c) The inner membrane is folded to form cristae, it has its own DNA and proteins.
(d) It helps in the formation of lysosomes.
(e) It imparts colour to the fruit and flowers.
Ans: (a) Cell wall
(b) Smooth endoplasmic reticulum
(c) Mitochondria
(d) Golgi apparatus
(e) Chromoplast

Value-Based Questions

Ques 1: Vasu was helping his mother in laying the table when they had some guest for dinner. Vasu was about to sprinkle salt on salad for dressing. His mother stopped him from doing so and told him that it is too early to sprinkle salt on the salad, he should do so only when they are seated for having the dinner.
(a) What would happen if salt is sprinkled on the salad?
(b) Which property of cells is seen in adding salt to it?
(c) What value of Vasu is seen?
Ans: (a) On sprinkling of some salt on the salad, the salad will release water.
(b) The salt outside the salad acts as hypertonic solution as it has less water concentration and therefore the cell looses water by osmosis.
(c) Vasu showed the value of being very helpful, caring and responsible.

Ques 2: Anu is a five year old girl who joined the swimming classes. After the first class she was worried when she saw her wrinkled fingers. She asked her elder sister about the wrinkling and shrinking of her fingers. Her sister explained Anu why it was so.
(a) Why did the fingers wrinkle after swimming?
(b) What caused the shrinking/wrinkling of fingers?
(c) What value of Anu is seen in the above cast?
Ans: (a) Fingers wrinkled because the cells of the skin lost some water.
(b) This happened because of the difference in the concentration of water in the skin cells and swimming pool’s water.
(c) Anu showed the value of aware citizen and a good learner who clarifies the doubts.

Ques 3: Two sailors got marooned on the island, both of them were very thirsty and one of the sailors tried to drink sea water. The other sailor immediately stopped him from drinking the salty water of sea and suggested to wait for some help, stay calm and patient.
(a) What would happen if the sailor drinks salty water?
(b) What is osmosis?
(c) What value of sailors is reflected in the above act?
Ans: (a) On drinking sally water, exosmosis in the intestine will cause dehydration and vomiting.
(b) The movement of water molecules through a permeable membrane is called osmosis.
(c) Sailors showed the value of being patient, composed, empathy and determination.

Ques 4: Sachin’s mother wanted to use some eggs for incubation. Sachin helped his mother in separating rotten and spoilt eggs from the good ones. He took a bucket of water to separate them.
(a) How can one separate the rotten eggs from the good ones using water?
(b) What is the shell of egg made up of?
(c) What value of Sachin is seen in this act?
Ans: (a) We can separate the rotten eggs by dipping them in water. The eggs that will float in water are rotten eggs and the one that sinks are good one.
(b) Egg shell is made up of calcium carbonate.
(c) Sachin showed the value of being helpful, responsible behaviour.

Very Short Answer Type Questions 

Q1:  Draw the atomic structure of the hydrogen atom.
Ans:

Protium (represented as ¹H₁) contains 1 proton, 1 electron and 0 neutrons.

Q2:  Why are some elements chemically inert?
Ans:  Because their outermost (valence) shell is completely filled, they have a stable electron arrangement and therefore do not readily gain, lose or share electrons.

Q3:  Why is an atom electrically neutral?
Ans:  An atom is electrically neutral because it contains equal numbers of protons (positive charge) and electrons (negative charge). These opposite charges balance each other, giving the atom a net charge of zero.

Q4:  What is the charge and mass of alpha particles?
Ans:  An alpha particle has a charge of +2 and a mass of 4 a.m.u. 

Q5:  What are valence electrons?
Ans:  Valence electrons are the electrons present in the outermost shell of an atom. 

Q6:  An atom has atomic number 12, what is its valency and name the element?
Ans:  Atomic number = 12 → Protons = Electrons = 12.
Electronic configuration: K, L, M = 2, 8, 2.
Valency = 2 (it tends to lose two electrons to attain a stable configuration).
Element: Magnesium.

Q7:  Find the number of neutrons in ²⁷X₁₃.
Ans:  Mass number (A) = 27, Atomic number (Z) = 13 (protons)
Number of neutrons = A – Z = 27 – 13 = 14.
Therefore, neutrons = 14.

Q8:  Where is the mass of an atom concentrated?
Ans:  The mass of an atom is concentrated in the nucleus because most of the atom’s protons and neutrons (which carry nearly all the mass) are located there.

Q9:  Name two elements with the same number of protons and neutrons.
Ans:  Carbon and Oxygen are examples where a common isotope has equal numbers of protons and neutrons:

• Carbon (for ¹²C): Protons = Neutrons = 6
• Oxygen (for ¹⁶O): Protons = Neutrons = 8

Q10:  Draw the atomic structure of a sodium atom.
Ans:

Electronic configuration of sodium: K, L, M = 2, 8, 1; valency = 1.

Q11:  Name the isotope used for the treatment of cancer.
Ans:  A commonly used isotope for cancer treatment is Cobalt-60 (an isotope of cobalt).

Q12:  ^AX_Z What does this symbol represent?
Ans:  X → Symbol of the element.
A → Mass number (total number of protons and neutrons).
Z → Atomic number (number of protons).

Q13:  Can the value of ‘Z’ be the same for two different atoms?
Ans:  No. The atomic number (Z) is unique to each element; two different elements cannot have the same atomic number because Z defines the element.

Q14:  Can the value of ‘A’ be the same for two different atoms?
Ans:  Yes. Different elements can have the same mass number (A). For example, calcium and argon both have mass number 40 (they are isobars).

Short Answer Type Questions

Q1:  Name the scientist who discovered protons and neutrons in an atom.
Ans:  Protons were discovered (observed) by E. Goldstein in 1886, neutrons were discovered by J. Chadwick in 1932.

Q2:  What is the contribution of Bohr and Bury together to the structure of the atom?
Ans:  Bohr and Bury proposed the distribution of electrons in shells using the formula 2n², where n is the shell number. This gives:

• First shell (K, n=1): 2 electrons
• Second shell (L, n=2): 8 electrons
• Third shell (M, n=3): 18 electrons
• Fourth shell (N, n=4): 32 electrons

Electrons fill shells beginning from the innermost shell outward.

Q3:  Draw the atomic structure of (i) an atom with the same number of sub-atomic particles, (ii) an atom with the same number of electrons in L and M shells.
Ans:  (i) Helium ( most common isotope ⁴He ) has Protons = 2, Electrons = 2, Neutrons = 2.

(ii) Argon has filled L and M shells: K, L, M = 2, 8, 8.

Q4:  What is an octet? Why would atoms want to complete their octet?
Ans:  An octet refers to a stable arrangement of eight electrons in the outermost shell (common for many elements). Atoms tend to complete their octet because a full valence shell gives a more stable, lower-energy electronic arrangement, achieving an octet reduces the atom’s tendency to react.

Q5:  Find the valency of ¹⁴N₇ and ³⁵Cl₁₇.
Ans:  The atomic number of nitrogen = 7, No. of protons = 7, No. of electrons = 7
Electronic configuration = K, L  =2 ,5 
Valency = 3
Because it will either gain three electrons or share 3 electrons to complete its octet.
The atomic number of chlorine = 17, p = 17, e=17
Electronic configuration = K, L ,M= 2 ,8 ,7
Valency = 1
Because it will gain 1 electron to complete its octet.

Q6:  Pick up the isotopes among the following and state the reason.

Ans: The isotopes are ³⁵X₁₇ and ³⁷X₁₇, because both have the same atomic number (17) but different mass numbers (35 and 37).

Q7:  Pick up atoms that have the same number of neutrons from the following:

Ans:

Q8:  What are nucleons? What is the name given to those atoms which have the same number of nucleons?
Ans:  Protons and neutrons present in the nucleus are called nucleons. Atoms of different elements that have the same total number of nucleons (same mass number) are called isobars.

Element	Protons	Neutrons	(Protons + Neutrons)
Argon	18	22	40
Calcium	20	20	40

Q9:  Give the difference between the three sub-atomic particles.
Ans:  The three sub-atomic particles are electron, proton and neutron. Key differences:

Q10.  Give the names of three atomic species of hydrogen.
Ans:  The three atomic species (isotopes) of hydrogen are: Protium (¹H), Deuterium (²H) and Tritium (³H).

Q11:  Atomic mass exists as a whole number. Why do we write the atomic mass of chlorine as 35.5 u?
Ans:  Chlorine occurs naturally as a mixture of two main isotopes, approximately 75% ³⁵Cl and 25% ³⁷Cl. The atomic mass given (≈ 35.5 u) is the weighted average of these isotopic masses:

Q12:  Give the difference between isotopes and isobars.
Ans:

Isotopes	Isobars
Atoms of the same element	Atoms of different elements
Have the same atomic number (Z)	Have different atomic numbers (Z)
Have different mass numbers (A)	Have the same mass number (A)
Number of protons and electrons are the same in each isotope of the element	Number of protons and electrons differ because elements are different, but total nucleons are the same

Q13:  The Number of protons and electrons are same in an atom. Then why is it wrong to say that the atomic number of an atom is equal to its number of electrons?
Ans:  Although in a neutral atom the number of electrons equals the number of protons, the number of electrons can change when the atom forms ions (by loss or gain of electrons). The atomic number (number of protons) remains fixed and uniquely defines the element, so atomic number should not be equated with the variable number of electrons.

Q14:  An atom is electrically neutral but on loss or gain of electrons why does it become charged?
Ans:  An atom is electrically neutral because it has an equal number of protons and electrons. However, when it loses or gains electrons, it becomes charged:

• If an atom loses electrons, it has more protons than electrons, resulting in a positive charge.
• If an atom gains electrons, it has more electrons than protons, leading to a negative charge. 

Q15:  In the structure of an atom, why are protons present in the centre and are not pulled outside by the electrons, as both are oppositely charged with the same unit of charge?
Ans:  Protons are located at the centre of an atom, forming the nucleus, due to their significant mass. Here are the key reasons:

• Protons have a mass of approximately 1 unit, making them much heavier than electrons.
• Electrons have a negligible mass, roughly 1/1800 that of protons.
• Despite being oppositely charged, the mass of electrons is insufficient to pull protons away from the nucleus.

This structure ensures stability within the atom.

Q16:  According to you, among the structures of an atom studied, which model is correct and why?
Ans:  Bohr’s model of the atom is the most accurate because it effectively explains the arrangement of nucleons (protons and neutrons) at the centre, with electrons orbiting around them in specific, discrete paths.

• Electrons in these orbits do not lose energy.
• This stability allows them to remain in their respective shells.

Long Answer Type Questions

Q1:  What are isotopes? State its characteristics, and give the uses of isotopes?

Answer:  Isotopes are atoms of the same element that have the same atomic number (Z) but different mass numbers (A).
Characteristics:
(1) Physical properties (such as mass and density) of isotopes differ.
(2) Chemical properties are effectively the same because they have the same electronic configuration.

Uses:
(1) Uranium-235 is used as fuel in nuclear reactors.
(2) Cobalt-60 is used in radiotherapy for the treatment of cancer.
(3) Iodine-131 is used in diagnosis and treatment of thyroid disorders (goitre).

Q2:  Explain Rutherford’s α-particle scattering experiment and give its observation and the conclusion drawn.
Ans:  Rutherford’s α-particle scattering experiment: Rutherford directed fast α-particles (helium nuclei) at a very thin gold foil and observed their behaviour on a fluorescent screen placed around the foil.

Observations:
(1) Most α-particles passed straight through the foil without deflection.
(2) Some α-particles were deflected at small angles.
(3) A very few (about 1 in 12,000) were deflected back (rebounded).

Conclusions:
(1) Most of the atom is empty space (explains why most α-particles pass through).
(2) The positive charge of the atom and most of its mass are concentrated in a very small central region – the nucleus (explains the large deflections and rare rebounds).
(3) Electrons move around this nucleus occupying the remaining space.

Q3:  Establish the relationship between atomic number, mass number, isotopes, isobars and valency of an atom.
Ans:  Atomic number — Gives the number of protons (Z)
Mass number — Gives the number of protons and neutrons (A)
Isotopes — When atoms of the same element have the same number of protons (Z) but a different number of neutrons (s) such atoms are called isotopes.
Isobars — When atoms of different elements have the same mass number (A) but different atomic number (Z) such atoms are called isobars.
Valency — It is the combining capacity of an atom.

Q4:  Why do Helium, Neon and Argon have zero valency?
Ans: Helium, Neon, and Argon have a zero valency because their outermost electron shells are full, making them stable and unreactive. They don’t need to gain, lose, or share electrons to achieve a stable electron configuration. 

• In Helium, the outermost orbit contains two electrons, forming a stable duplet configuration and remains inert in most reactions.
• Both Neon and Argon have eight electrons in their outermost shell, resulting in a stable octet configuration, which makes them unreactive under normal conditions.

Q5:  What were the drawbacks of Rutherford’s model of an atom?
Ans:   The drawbacks of Rutherford’s model of an atom are significant in understanding the limitations of his atomic theory. Rutherford conducted an experiment where he directed alpha particles at a thin gold foil and observed their deflection patterns. However, his model faced several challenges:

1. Stability of Electrons: Rutherford’s model couldn’t explain how electrons, which are charged particles, could continuously move in orbits around the nucleus without losing energy

2. Radiation Concerns: According to classical physics, any accelerating charged particle emits electromagnetic radiation. If this were the case, the electron would lose energy, spiral inward, and collapse into the nucleus, contradicting the stability of the atom.

Q6:  Compare the properties of electrons, protons and neutrons.
Ans:  

Very Short Answer Type Questions

Q1. Define the law of conservation of mass.

Ans: The law of conservation of mass states that in a chemical reaction mass can neither be created nor destroyed. This means the total mass of the reactants equals the total mass of the products.
For example:
2Na + Cl₂ —> 2NaCl
Mass of reactants = 2 × 23 + 2 × 35.5 = 46 + 71 = 117 u
Mass of products = 2 × (23 + 35.5) = 2 × 58.5 = 117 u
Thus, mass is conserved before and after the reaction.

Q2. Explain the law of constant proportion.

Ans: The law of constant proportion (law of definite proportions) states that a chemical compound always contains the same elements in the same fixed proportion by mass, irrespective of its source or method of preparation.
Example: For example, in water (H₂O), the ratio of hydrogen to oxygen by mass is always 1 : 8.

Q3. Who coined the term atom?

Ans: The word “atom” comes from the ancient Greek word atomos and was used by early Greek philosophers such as Democritus. John Dalton, however, proposed the modern atomic theory and gave a scientific basis to the idea of atoms.

Q4. Define atom.

Ans: An atom is the smallest particle of an element that can take part in a chemical reaction and retain the chemical properties of that element.

Q5. Define molecule.

Ans: A molecule is the smallest particle of an element or compound that can exist independently and show all the chemical properties of that substance. 
Examples: O₂ is a molecule of oxygen; H₂O is a molecule of water.

Q6. Define atomicity.

Ans: Atomicity is the number of atoms present in one molecule of an element.
Example: Atomicity of O₂ is 2; atomicity of O₃ is 3.

Q7. What is an atomic mass unit?

Ans: An atomic mass unit (amu), also written as unified atomic mass unit (u), is defined as one twelfth (1/12) of the mass of a carbon-12 atom. It is the standard unit for expressing atomic and molecular masses.
Example: Molecular mass of H₂O = (2 × 1) + 16 = 18 u.

Q8. Give one example of cation and anion.

Ans: Cation => Na⁺
Anion => Cl^–

Q9. Give the chemical formula for ammonium sulphate.

Ans: Ammonium ion = NH₄⁺
Sulphate ion = SO₄^2-
Chemical formula: (NH₄)₂SO₄.

Q10. Find the molecular mass of H₂O.

Ans: Molecular mass of H₂O
= (2 × 1) + 16
= 2 + 16 = 18 u

Short Answer Type Questions

Q1. Give the unit to measure the size of an atom and give the size of a hydrogen atom.

Ans: Atomic size is measured in nanometres (nm). The size of a hydrogen atom is about 10⁻¹⁰ m.

Q2. What is IUPAC, give its one function?

Ans: IUPAC is the International Union for Pure and Applied Chemistry. One of its functions is to set standard rules for naming chemical elements and compounds and to approve official chemical names, symbols and units.

Q3. Give the Latin name for sodium, potassium, and iron.

Ans: Sodium -> Natrium (Na)
Potassium -> Kalium (K)
Iron -> Ferrum (Fe)

Q4. What is the ratio by mass of combining elements in H₂O, CO₂and NH₃?

Ans: Mass ratios of combining elements are:
H₂O: H : O = 2 : 16 → simplest mass ratio H : O = 1 : 8
CO₂: C : O = 12 : 32 → simplest mass ratio C : O = 3 : 8
NH₃: N : H = 14 : 3 → simplest mass ratio N : H = 14 : 3

Q5. What is a polyatomic ion? Give one example.

Ans: A polyatomic ion is a group of two or more atoms bonded together that carry an overall electrical charge and act as a single charged species. 
Examples: Ammonium – NH₄⁺; Nitrate – NO₃^–.

Q6. Write down the formula for:
Copper nitrate, calcium sulphate and aluminium hydroxide.

Ans: Chemical formulae:
Copper nitrate -> Cu(NO₃)₂
Calcium sulphate -> CaSO₄
Aluminium hydroxide -> Al(OH)₃

Q7. What is formula unit mass? How is it different from molecular mass?

Ans: The formula unit mass of an ionic compound is the sum of the atomic masses of all atoms present in its simplest formula unit. Formula unit mass applies to ionic substances whose smallest repeating unit is a formula unit (made of ions). Molecular mass is the sum of the atomic masses of all atoms in a molecule and applies to molecular (covalent) substances. In short, formula unit mass = ionic compounds; molecular mass = molecular compounds.

Q8. What are the rules for writing the symbol of an element?

Ans: IUPAC approves the symbols of elements. Rules for writing symbols:
– Symbols are one or two letters, usually taken from the element’s English name.
– The first letter is always a capital (upper case) and, if there is a second letter, it is lower case.
Examples: Hydrogen – H; Helium – He.
– Some symbols come from Latin, e.g., Iron – Fe (Ferrum); Sodium – Na (Natrium).

Q9. Explain relative atomic mass and relative molecular mass.

Ans: Relative atomic mass of an element is the ratio of the average mass of atoms of the element to 1/12th of the mass of a carbon-12 atom. Relative molecular mass is the sum of the relative atomic masses of all atoms in a molecule; it gives how many times a molecule is heavier than 1/12th of a carbon-12 atom.

Q10. The formula of carbon dioxide is CO₂. What information do you get from this formula?

Ans: The chemical formula CO₂ gives the following information:

1. CO₂ represents carbon dioxide, which is a compound.
2. One molecule of carbon dioxide contains one atom of carbon and two atoms of oxygen.
3. The ratio by number of atoms of carbon to oxygen in CO₂ is 1 : 2.
4. The formula shows that carbon and oxygen are chemically combined in a fixed proportion, which is characteristic of a compound.

Q11. State 3 points of difference between an atom and an ion.

Ans:

Atom	Ion
Is electrically neutral (no net charge).	Has a net electrical charge (positive or negative).
Number of electrons = number of protons.	Number of electrons ≠ number of protons (formed by loss or gain of electrons).
May be chemically reactive depending on element.	Usually formed to achieve greater stability (by gaining or losing electrons).

Q12. Calculate the formula unit mass of NaCl and CaCl₂.
(Na = 23, Cl = 35.5, Ca = 40)

Ans: Formula unit mass of NaCl = 23 + 35.5
= 58.5 u
Formula unit mass of CaCl₂ = 40 + (2 × 35.5)
= 40 + 71 = 111 u

Q13.The ratio by mass for hydrogen and oxygen in water is given as 1 : 8 respectively. Calculate the ratio by number of atoms for a water molecule.

Ans: To convert mass ratio to number of atoms, divide the mass ratio by the atomic masses:
Mass ratio H : O = 1 : 8
Atomic masses H = 1, O = 16
Divide each by its atomic mass: H → 1/1 = 1; O → 8/16 = 0.5
Make whole numbers by multiplying by 2: H → 1 × 2 = 2; O → 0.5 × 2 = 1
Thus, the ratio by number of atoms in water is H : O = 2 : 1.

Q14. Write down the chemical formula for the following compounds:
(a) Aluminium carbonate
(b) Calcium sulphide
(c) Zinc carbonate
(d) Copper phosphate
(e) Magnesium bicarbonate
(f) Aluminium hydroxide.

Ans:The chemical formulae are:
(a) Aluminium carbonate -> Al₂(CO₃)₃
(b) Calcium sulphide -> CaS
(c) Zinc carbonate -> ZnCO₃
(d) Copper phosphate -> Cu₃(PO₄)₂
(e) Magnesium bicarbonate -> Mg(HCO₃)₂
(f) Aluminium hydroxide -> Al(OH)₃

Q15. Explain the difference between 2[O] , O₂and O₃.

Ans: 2[O] -> Represents two separate oxygen atoms (these single atoms cannot exist freely under normal conditions).
O₂ -> Represents one oxygen molecule made of two oxygen atoms; this diatomic molecule exists freely and is the common form of oxygen in air.
O₃ -> Represents one molecule of ozone made of three oxygen atoms; ozone is a distinct triatomic form of oxygen with different properties.

Long Answer Type Questions

Q1. (a) How do atoms exist?
(b) What is atomicity?
(c) What are polyatomic ions?

(a) Atoms of some elements cannot exist independently under normal conditions and combine to form molecules or ions. For example, non-metals like oxygen form O₂ molecules. Noble gases can exist independently; most other elements exist as molecules or ions.

(b) The number of atoms constituting a molecule is known as its atomicity.
Examples: O₃ -> atomicity is 3; O₂ -> atomicity is 2.
(c) Polyatomic ions: When more than one atom combine and together carry a net charge, they form a polyatomic ion which behaves as a single charged particle.
Examples: OH^–, NO₃^–, NH₄⁺.

Q2. Which are the six postulates of Dalton’s atomic theory?

Ans: Postulates of Dalton’s atomic theory:
1. Matter is made of indivisible particles called atoms.
2. Atoms of the same element are similar in mass and properties, but differ from atoms of other elements.
3. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
4. Atoms of different elements combine in fixed, simple whole-number ratios to form compounds.
5. Atoms of the same element can combine in different ratios to form different compounds.
6. The atom is the smallest unit of matter that can take part in a chemical reaction.

Q3. What is an atomic mass unit’? How is it linked with relative atomic mass?

Ans: An atomic mass unit (amu or u) is defined as one twelfth of the mass of a carbon-12 atom. The relative atomic mass of an element states how many times an atom of that element is heavier than 1/12th the mass of a carbon-12 atom. Thus, relative atomic mass is a dimensionless number comparing an atom’s mass to the standard amu.

Q4. Write the formulae for the following and calculate the molecular mass for each of them.
(a) Caustic potash
(b) Baking powder
(c) Limestone
(d) Caustic soda
(e) Ethanol
(f) Common salt

Ans:

(a) Caustic potash: KOH.
Molecular mass = 39 + 16 + 1 = 56 u.

(b) Baking powder (sodium hydrogen carbonate): NaHCO₃.
Molecular mass = 23 + 1 + 12 + (3 × 16) = 23 + 1 + 12 + 48 = 84 u.

(c) Limestone (calcium carbonate): CaCO₃.
Molecular mass = 40 + 12 + (3 × 16) = 40 + 12 + 48 = 100 u.

(d) Caustic soda: NaOH.
Molecular mass = 23 + 16 + 1 = 40 u.

(e) Ethanol: C₂H₅OH.
Molecular mass = (2 × 12) + (6 × 1) + 16 = 24 + 6 + 16 = 46 u.

(f) Common salt: NaCl.
Molecular mass = 23 + 35.5 = 58.5 u.

Q5. Write chemical formulae of the sulphates of Aluminium, Ammonium and Zinc.

Ans: 

Aluminium sulphate -> Al₂(SO₄)₃
Ammonium sulphate -> (NH₄)₂SO₄
Zinc sulphate -> ZnSO₄

Very Short Answer Type Questions

Q1. Define solvent.

A solvent is the substance in a solution that dissolves the solute. Example: In a saltwater solution, water is the solvent.

Q2. Define solute.

A solute is the substance that gets dissolved in a solvent to form a solution. Example: In a saltwater solution, salt is the solute.

Q3. What is ‘tincture of iodine’?

A solution of iodine in alcohol is known as tincture of iodine. It has iodine (solid) as the solute and alcohol (liquid) as the solvent.

Q4. What are alloys?

Alloys are mixtures of two or more metals, or a metal and a non-metal. They cannot be separated into their individual components through physical methods.
Example: Steel is an alloy of iron and carbon.

Q5. Give one example of a gas in a liquid solution.

In cold drinks, carbon dioxide gas is the solute and water is the solvent.

Q6. How can a solution be dilute or concentrated?

A solution is dilute if it contains a small amount of solute in the solvent, and concentrated if it contains a large amount of solute in the solvent.

Q7. What is “concentration of a solution”?

The concentration of a solution refers to the amount of solute present in a given amount of solvent or solution. It indicates how concentrated or dilute a solution is.

Q8. State the difference between aqueous and non-aqueous solutions.

Aqueous solutions use water as the solvent, while non-aqueous solutions do not contain water as the solvent.

Q9. What is the “solubility” of a solute?

The amount of a solute that can dissolve in a given amount of solvent at a specific temperature is known as its solubility.

Q10. What is a saturated solution?

A saturated solution is one in which the maximum amount of solute is dissolved in a solvent at a given temperature, and no more solute can dissolve further.

Q11. What is an unsaturated solution?

An unsaturated solution contains less solute than the maximum amount it can dissolve at a given temperature.

Q12. How can you convert the saturated solution into an unsaturated one or vice versa?

A saturated solution can be converted to an unsaturated solution by:

• To convert a saturated solution into an unsaturated one, heat the solution so more solute can dissolve.
• To convert an unsaturated solution into a saturated one, cool the solution or add more solute until no more dissolves.

Q13. Why is water called a universal solvent?

Water is called a universal solvent because:

• It can dissolve a wide variety of substances due to its polar nature, which allows different molecules to interact with it effectively.
• Example: In our body, water dissolves salts, sugars, and gases to help with digestion, transport, and cell functions.

Q14. What is the Tyndall effect?

The scattering of light by colloidal particles is known as Tyndall effect.

Q15. How can we separate colloidal mixtures?

Colloidal mixtures can be separated using centrifugation. This process involves:

• Placing the colloidal solution in a test tube.
• Rotating the test tube rapidly in a centrifuge machine.
• Utilising centrifugal force to separate the colloidal particles from the mixture.

Q16. What is an emulsion?

An emulsion is a mixture where both the dispersed phase and the dispersing medium are liquids. 
Example: Milk, Face Cream.

Q17. What is aerosol?

An aerosol is a mixture where solid or liquid particles are dispersed in a gas.
Example: Smoke, Fog.

Q18. What is the principle for the separation of immiscible liquids?

The principle of separation of immiscible liquids is based on their density differences. The less dense liquid collects at the top, and the denser one settles at the bottom.

Q19. How can you separate two liquids that have less than 25 K difference of boiling points?

To separate a mixture of two or more miscible liquids with a boiling point difference of less than 25 K, the method used is fractional distillation.

Q20. What is crystallisation?

When a saturated solution is heated and then allowed to cool slowly, crystals of the dissolved solute separate out. This process, called crystallisation, is used to purify solids.

Short Answer Type Questions

Q1. Why is a mixture called an impure substance?

A mixture is called an impure substance because it contains different components that:

• Retain their individual properties
• Can be easily separated by physical processes

Unlike pure substances, which have a uniform composition, mixtures consist of two or more pure substances combined together.

Q2. Give the differences between a mixture and a compound.

Q3. Distinguish between a physical change and a chemical change.

Q4. State the properties of a solution.

Properties of a solution are:

• A solution is a homogeneous mixture.
• Particles in a solution are smaller than 1 nm and are invisible to the naked eye.
• They do not scatter a beam of light, making the path of light invisible.
• Solute particles cannot be separated by filtration, indicating that a solution is stable.

Q5. State the properties of a suspension.

Properties of a Suspension:

• Heterogeneous mixture: A suspension consists of different components that are not uniformly mixed.
• Visible particles: The particles in a suspension can be seen with the naked eye.
• Tyndall effect: When light passes through a suspension, it scatters, making the path of the light visible.
• Unstable: The particles will eventually settle at the bottom if left undisturbed, indicating that a suspension is not stable.
• Separation: The components can be separated using filtration.

Q6. What is a colloidal solution?

A colloidal solution is a type of heterogeneous mixture that looks homogeneous. Its particles are very small, typically less than 1 micrometre, making them invisible to the naked eye. However, these particles remain suspended and do not settle.
Example: Milk and Blood.

Q7. State the properties of a colloidal solution.

Properties of colloidal solution:

• A colloidal solution is a heterogeneous mixture with particle sizes ranging from 1 nm to 100 nm.
• The particles are so small that they cannot be seen with the naked eye.
• Colloidal solutions can scatter light, which is known as the Tyndall effect.
• They are stable because the particles do not settle when undisturbed.

Q8. Give the applications of centrifugation.

Applications of centrifugation are:

• Centrifugation is used in diagnostic laboratories for blood and urine tests.
• It is applied in dairies and at home to separate butter from cream.
• It is also used in washing machines to remove water from wet clothes.

Q9. Why is crystallisation better than evaporation?

Crystallisation is a process that separates a pure solid in the form of its crystals from a solution. It is often preferred over evaporation for the following reasons:

• During evaporation, some solids may decompose or, like sugar, become charred when heated to dryness.
• Some impurities can remain dissolved in the solution, which may contaminate the solid after evaporation.

Q10. A student is given a mixture of naphthalene balls’ powder and common salt. He needs to separate this mixture. How will he do this? 

• The properties of both naphthalene and common salt should be known before we choose the separation technique.
• Naphthalene is a sublimate which on heating changes to gaseous state directly. Hence to separate a volatile compound (sublimate) from a non-volatile compound (non-sublimate), the sublimation process is used.
  Sublimation of Naphthalene
• In a China dish, the mixture is kept and is placed on a stand. An inverted funnel is kept over the mixture in China dish with plugged stem. The sublimate on heating gets collected on the funnel and common salt remains in the China dish.

Q11. How can we obtain different gases from the air?

Air is a mixture of gases, and we can separate its components using fractional distillation. This process involves:

• Cooling air to a liquid state.
• Gradually warming the liquid to allow different gases to evaporate at their specific boiling points.
• Collecting the gases separately.

Below is a flow diagram illustrating the steps involved in this process:

Q12. Draw a flow diagram to show the water purification system in waterworks.

The water purification process involves several key steps:

• Collection: Water is collected in a reservoir.
• Sedimentation: Water is sent to a sedimentation tank where solids settle at the bottom.
• Loading Tank: The water moves to a loading tank, allowing suspended impurities to settle as sediment.
• Filtration: Water passes through a filtration tank, where it goes through layers of sand and gravel to remove impurities.
• Chlorination: The clear water is mixed with chlorine or bleaching powder in a chlorinated tank to kill bacteria.
• Distribution: Finally, the purified water is supplied to homes.

Q13. Why is air considered a mixture and not a compound?

Air is considered as a mixture because it exhibits following properties:

1. Each component present in air retains its properties.

2. Each component can be separated by simple physical processes.

3. The components do not have any fixed proportion. All gases are present in different amount. 
Example: In greener area—more oxygen and water vapour is present; near industrial area—air consists of a lot of impurities and smoke suspended in it.

Q14. How can you prove that water is a compound?

Water is a compound because if we pass electricity through it then at two different electrodes, we get two different gases i.e., oxygen and hydrogen during the electrolysis of water. The ratio of oxygen: hydrogen is 1: 2 by number of molecules.
(i) The properties of oxygen and hydrogen gases are entirely different from liquid water.
(ii) The ratio of oxygen: hydrogen combination is always constant i.e., 1: 2 by volume.
(iii) To separate the components of water, we need electrolytic cell, and it is not a simple process.

Q15. How can we convert a saturated solution into an unsaturated one by heating? 

A saturated solution contains the maximum amount of solute that can dissolve at a specific temperature. When heated:

• The solvent molecules gain kinetic energy.
• They vibrate and move apart, creating more space.
• This allows additional solute particles to dissolve.
• As a result, the solution becomes unsaturated.

Thus, heating a saturated solution enables it to accommodate more solute, transforming it into an unsaturated solution.

Q16. What is the difference between fog and smoke?

Fog is a colloidal solution consisting of tiny liquid droplets suspended in gas. In contrast, smoke is a colloidal solution made up of tiny solid particles dispersed in gas.

• Fog: Liquid droplets in gas.
• Smoke: Solid particles in gas.

Q17: If 20g of salt is present in 220 g of solution, calculate the concentration of the solution.

Concentration of solution = Mass of solute/(Mass of solute + Mass of solvent) × 100
Mass Solute = 20g
Mass of solute + solvent = 220g
∴ Concentration of solution = 20/220 × 100 = 9.09%

Also read: Short Notes: Is Matter Around Us Pure

Long Answer Type Questions

Q1. Give the difference between a true solution, a colloidal solution and a suspension.

The difference between true solution, colloidal solution and suspension

Q2. State the different types of colloids with examples.

Q3. (a) Define solution.
(b) Give different types of solutions with one example each. 

(a) Solution: It is a homogeneous mixture of two or more substances. It consists of solute and solvent.
(b) Different types of solution:

• Based on solvent—Aqueous and non-aqueous Aqueous solution has water as solvent (sugar + water) Non-aqueous solution has some other solvent but not water.
  Example: (sulphur + carbon disulphide)
• Depending on the amount of solute dissolved in solvent—Dilute solution and concentrated solution.
• Dilute solution—Less amount of solute particles are present in a solvent.
  Concentrated solution—Amount of solute present in its maximum capacity in a solvent.
• Amount of solute present in its maximum capacity at a given temperature—Saturated and unsaturated solution.
• Saturated Solution—It is a solution in which no more solute can further dissolve in a given solvent at a given temperature.
• Unsaturated Solution—It is a solution in which some more solute can dissolve in a solvent at a given temperature.
• Depending on the size of solute particles:
  (i)True solution Size is very small and particles cannot be seen through naked eyes
  (ii) Suspension Size is very big and can be seen through naked eyes
  (iii) Colloid Size is intermediate between true solution and suspension

Q4. How can you separate the following mixtures?
(a) Sand + iron 
(b)Cream from milk
(c) Salt + water 
(d) Ammonium chloride + NaCl
(e) Copper sulphate + water 
(f) Rice and dal (uncooked)
(g) Gases from air 
(h) Petrol and diesel from crude oil
(i) Drugs from blood 
(j) Acetone from water

Value-Based Questions

Q1. Anil’s sister accidentally added some water to the bottle containing olive oil, and she was afraid of the scolded. Anil helped his sister and separated the water from the olive oil using a bottle as a separating funnel.
(a) What is the principle of using and working of a separating funnel?
(b) Suggest two separation techniques used to separate liquid mixtures.
(c) What value of Anil is seen in the above case? 

(a) The principle of the separating funnel is the difference in the densities of two immiscible liquids.
(b) Liquid mixtures can be separated by distillation and fractional distillation.
(c) Anil showed the value of helping, caring and responsible behaviour.

Q2. Preeti saw a labourer entering the sewage manhole immediately after removing the lid. She promptly stopped the labour from entering the manhole and told him to wait for some time before he entered it.
(a) What will happen if the labourer immediately enters the manhole for cleaning after removing the lid?
(b) Name the main gases that are released from the manhole.
(c) What value of Preeti is seen in the above act? 

(a) He could die from suffocation due to inhalation of poisonous gases released from sewage.
(b) Gases released from the sewage manhole are methane, carbon dioxide and hydrogen sulphide.
(c) Preeti showed moral responsibility and awareness as a citizen.

Q3. Prasanna wanted to buy a deodorant from the shop. While buying a bottle, he felt that it was slightly heavier than the usual deodorant bottle that he purchased every time. He read the weight mentioned on the bottle and told the shopkeeper to weigh the same. He found the bottle was heavy, and on opening the deodorant bottle, he found it half-filled with water. He complained about the matter to the consumer authority.
(a) Define density.
(b) Apart from water, what is the other substance that some shopkeepers add to the deodorant?
(c) What value of Prasanna is reflected in this act?

(a) Density of any substance is defined to be the mass of the substance per unit volume.
(b) One can add some cheap gases or compressed air in the deodorant bottles.
(c) Prasanna showed leadership, awareness of consumer rights, and responsibility.

Q4. Rita’s father always got his vehicle checked for pollution control. He got it tested for the aerosol if released by his car. He also uses unleaded petrol and makes use of public transport wherever possible. He sparingly uses his car.
(a) What is aerosol?
(b) What happens when smoke released from a vehicle mixes with fog?
(c) What values of Rita’s father are reflected here? 

(a) An aerosol is a colloid in which solid or liquid particles are dispersed in a gas.
Example: Smoke.
(b) When smoke mixes with fog it forms smog.
(c) Rita’s father reflects environmental awareness, responsibility, and good citizenship.

Very Short Answer Type Questions

Q.1. Define matter.
Ans. Anything that occupies space and has mass is called matter.

Q.2. State different states of matter with an example.
Ans. Matter exists in three common states: solid (e.g., a pen), liquid (e.g., water) and gas (e.g., oxygen).

Q.3. What is diffusion?

Ans. Diffusion is the process by which particles of one substance spread and mix with particles of another due to their random motion. For example, a drop of ink spreading in water is a result of diffusion.

Diffusion

Q.4. What happens to the rate of diffusion if the temperature is increased?

Ans. With an increase in temperature, the rate of diffusion increases because particles gain more kinetic energy, move faster and mix more quickly.

Q.5. Name the state of matter that has the tendency to maintain its shape when subjected to outside force.

Ans. A solid tends to maintain its shape when subjected to an external force. Examples include a pen, a book or a wooden stick.

Q.6. Define melting point.

Ans. The melting point is the temperature at which a solid turns into a liquid at the normal atmospheric pressure.

Q.7. Define boiling point.

Ans. The boiling point is the temperature at which a liquid changes into vapour (gas) at the normal atmospheric pressure.

Q.8. Define latent heat of vaporization.

Ans. Latent heat of vaporisation is the heat energy required to convert 1 kg of a liquid into its vapour at its boiling point, at atmospheric pressure.

Q.9. Define latent heat of fusion.

Ans. Latent heat of fusion is the amount of heat energy required to change 1 kg of a solid into a liquid at its melting point, without any change in temperature.

Q.10. Define sublimation.

Ans. Sublimation is the process in which a solid changes directly into a gas without passing through the liquid state.
The reverse process, in which a gas changes directly into a solid, is called deposition.

Sublimation

Q.11. What is dry ice?

Ans. Dry ice is the solid form of carbon dioxide (CO₂). It does not melt at atmospheric pressure but sublimates directly to carbon dioxide gas. Its temperature is about -78.5°C.

Q.12. What is humidity?

Ans. Humidity is the amount of water vapour present in the air. When the air contains a large quantity of water vapour, it feels humid.

Q.13. Give two properties of a solid.

Ans.

(i) Solids have a fixed shape and are rigid.

(ii) Solids cannot be easily compressed because their particles are closely packed.

Q.14. What will happen if the pressure is reduced on solid carbon dioxide (dry ice)?

Ans. If the pressure on solid carbon dioxide is reduced, it will sublimate – that is, it will change directly from solid to gas without melting into a liquid.

Q.15. Name any three substances that show sublimation.

Ans. Ammonium chloride, camphor and naphthalene are examples of substances that undergo sublimation.

Q.16. Sponge is solid, but we can still compress it. Why?

Ans. A sponge is a solid with many tiny pores that trap air. When compressed, the air in these pores is pushed out and the sponge’s volume decreases, which makes it appear compressible even though the material is solid.

Q.17. What is normal atmospheric pressure?

Ans. Normal atmospheric pressure at sea level is defined as:

• 1 atmosphere (1 atm)
• which is equal to 1.013 × 10⁵ Pa.

Thus, standard atmospheric pressure is 1 atm at sea level.

Q.18. What is Kelvin?

Ans. Kelvin is the SI unit of temperature.

• 0°C is equivalent to 273.15 K.
• To convert from Kelvin to Celsius, subtract 273.15.
  °C = K − 273
• To convert from Celsius to Kelvin, add 273.15.
  K = °C + 273

Temperature Scale in Degree and Kelvin

Q.19. Give two examples of diffusion.
Ans. Examples of diffusion:

• Milk drops spreading in water.
• Perfume sprayed in a room spreading its smell.

Q.20. Give the temperature at which water exists in two different phases/states.

Ans.

(i) At 0°C (273.15 K), water can exist as ice (solid) or as liquid water.

(ii) At 100°C (373.15 K) at 1 atm pressure, water can exist as liquid or as vapour (gas).

Short Answer Type Questions

Q.1. Why do we see water droplets collected on the outer surface of a glass container containing ice?

Ans. Water vapour in the air comes into contact with the cold outer surface of the glass, loses heat and changes into tiny liquid droplets. This process is called condensation.

Q.2. Explain why solids have a fixed shape but liquids and gases do not have a fixed shape.

Ans.Solids have particles held tightly in fixed positions by strong intermolecular forces, so they keep a definite shape. Liquids have weaker forces, allowing particles to move past each other; therefore liquids take the shape of their container. Gases have very weak forces and particles are far apart and move freely, so gases expand to fill any available space.

Q.3. Liquids and gases can be compressed but it is difficult to compress solids. Why?

Ans. Liquids and gases have noticeable space between their molecules, so applying pressure can push these molecules closer together – this makes them compressible. Solids have particles closely packed with very little space between them, so they cannot be easily compressed.

Q.4. A balloon, when kept in the sun, bursts after some time. Why?

Ans. The air inside the balloon heats up in sunlight. As temperature rises, air molecules gain kinetic energy and the gas expands, increasing the internal pressure on the balloon’s walls. If the pressure exceeds the elastic strength of the balloon, it bursts.

Q.5. Why do people perspire a lot on a hot, humid day?

Ans. The body sweats to cool down by evaporation of sweat. On a humid day the air already contains a lot of water vapour, so sweat evaporates more slowly and cooling is less effective; consequently, more sweat accumulates and we appear to perspire more.

Q.6. Distinguish between evaporation and boiling.

Ans. Evaporation is the slow vapourisation of a liquid from its surface at any temperature below its boiling point. Boiling is rapid vapourisation that occurs throughout the liquid at a specific temperature called the boiling point. Evaporation occurs at the surface and causes cooling; boiling occurs throughout the liquid and produces bubbles of vapour.

Q.7. Why is it advisable to use a pressure cooker at higher altitudes?

Ans. At higher altitudes, atmospheric pressure is low, so the boiling point of water decreases. As a result, food cooks slowly. A pressure cooker increases pressure and raises the boiling point, allowing faster cooking.

Q.8. What are fluids?

Ans. Fluids are substances that can flow and take the shape of their container. They include:

• Liquids – definite volume but no fixed shape.
• Gases – neither definite volume nor definite shape.

Q.9. One kg cotton and one kg sand, which is more denser? Why?

Ans. One kilogram of sand is denser than one kilogram of cotton because density = mass/volume. Both masses are equal, but cotton occupies a much larger volume (because of trapped air), so its density is lower than that of sand.

Q.10. Why is water liquid at room temperature?

Ans. At room temperature the intermolecular forces in water are not fully overcome by thermal energy, so water molecules remain close enough to form a liquid phase. The available thermal energy at room temperature is insufficient for water to become gas.

Q.11. State the differences between solid, liquid and gas.

Ans.

Q.12. Cotton is solid but it floats on water. Why?

Ans. Cotton contains many pores filled with air, which greatly increases its volume and reduces its average density. Because its density is lower than that of water, cotton floats. If the pores fill with water, the cotton becomes heavier and may sink.

Q.13. Why are solids generally denser than liquids and gases?

Ans. Density = mass/volume. In solids particles are packed closely, so a given mass occupies a small volume, producing higher density. Liquids and gases have larger intermolecular spaces, so for the same mass their volume is larger and density is lower.

Q.14. On a hot sunny day, why do people sprinkle water on the roof or open ground?

Ans. When water evaporates, it absorbs latent heat of vapourisation from the surrounding surface, causing cooling. Sprinkling water increases evaporation and thereby cools the roof or ground.

Q.15. On a hot sunny day, why do we feel pleasant sitting under a tree?

Ans. Leaves lose water by transpiration. The evaporating water cools the air nearby, making the shaded area under the tree feel cooler and more pleasant.

Q.16. The temperature at which liquids change into vapours is very high, for example, if water evaporates at 100°C, then how is water to evaporate at room temperature or at other temperatures?

Ans. Although the boiling point of water is 100°C, surface molecules of a liquid can gain enough energy from surroundings to escape into the vapour state at lower temperatures. This surface vapourisation at temperatures below the boiling point is evaporation. Examples: uncovered water drying and wet clothes drying at room temperature.

Q.17. Name the factors that affect evaporation.

Ans. The rate of evaporation depends on:

• Surface area: Larger area → faster evaporation.
• Temperature: Higher temperature → faster evaporation.
• Humidity: Lower humidity → faster evaporation.
• Wind speed: Higher wind speed removes vapour from the surface → faster evaporation.

Q.18. The melting point of ice is 273.16 K. What does this mean? Explain in detail.

Ans. The given value 273.16 K is essentially equivalent to 273.15 K (commonly used). This means:

• 273.15 K = 0°C, the temperature at which pure ice and water coexist in equilibrium at 1 atm pressure.
• When ice at 0°C is heated, it absorbs heat but its temperature stays constant until all ice melts; the absorbed heat is used to overcome attractive forces between molecules (latent heat of fusion).
• After all ice melts, further heating raises the temperature of the water.

Q.19. How is the high compressibility property of gas useful to us?

Ans. High compressibility allows large volumes of gas to be stored in small cylinders for transport and use. Examples include LPG and CNG for cooking and vehicles, and medical oxygen cylinders. Compressibility saves space and makes handling easier.

Q.20. With the help of an example, explain how diffusion of gases in water is essential.

Ans. Diffusion of gases into water is essential for aquatic life. For example:

• Oxygen from air dissolves into pond water and diffuses to fish and other organisms for respiration.
• Carbon dioxide dissolves and is used by aquatic plants for photosynthesis.

Long Answer Type Questions

Q.1. Pressure and temperature determine the state of a substance. Explain this in detail.
Ans.
(a) Increasing temperature gives particles more energy. For example, heating ice causes its temperature to rise until it reaches its melting point. At that temperature the solid melts to a liquid; further heating converts the liquid to vapour.

(b) Decreasing temperature removes energy from particles. For example, cooling steam causes it to condense into water, and further cooling of water leads to freezing into ice.

(c) Changing pressure can also change the state. Increasing pressure can liquefy gases (as in LPG cylinders where gas is stored as liquid under pressure). Reducing pressure can cause solids like dry ice (solid CO₂) to sublimate directly into gas. Thus the combination of temperature and pressure determines whether a substance is solid, liquid or gas.

Q.2. Explain, giving examples, the various factors on which the rate of evaporation depends.

Ans. The rate of evaporation depends on:

1. Surface area: A larger surface area exposes more molecules to escape into the vapour and so increases evaporation. For example, spreading wet clothes or placing tea in a saucer helps them cool and dry faster.
2. Temperature: Higher temperature supplies more energy to surface molecules so more of them escape as vapour. Water evaporates quicker in sunlight than in the shade.
3. Humidity: If the air already contains a lot of water vapour (high humidity) its capacity to take more vapour is reduced and evaporation slows down. This is why wet clothes dry slowly on humid days.
4. Wind speed: Air movement carries away the vapour near the surface, lowering local humidity and increasing evaporation. A breeze speeds up drying.

Value-Based Answer Type Questions

Q.1. Adil parked his bicycle on a sunny day in a parking stand on his school campus. When the school got over, Adil saw his burst cycle type. Thereafter, he kept less air in his cycle types and did not inflate them fully.
(a) Why did the tyre burst?
(b) Why is air compressible?
(c) What value of Adil is reflected in the above act?
Ans.
(a) The tyre burst because the air inside it heated up in the sun, expanded and exerted extra pressure on the tyre walls. If this pressure exceeded the tyre’s strength, it caused the tyre to burst.
(b) Air is compressible because its molecules are far apart with significant space between them, so they can be pushed closer together under pressure.
(c) Adil showed the values of prudence and responsibility by deciding to keep less air to avoid future bursts. He demonstrated awareness of safety.

Q.2. Akshay’s friend visited his house in Mumbai and he was surprised to see air conditioners installed in all of his rooms. His friend advised Akshay to use water coolers and save electricity. On this, Akshay told him that the water-cooler is not at all effective in coastal areas.
(a) Why are water-coolers not effective in coastal areas?
(b) What are the other two factors on which evaporation of water depends?
(c) What value of Akshay’s friend is seen in this act?
Ans.
(a) Water coolers work by evaporative cooling. In coastal areas the humidity is already high, so the air cannot accept much more moisture and evaporation (and hence cooling) is reduced.
(b) Two other factors affecting evaporation are temperature and surface area.
(c) Akshay’s friend showed the values of environmental concern and thoughtfulness by suggesting an energy-saving alternative.

Q.3. Sita lived in a village and could not afford a refrigerator in her house. She knew how to keep water cold and preserve all perishable items in her house. She kept an ivet cloth surrounding the earthen pot to keep water cool. She also kept vegetables fresh by keeping them in wet gunny bags and sprinkled water over them in a timely manner.
(a) Why did Sita keep wet cloth surrounding the earthen pot?
(b) Suggest one more method of keeping the house cool in summer.
(c) What value of Sita is reflected in the above case?
Ans.
(a) The wet cloth surrounding the earthen pot keeps water cool because the water in the cloth evaporates, and evaporation absorbs heat from the pot, producing a cooling effect.
(b) Sprinkling water on the courtyard or veranda or keeping windows shaded with curtains are simple methods to keep the house cooler in summer.
(c) Sita showed practical wisdom, resourcefulness and a responsible attitude towards using simple, low-cost methods for cooling and preservation.

Q.4. Shreya commutes in a CNG-fitted van to school every day along with many other students. She told the van driver to get the CNG connection certified and timely check it for any leakage or loose connection of pipes. She told the driver to be more careful during summer.
(a) What is CNG?
(b) Why should one be more careful with CNG cylinders during summer?
(c) What value of Shreya is seen in the above act?
Ans.
(a) CNG stands for Compressed Natural Gas, a fuel stored at high pressure in cylinders.
(b) In summer the temperature rises and gas inside cylinders expands, increasing pressure. Any leakage or faulty connection can lead to fire or explosion, so regular checks and certification are necessary for safety.
(c) Shreya demonstrated responsibility, care for others’ safety and civic-minded behaviour by ensuring regular checks and safe use of fuel systems.

Q1: What are manures? Give its classification.
Ans: Substance rich in organic matter and also supplies small quantities of nutrients to soil is called manure. Manure is classified based on the kind of biological material used to make it as :
(i) Compost,
(ii) Vermi-compost,
(iii) Green manure. 

• Compost : The manure prepared by decomposing farm waste, livestock excreta, plant waste, etc. in a pit is known as compost.
• Vermi-compost : When the above given matter is allowed to decompose in the pit along with some earthworms to fasten the process of decomposition is called vermi-composting. 
• Green manure : Some plants like sun-hemp are used to prepare manure by mulching them into soil by plough is known as green manure.

Q2: What are the various methods of irrigation in India?
Ans: Most of agriculture in India is rain-fed, several different kinds of irrigation system are adopted to supply water to agricultural lands. The resources are-wells, canals, rivers and tanks. 

• Wells : Dug wells and tube wells. In dug wells water is collected from water— bearing strata. 
• Tubewells : Water from deeper strata. 
• Canals : Most extensive irrigation system. Canals receive water from reservoirs or rivers. The main canal is divided into branch canals having further distributaries to irrigate fields.
• River lift system : Water is directly drawn from the river for supplementing irrigation in areas close to rivers. 
• Tanks : These are small storage reservoirs, which intercept and store the run-off of smaller catchment areas.

Q3: What are the different patterns of cropping? 

or 

What are the different cropping systems?
Ans: Different systems of growing crop : 

• Mixed cropping : Two or more crops grow simultaneously on the same piece of land, is called mixed cropping.  
  Example : Wheat + grain, wheat + mustard.
• Inter-cropping : It is a method of growing two or more crops simultaneously on the same field in a definite pattern. A few row of one crop alternate with a few rows of second crop.  
  Example : Soyabean + Maize or Bajra + Lobia.(
• Crop rotation : The growing of different crops on a piece of land in a succession is known as crop rotation.

Q4: What are weeds? How can we control them? Give different methods of weed control.
Ans: Unwanted plants which grow along with crops are called weeds. Weeds compete with crops for natural resources; like sunlight, water and nutrients. Thus, weeds hamper the growth of crops. Weeds are usually removed manually by hands and by sickles. This process is called weeding. Sometimes, weedicides are also sprayed to kills weeds.
Weeds can be controlled by different methods :

• Weedicides : These are the chemicals sprayed on the weeds to kill them. Excessive use is poisonous and causes environmental pollution. 
• Mechanical removal : In this method weeds are uprooted by removing manually or by machines.
• Preventive methods : Proper seed bed preparation, timely sowing of crops, intercropping and crop rotation helps in weed control.

Q5: What are the factors for which variety improvement of crop is done?
Ans:

• Higher yield : It increases production of crop. 
• Biotic and abiotic resistance : Crop should be resistant to biotic factors like diseases, insects, pests and abiotic factors like drought, salinity, heat, etc. 
• Change in maturity duration : Short-duration maturity allows farmer to grow more crops in a year with short duration maturity and reduces the crop production cost. 
• Wider adaptability : Crop should be able to adapt to changing environmental conditions. 
• Desirable agronomic characteristics : Crop should have tallness and dwarfness as per need. Dwarfness is required for cereals, so that few nutrients are consumed.

Q6: Large amount of food grains get spoiled every year in India due to improper storage of food grains. How can this be avoided?
Ans: Food grains get spoiled by insects, fungi, rodents, bacteria, moisture at the place of storage.
Storage losses can be reduced by taking some preventive and control measures. 

• The seeds that are to be stored should be dry
• The grains should be cleaned.
• The grains should be fumigated using chemicals that kills pest.
• The storage houses should be waterproof.
• The grains should be stored in sealed gunny bags.
• The bags should be kept few centimetres away from the wall.
• The walls and the floor should be water-proof with no holes in it, to avoid rodents, pests.

Q7: Explain different types of fisheries.
Ans: The different types of fisheries are marine fisheries; inland fisheries, capture fishing, mariculture and aquaculture.

• Marine fisheries : Marine fishes are caught using fishing nets. 
• Mariculture : Marine .fishes are cultured in seawater. 
• Inland fisheries : The fisheries done in freshwater resources like canals, ponds, reservoirs and rivers. 
• Capture fishing : It is done in sea-water, estuaries and lagoons. 
• Aquaculture : Culture of fish done in different water bodies is called aquaculture.

Q8: What are the practices used for dairy industry?
Ans: The practices used for dairy industry to get the optimum yield are : 

• Shelter : The shelter should be clean, spacious and airy. (
• Feeding : Proper food at proper time is essential for dairy animals. 
• Rearing of animals : Providing them proper health care and protection from pathogens, diseases and proper vaccination. 
• Breeding : The crossing of different variety of milch animals to obtain a breed that can produce more yield of milk.

Q9: Write a paragraph in your own words on each of the following.
(a) Preparation of soil,
(b) Sowing,
(c) Weeding,
(d) Threshing
Ans:
(a) Preparation of soil : Preparation of soil is the first step of farming. Soil is loosened and turned over. This helps in making the soil more airy so that roots can breathe in air. Moreover, loosening of soil also facilitates better penetration of roots into the soil. Seeds can be easily sown in loosened soil.
(b) Sowing : The method of putting the seeds into soil is called sowing. Traditionally, seed is sown manually by spreading the seeds by hands. This process is called broadcasting. Seed drills are used when sowing needs to be done on a large scale.
(c) Weeding : Removal of weeds is called weeding. Unwanted plants which grow along with the crop are called weeds. They compete for resources; like sunlight, water and air; with the main crop. So, it is necessary to remove weeds for proper growth of crops. Weeding is usually done manually by using hands and sickles. Sometimes weedicides are also sprayed.
(d) Threshing : Separation of grains from harvested stems is called threshing. For smaller quantity, threshing is done by hands. For somewhat bigger quantity, threshing is done using animal; especially bullocks. Animals are made to trample over the harvested stock which helps in separation of grains. Threshing machines are used for bigger quantities.

Q10: If wheat is sown in the kharif season, what would happen? Discuss.
Ans: The crop of wheat needs mild to moderate temperature and frost free days; along with irrigation but no water logging. Winters are suitable for growing wheat. In the kharif season; which coincides with the peak summer months in India, temperature is at its peak which is not suitable for wheat. Moreover, during rainy season lot of water accumulates in fields which would be harmful for wheat crop. Hence, if wheat is sown in the kharif season; the productivity would be minuscule and would not be profitable for the farmers.

Q11: Give brief sketch on advantages and disadvantages on manure and fertilizers.
Ans:
Advantages of manure 

• It increases the number of friendly microbes. 
• It improves the texture of soil by adding organic matter (humus). 
• It increases soil fertility, water holding capacity and aeration. 
• It reduces soil erosion. 
• It is cheap.

Disadvantages of manure

• They have fewer amounts of nutrients as compared to fertilizers.
• Manures are bulky and not easy to store and transport.

Fertilizers : These are commercially manufactured inorganic salts containing one or more essential plant nutrients like NPK, which are used to increase soil fertility.
Advantages of fertilizers

•  They are nutrient specific and required in small amounts. 
• They are water soluble and absorbed by the plant easily. 
• They are easy to store and transport. 

Disadvantages of fertilizers

• Fertilizers can change the soil structure by killing the soil microbes. 
• Fertilizers can change the chemical composition of soil. 
• Accumulation of fertilizers in water bodies causes eutrophication.

Q1: (i) Draw the sound waves for a low pitched and the high pitched sound.
(ii) Write one use of ultrasonography.
(iii) Which wave property determines pitch?
Ans:
(i) The diagram is as shown:
Frequency of sound wave(ii) Ultrasonography is used for examination of the fetus during pregnancy to detect congenital defects and growth abnormalities.
(iii) The pitch depends on frequency.

Q2: The stem of a tuning fork is pressed against a table top. Answer the following questions :
(i) Would the above action produce any audible sound?
(ii) Does the above action cause the table to set into vibrations?
(iii) If the answer above is yes, what type of vibrations are they? 
(iv) Under what conditions does the above action lead to resonance?
Ans:
(i) Yes, there is an audible sound produced.
(ii) Yes, the table top is set into ‘forced vibrations’ by this.
(iii) The vibrations are forced vibrations.
(iv) P ressing the stem of a vibrating tuning fork against a table top, would lead to resonance if the frequency of the tuning fork equals the natural frequency of oscillations of the table top.

Q3: How does the sound produced by a vibrating object in a medium reach your ear?
Ans : When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure called compression. This compression starts to move away from the vibrating object. When vibrating object moves backwards, it creates a region of low pressure called refraction. As the object moves forth and back rapidly, a series of compressions and refractions are created in the air. These produce the sound wave that propagates through the medium. This continues until the sound wave reaches to the ear of the listener.
Q4: Write conditions for the production of an echo. 
Ans: Conditions for the production of an echo are :

• Time gap: The echo is heard if the original sound reflected by an obstacle reaches our ears after at least 0.1 seconds.
• Distance: The minimum distance between the sound source and the obstacle must be 17.2 metres in air at 25°C for a clear echo.
• Nature of the obstacle: The reflecting surface must be rigid, such as a building, hill, or cliff.
• Size of the obstacle: The obstacle should be large enough to effectively reflect the sound.

Q5: With the help of a labelled diagram show that sound needs a material medium for its propagation.
Ans: A well labelled diagram is as shown :

Bell jar experiment

(i) Take an electric circuit which consists of a cell, a switch and an electric bell arranged inside a bell jar, which stands on the platform of an evacuating pump.
(ii) The switch of the bell is pressed to close the electric circuit. When there is air within the bell jar, sound is heard. Air is now pumped out of the bell jar. When the air is completely removed from the bell jar, no sound is heard as it is obvious from fig. because the medium of air which has to carry energy from the bell to the bell jar is removed. It shows that sound needs material medium for its propagation.

Q6: A particular transmitter of Aakashvani broadcasts at 420.5 m wavelength. (Given the speed of radio waves 3 × 108 ms^–1) Calculate the frequency at which the radio station broadcasts its program. What is the direction of oscillations of the medium particles through which a :
(i) Transverse w ave is propagating?
(ii) Longitudinal wave is propagating?
Ans:
Given
λ = 420.5 m, v = 3 × 108 ms^–1, n = ?
Using the expression v = nλ

= 7 × 10⁵ Hz
(i) The particles oscillate perpendicular to the direction of propagation of the wave.
(ii) The particles oscillate parallel to the direction of propagation of the wave.

Q7: (i) State two characteristics of wave motion.
(ii) W hat is the relation between frequency, wavelength and speed of a wave?
Ans:
(i) A wave motion is periodic in nature. The particles of the medium do not move from their mean position but execute vibration but only the energy is transmitted from one point to another.
(ii) Speed = wavelen gth  × frequency
v = λ x v

Q8:Give uses of multiple reflection of sound.
Ans: There are several uses of multiple reflection of sound :

• Megaphone: This horn-shaped device is used to address public gatherings. When you speak into a megaphone, sound waves are reflected and directed towards the audience, minimising spreading.
• Concert Halls: The ceilings in concert halls and auditoriums are often curved. This design helps sound reflect evenly throughout the space, ensuring all areas receive sound. Additionally, these ceilings use sound-absorbing materials to reduce reverberation.
• Stethoscope: Doctors use this instrument to listen to sounds from the heart and lungs. The sounds produced by these organs reach the doctor’s ears through multiple reflections.
• Sound Boards: These are curved surfaces (concave) placed in large halls to direct sound waves towards the audience. The speaker is positioned at the focus of the sound board, enhancing sound delivery.
• Hearing Aids: For individuals with hearing difficulties, hearing aids concentrate incoming sound waves into a narrow beam through reflection. This beam vibrates the ear’s diaphragm with greater amplitude, improving hearing ability.

Q9: Give application of ultrasound (ultrasonic waves).
Ans: Ultrasonic waves have number of uses :

• Ultrasonic vibrations are used to homogenise milk, breaking down larger fat particles into smaller ones.
• In dishwashing machines, ultrasonic vibrations help clean utensils by agitating detergent particles.
• These vibrations can create a depression effect in pests like rats and cockroaches.
• Ultrasonic waves are essential for monitoring fetal growth during pregnancy.
• They are also effective in relieving pain in joints and muscles.
• In industry, ultrasonic vibrations detect flaws in metal products and measure their thickness.

Q10: A tuning fork produces 1024 waves in 4 seconds. Calculate the frequency to the tuning fork.
Ans: The tuning fork produces 1024 waves in 4 seconds. To find the frequency:

• Frequency (v) is the number of vibrations per second.
• Calculate by dividing the total number of waves by the time:
• Frequency = 1024 waves / 4 seconds
• Thus, Frequency = 256 Hz.

Q1: A force of 10N causes a displacement of 2 m in a body in its own direction. Calculate the work done by force.

Ans:

The work done by a constant force acting in the direction of displacement is given by

Work = F × s

= 10 N × 2 m

= 20 J

The work is positive because the force and displacement are in the same direction.

Q2: How much force is applied on the body when 150 joule of work is done in displacing the body through a distance of 10 m in the direction of force?

Ans: 

Given W = 150 J and s = 10 m.

Using W = F × s, we get

F = W / s

= 150 J / 10 m

= 15 N

The force applied in the direction of displacement is 15 N.

Q3: A body of 5 kg raised to 2 m find the work done.

Ans:

The work done to raise a body against gravity equals the gain in gravitational potential energy (PE).

PE = mgh

= 5 kg × 9.8 m/s² × 2 m

= 98 J

Therefore, the work done in raising the body is 98 J.

Q4: A work of 4900 J is done on load of mass 50 kg to lift it to a certain height. Calculate the height through which the load is lifted.

Ans: 

Work done in lifting = increase in potential energy = mgh.

4900 J = 50 kg × 9.8 m/s² × h

h = 4900 / (50 × 9.8) = 4900 / 490 = 10 m

The load is lifted through a height of 10 m.

Q5: An engine work 54,000 J work by exerting a force of 6000 N on it. What is the displacement of the force?

Ans:

Displacement s = W / F

= 54000 J / 6000 N= 9 m

Thus, the displacement is 9 m.

Q6: A body of mass 2 kg is moving with a speed of 20 m/s. Find the kinetic energy.

Ans:

Kinetic energy (KE) = ½ mv²

= 0.5 × 2 kg × (20 m/s)²

= 0.5 × 2 × 400

= 400 J

The kinetic energy of the body is 400 J.

Q7: A hammer of mass 1 kg falls freely from a height of 2 m. Calculate (I) The velocity and (II) The kinetic energy of the hammer just before it touches the ground. Does the velocity of hammer depend on the mass of hammer?

Ans: 

Use energy conservation (neglecting air resistance): loss of potential energy = gain in kinetic energy.
PE at top = mgh = 1 kg × 9.8 m/s² × 2 m = 19.6 J
Thus KE just before impact = 19.6 J.
Let v be the speed just before impact. Then ½ m v² = 19.6 J.
19.6 = 0.5 × 1 × v² → v² = 39.2 → v ≈ 6.26 m/s.
(I) Velocity just before touching ground ≈ 6.26 m/s.
(II) Kinetic energy just before impact = 19.6 J.
The velocity does not depend on the mass of the hammer (neglecting air resistance) because v for a freely falling object from rest after falling height h is v = √(2gh), which contains no mass term.

Q8: A weight of 50 kg runs up a hill rising himself vertically 10 m in 20 sec. Calculate power. (Given g = 9.8 ms^-1)

Ans: Work done = increase in potential energy = mgh 
= 50 kg × 9.8 m/s² × 10 m 
= 4900 J.

Power = Work / time = 4900 J / 20 s = 245 W.

The required power is 245 W.

Q9: A rickshaw puller pulls the rickshaw by applying a force of 100 N. If the rickshaw moves with constant velocity of 36 kmh-1. Find the power of rickshaw puller.

Ans:

Force = 100 N.

Velocity = 36 km/h = 36 × 5 / 18 = 10 m/s.

Power = Force × Velocity = 100 N × 10 m/s = 1000 W.

Therefore, the rickshaw puller supplies 1000 W of power. At constant velocity this power balances resistive forces such as friction and air resistance.

Q10: An athlete weighing 60 kg runs up a staircase having 10 steps each of 1 m in 30 sec. Calculate power (g = 9.8ms^-1).

Ans:

Total vertical height climbed h = 10 × 1 m = 10 m.

Work done = increase in potential energy = mgh = 60 kg × 9.8 m/s² × 10 m = 5880 J.

Power = Work / time = 5880 J / 30 s = 196 W.

The athlete’s power while climbing is 196 W.

Q11: A horse exert a force of 200N to pull the cart. If the horse cart system moves with velocity 36 kmh^-1 on the level road, then find the power of horse in term of horse power (1 HP = 746 W).

Ans:

Velocity = 36 km/h = 10 m/s.

Power = F × v = 200 N × 10 m/s = 2000 W.

In horsepower: 2000 W ÷ 746 W per HP ≈ 2.68 HP.

So the horse develops about 2.68 horsepower.

Q12: An electric kettle of 500W is used to heat water everyday for 2 hours. Calculate the number of unit of electrical energy consumed by it in 10 days

Ans:

Power = 500 W = 0.5 kW. Daily usage = 2 h, for 10 days total time = 2 × 10 = 20 h.

Energy = P × t = 0.5 kW × 20 h = 10 kWh = 10 units.

The kettle consumes 10 units of electrical energy in 10 days.

Q13: Calculate the cost of using a 2 kWh immersion rod for heating water in a house for one hour each day for 60 days if the rate is 1.50 per unit kWh.

Ans:

Power = 2 kW, daily time = 1 h, period = 60 days → total energy = 2 kW × 60 h = 120 kWh = 120 units.

Cost = 120 units × Rs 1.50 per unit = Rs 180.

The total cost is Rs 180.

Q14: In an experiment to measure his power, a student records the time taken by him in running up a flight of steps on a staircase.

Ans:

Use the following data to calculate the power of the student :

Number of steps = 28,

Height of each step = 20 cm,

Time taken = 5.4 s,

Mass of student = 55 kg,

Acceleration due to gravity = 9.8 ms^-2

The total vertical height climbed = 28 × 0.20 m = 5.6 m.

Work done = mgh = 55 kg × 9.8 m/s² × 5.6 m = 3018.4 J.

Power = Work / time = 3018.4 J / 5.4 s ≈ 559 W.

The student’s power while climbing the steps is approximately 559 W.

Q15: The power of a heart which beats 72 times in a minute is 1.2 kW. Calculate the work done by heart for each beat. (1 kJ)

Ans: 

Power P = 1.2 kW = 1200 W. Time interval considered = 1 minute = 60 s.

Total work done in 60 s = P × t = 1200 W × 60 s = 72000 J.

Number of beats in 60 s = 72. Work done per beat = 72000 J / 72 = 1000 J = 1 kJ.

Work done by the heart in each beat is 1 kJ.

Q16: Calculate the electricity bill amount for a month of 31 days, if the following devices are used as specified. 
(a) 3 bulbs of 40 W for 6 hours. 
(b) 4 tubelights of 50 W for 8 hours, 
(c) A TV of 120 W for 6 hours.
The rate of electricity is Rs 2.50 per unit.

Ans: 

Convert powers to kW and find daily energy consumption:

E₁ (3 bulbs) = 3 × 40 W × 6 h = 720 Wh = 0.72 kWh per day.

E₂ (4 tubelights) = 4 × 50 W × 8 h = 1600 Wh = 1.60 kWh per day.

E₃ (TV) = 120 W × 6 h = 720 Wh = 0.72 kWh per day.

Total per day = 0.72 + 1.60 + 0.72 = 3.04 kWh per day.

For 31 days: Energy = 3.04 kWh × 31 = 94.24 kWh (units).

Cost = 94.24 × Rs 2.50 = Rs 235.60.

The electricity bill for the month is Rs 235.60.

Q17: (a) What is meant by mechanical energy? State its two forms. State the law of conservation of energy. Give an example in which we observe a continuous change of one form of energy into another and vice-versa.
(b) Calculate the amount of work required to stop a car of 1000 kg moving with a speed of 72 km/h.

Ans:

(a) Mechanical energy of a body is the sum of its kinetic energy and gravitational potential energy. Its two forms are:

– Kinetic energy (energy of motion).

– Potential energy (energy due to position in a force field, e.g., gravity).

Law of conservation of energy: Energy can neither be created nor destroyed; it can only be transformed from one form to another.

Example: In a simple pendulum, energy continuously converts between kinetic and potential energy. At the mean position the energy is mainly kinetic, and at the extreme positions it is mainly potential. During oscillation the total mechanical energy (neglecting friction) remains constant.

(b) Given: m = 1000 kg, initial speed u = 72 km/h = 20 m/s, final speed v = 0.

Work required to stop = change in kinetic energy = ½ m (v² – u²) = -½ m u².

Magnitude of work done by the brakes = ½ × 1000 kg × (20 m/s)² = 0.5 × 1000 × 400 = 200000 J = 2 × 10⁵ J.

The brakes must do 2 × 10⁵ J of work (energy removed) to bring the car to rest.

Q1: A ball thrown up is caught by the thrower after 4 s. With what velocity was it thrown up? How high did it go? Where was it after 3 s ? (g = 9.8 ms^–2) 
Ans: In case of upward motion of the ball from A to B

• Initial velocity, u = ?
• Final velocity, v = 0 (at maximum height)
• Time taken by the ball to reach the highest point = 2s (time of ascent = time of descent)
• Acceleration due to gravity g = -9.8 m/s² (upward motion)

Finding the initial velocity of the ball

Using the first equation of motion, v = u + gt:

v = u – gt

0 = u – 9.8 × 2

u = 19.6 m/s

The initial velocity of the ball is 19.6 m/s.

Finding the maximum height (h) attained by the ball

Using the second equation of motion, h = ut + 1/2 gt²:

h = 19.6 × 2 – 1/2 × 9.8 × (2)²

h = 39.2 – 19.6

h = 19.6 m

Let the ball be at C after t = 3 sec

Consider motion from A to C

• u = 19.6 m/s
• t = 3 s
• g = -9.8 m/s²
• s = h’

s = ut + 1/2 gt²

h’ = 19.6 × 3 – 1/2 × 9.8 × (3)²

h’ = 58.8 – 44.1 = 14.7 m

Distance from top

x = h – h’

x = 19.6 – 14.7 = 4.9 m

Hence, the ball goes to the maximum height of 19.6 m, the velocity at which it was thrown is 19.6 m/s, and the distance below its highest point after 3 sec is 4.9 m.

Q2: Calculate the force of gravitation due to a child of mass 25 kg on his mother of mass 75 kg if the distance between their centres is 1 m from each other. Given G = (20/3) × 10^–11 Nm² kg^–2
Ans: Here m₁ = 25 kg; m₂ = 75 kg; d = 1 m;
Using,

F = 12,500 × 10^–11
or F = 1.25 × 10^–7 N

Q3: A sealed tin of Coca-Cola of 400 g has a volume of 300 cm³. Calculate the density of the tin.
Ans: Here,
mass of tin, M = 400 g
Volume of tin, V = 300 cm³
Density of tin,


Q4: A sealed can of mass 600 g has a volume of 500 cm3. Will this can sink in water? Density of water is 1 g cm^–3.
Ans: Here,
mass of can, M = 600 g
Volume of can, V = 500 cm³
Density of can, 
Since, density of the can is greater than the density of water, so the can will sink in water.

Q5: The gravitational force between two objects is 49 N. How much distance between these objects be decreased so that the force between them becomes double?
Ans: Let ‘r’ be the distance between the object of mass m₁ and m₂

Now, the distance is reduced to ‘x’ so that the force become twice, then

Dividing eq. (i) by (ii)

So, the distance must decrease by  times the original distance.

Q6: A force of 200 N is applied perpendicular to its surface having area 4 square metres. Calculate the pressure.
Ans:
Thrust = 200 N
Area = 4 m2  
Pressure = ?  
Pressure = Thrust / Area = 
= 50 Nm^–2 = 50 Pa

Q7: The density of water is 1000 kg m³. If relative density of iron is 7.874, then calculate the density of iron.
Ans:  
Density of water = 1000 kg/m³
Relative density (R.D.) of iron = 7.874
Using, R.D. of iron we get  
Density of iron = R.D. of iron   × density of water  
= 7.874 × 1000 kg/m³
= 7874 kg/m³.

Q8: What is the force of gravitation between two point masses of 1 kg and 2 kg kept 1 m apart?
Ans:
m = 1 kg, m₂ = 2 kg, r = 1 m

= 13.34 x 10^-11N
This is an extremely small force.

Q9: A particle is thrown up vertically with a velocity of 50 m/s. (a) What will be its velocity at the highest point of its journey? (b) How high would the particle rise? (c) What time would it take to reach the highest point?
Ans: 
At the highest point the velocity will be zero. Considering activity A to B
Using
 = u + at
0 = 50 – 9.8 × t
t = 5.1 sec
Also v² – u² = 2as s = 127.5 m

Q10: Weight of a girl is 294 N. Find her mass.
Ans: 
W = mg  
294 = m × 9.8  m
Q11: How much force should be applied on an area of 1 cm² to get a pressure of 15 Pa?
Ans: 
Here,
Area, A = 1 cm² = 10–4 m²
Pressure (P) = 15 Pa = 15 N/m²
As F = P × A  
= (15 N/m²) × (10–4 m²)  
= 1.5 × 10^–3 N

Q12: A force of 20N acts upon a body whose weight is 9.8N. What is the mass of the body and how much is its acceleration? Given, Force = 20 N, Weight W = 9.8 N. We know, W = mg; 9.8 = m × 9.8 m = 1 kg 
Ans: 
We know
F = ma
20 = 1 × a  
a = 20 m/s²

Q13: An object is thrown vertically upwards and reaches a height of 78.4 m. Calculate the velocity at which the object was thrown? (g = 9.8 m/s²)
Ans: Given,
h = 78.4 m  
v = 0  
g = –9.8 m/s²
Now, v² = u² – 2ghu = 39.2 m/s²

Q14: Two bodies A and B having masses 2 kg and 4 kg respectively are separated by 2 m. Where should a body of mass 1 kg be placed so that the gravitational force on this body due to bodies A and B is zero?
Ans:  
Mass of body a is M_a = 2 kg
Mass of body b is M_b = 4 kg
Mass of body c is M_c = 1 kg
Separation between a and b = 2 m
Let the body C be placed at a distance d from body A Gravitational force between A and C

Gravitational force between B and C is

For body C the gravitational force is 0.

Hence, F_AC = F_BC

 d = 0.83

Q15: Let us find force of attraction between two blocks lying 1 m apart. Let the mass of each block is 40 kg.
Ans:
F = ?
m₁ = 40 kg
m₂ = 40 kg
d = 1 m
G = 6.67 × 10–11 Nm²kg^–2

= 1.0672 x 10^-7N

Q1: State all 3 Newton’s law of motion.
Ans: 

• Newton’s I law of motion : An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.
• Newton’s II law of motion : The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
• Newton’s III law of motion : To every action, there is an equal and opposite reaction and they act on two different bodies.

Q2: Explain inertia and momentum.
Ans :

• Inertia : The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia. For example : A book lying on a table will remain there until an external force is applied on it to remove or displace it from that position.
• Momentum : Momentum of body is the quantity of motion possessed by the body. It is equal to the product of the mass and velocity of the body and is denoted by p. p = mv Momentum is a vector quantity and its direction is same as the direction of velocity of the object. Its SI unit is kilogram metre per second (kg ms^–1).

Q3: Define force. What are different types forces?
Ans: 

• Force : It is a push or pull on an object that produces acceleration in the body on which it acts. The S.I. unit of force is Newton.
• Types of forces : Balanced force : When the forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.
• Unbalanced force : When two opposite forces acting on a body move a body in the direction of the greater force or change the state of rest, such forces are called as unbalanced forces.
• Frictional force : Force of friction is the force that always opposes the motion of object.

Q4: What is inertia? Explain different types of inertia.
Ans: 

• Inertia : The natural tendency of an object to resist change in their state of rest or of motion is called inertia. The mass of an object is a measure of its inertia. Its S.I. unit is kg.

Types of inertia :

• Inertia of rest : The object remain in rest unless acted upon by an external unbalanced force.
• Inertia of motion : The object in the state of uniform motion will continue to remain in motion with same speed and direction unless external force is not applied on it.

Q5: Give example to show the effects of force.
Ans: 
(i) Place a ball in the ground. Kick it with your foot. The ball starts moving. The ball moves because of the force applied to it.
(ii) If ball is coming towards you, you can kick it in any direction. The direction of motion of the ball changes because of the force applied to it.
(iii) Place a rubber on the ground. Press it with your foot. It is found that the ball is no longer round but takes the shape of an egg, i.e. it is oblong. The shape of the ball has changed because of the force applied on the ball.

Q6: (a) What is friction?
(b) What are the advantages of friction? or Why is friction necessary?
Ans: 
(a) The force which opposes the motion of one body over the surface of another is called friction or the force of friction.
(b) Friction is very desirable and an important force in our daily life.
Some advantages of friction are :

• The nails and screws hold the wooden boards together due to friction. 
• It is due to the friction between the ground/road and the soles of our shoes that we are able to walk. 
• The friction between the road and the surface of the tyres permits safe driving. 
• W e are able to write on paper because of the friction between the pen/pencil and the paper. That is why, it is very difficult to write on a glazed/waxed paper. 
• The application of brakes to stop a cycle, scooter or car, etc., is possible due to friction between the brake-lining and the rim of the wheel.

Q7: (i)  What are the causes of friction?
(ii) How can friction between any two surfaces can be reduced?  or  Describe some method for reducing friction.
Ans: The friction rises due to the following factors : 

• Due to the force of attraction between the molecules of the two surfaces in contact. This is called the force of adhesion. 
• Due to the interlocking of the surface irregularities. 
• Thus, friction is due to the roughness of the two surfaces in contact. 
• The friction between two surfaces can be reduced by following methods :
  • By polishing surfaces : Rough surfaces can be made smooth by polishing. Therefore, polishing reduces friction. 
  • By applying oil or grease on the surfaces : Oil/ grease forms a thin layer between the two surfaces and reduces friction.

Long Question Answer: Force and Law of Motion

Q1: State all 3 Newton’s law of motion.
Ans: 

• Newton’s I law of motion : An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.
• Newton’s II law of motion : The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
• Newton’s III law of motion : To every action, there is an equal and opposite reaction and they act on two different bodies.

Q2: Explain inertia and momentum.
Ans :

• Inertia : The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia. For example : A book lying on a table will remain there until an external force is applied on it to remove or displace it from that position.
• Momentum : Momentum of body is the quantity of motion possessed by the body. It is equal to the product of the mass and velocity of the body and is denoted by p. p = mv Momentum is a vector quantity and its direction is same as the direction of velocity of the object. Its SI unit is kilogram metre per second (kg ms^–1).

Q3: Define force. What are different types forces?
Ans: 

• Force : It is a push or pull on an object that produces acceleration in the body on which it acts. The S.I. unit of force is Newton.
• Types of forces : Balanced force : When the forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.
• Unbalanced force : When two opposite forces acting on a body move a body in the direction of the greater force or change the state of rest, such forces are called as unbalanced forces.
• Frictional force : Force of friction is the force that always opposes the motion of object.

Q4: What is inertia? Explain different types of inertia.
Ans: 

• Inertia : The natural tendency of an object to resist change in their state of rest or of motion is called inertia. The mass of an object is a measure of its inertia. Its S.I. unit is kg.

Types of inertia :

• Inertia of rest : The object remain in rest unless acted upon by an external unbalanced force.
• Inertia of motion : The object in the state of uniform motion will continue to remain in motion with same speed and direction unless external force is not applied on it.

Q5: Give example to show the effects of force.
Ans: 
(i) Place a ball in the ground. Kick it with your foot. The ball starts moving. The ball moves because of the force applied to it.
(ii) If ball is coming towards you, you can kick it in any direction. The direction of motion of the ball changes because of the force applied to it.
(iii) Place a rubber on the ground. Press it with your foot. It is found that the ball is no longer round but takes the shape of an egg, i.e. it is oblong. The shape of the ball has changed because of the force applied on the ball.

Q6: (a) What is friction?
(b) What are the advantages of friction? or Why is friction necessary?
Ans: 
(a) The force which opposes the motion of one body over the surface of another is called friction or the force of friction.
(b) Friction is very desirable and an important force in our daily life.
Some advantages of friction are :

• The nails and screws hold the wooden boards together due to friction. 
• It is due to the friction between the ground/road and the soles of our shoes that we are able to walk. 
• The friction between the road and the surface of the tyres permits safe driving. 
• W e are able to write on paper because of the friction between the pen/pencil and the paper. That is why, it is very difficult to write on a glazed/waxed paper. 
• The application of brakes to stop a cycle, scooter or car, etc., is possible due to friction between the brake-lining and the rim of the wheel.

Q7: (i)  What are the causes of friction?
(ii) How can friction between any two surfaces can be reduced?  or  Describe some method for reducing friction.
Ans: The friction rises due to the following factors : 

• Due to the force of attraction between the molecules of the two surfaces in contact. This is called the force of adhesion. 
• Due to the interlocking of the surface irregularities. 
• Thus, friction is due to the roughness of the two surfaces in contact. 
• The friction between two surfaces can be reduced by following methods :
  • By polishing surfaces : Rough surfaces can be made smooth by polishing. Therefore, polishing reduces friction. 
  • By applying oil or grease on the surfaces : Oil/ grease forms a thin layer between the two surfaces and reduces friction.

Q8: When a force of 40 N is applied on a body it moves with an acceleration of 5 ms2. Calculate the mass of the body.
Ans: 
Let m be the mass of the body.
Given : F = 40 N, a = 5 ms² 
From the relation F = m a, we have  
40 = m × 5
m = 40/5 = 8 kg

Q9: An object undergoes an acceleration of 8 ms–2 starting from rest. Find the distance travelled in 1 second.
Ans: Given,
Acceleration, a = 8 ms^–2 
Initial velocity, u = 0
Time interval, t = 1 s
Distance travelled, s = ?
Using the equation of motion,
s = ut + 1/2 at², one gets  
s = 0 × 1 + 1/2 × 8 × 1² = 4 m
The object travels a distance of 4 m.

Q10: Calculate the force required to impact to a car, a velocity of 30 ms–1 in 10 seconds. The mass of the car is 1,500 kg.
Ans:
Here
u = 0 ms^–1;
v = 30 ms^–1;
t = 10 s;
a = ?
Using v = u + at, we have  
30 = 0 + a (10) a
= 3 ms–2
Now
F = ma
= 1,500 × 3 or F = 4,500 NLong Question Answer: Force and Law of Motion

Q1: State all 3 Newton’s law of motion.
Ans: 

• Newton’s I law of motion : An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.
• Newton’s II law of motion : The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
• Newton’s III law of motion : To every action, there is an equal and opposite reaction and they act on two different bodies.

Q2: Explain inertia and momentum.
Ans :

• Inertia : The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia. For example : A book lying on a table will remain there until an external force is applied on it to remove or displace it from that position.
• Momentum : Momentum of body is the quantity of motion possessed by the body. It is equal to the product of the mass and velocity of the body and is denoted by p. p = mv Momentum is a vector quantity and its direction is same as the direction of velocity of the object. Its SI unit is kilogram metre per second (kg ms^–1).

Q3: Define force. What are different types forces?
Ans: 

• Force : It is a push or pull on an object that produces acceleration in the body on which it acts. The S.I. unit of force is Newton.
• Types of forces : Balanced force : When the forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.
• Unbalanced force : When two opposite forces acting on a body move a body in the direction of the greater force or change the state of rest, such forces are called as unbalanced forces.
• Frictional force : Force of friction is the force that always opposes the motion of object.

Q4: What is inertia? Explain different types of inertia.
Ans: 

• Inertia : The natural tendency of an object to resist change in their state of rest or of motion is called inertia. The mass of an object is a measure of its inertia. Its S.I. unit is kg.

Types of inertia :

• Inertia of rest : The object remain in rest unless acted upon by an external unbalanced force.
• Inertia of motion : The object in the state of uniform motion will continue to remain in motion with same speed and direction unless external force is not applied on it.

Q5: Give example to show the effects of force.
Ans: 
(i) Place a ball in the ground. Kick it with your foot. The ball starts moving. The ball moves because of the force applied to it.
(ii) If ball is coming towards you, you can kick it in any direction. The direction of motion of the ball changes because of the force applied to it.
(iii) Place a rubber on the ground. Press it with your foot. It is found that the ball is no longer round but takes the shape of an egg, i.e. it is oblong. The shape of the ball has changed because of the force applied on the ball.

Q6: (a) What is friction?
(b) What are the advantages of friction? or Why is friction necessary?
Ans: 
(a) The force which opposes the motion of one body over the surface of another is called friction or the force of friction.
(b) Friction is very desirable and an important force in our daily life.
Some advantages of friction are :

• The nails and screws hold the wooden boards together due to friction. 
• It is due to the friction between the ground/road and the soles of our shoes that we are able to walk. 
• The friction between the road and the surface of the tyres permits safe driving. 
• W e are able to write on paper because of the friction between the pen/pencil and the paper. That is why, it is very difficult to write on a glazed/waxed paper. 
• The application of brakes to stop a cycle, scooter or car, etc., is possible due to friction between the brake-lining and the rim of the wheel.

Q7: (i)  What are the causes of friction?
(ii) How can friction between any two surfaces can be reduced?  or  Describe some method for reducing friction.
Ans: The friction rises due to the following factors : 

• Due to the force of attraction between the molecules of the two surfaces in contact. This is called the force of adhesion. 
• Due to the interlocking of the surface irregularities. 
• Thus, friction is due to the roughness of the two surfaces in contact. 
• The friction between two surfaces can be reduced by following methods :
  • By polishing surfaces : Rough surfaces can be made smooth by polishing. Therefore, polishing reduces friction. 
  • By applying oil or grease on the surfaces : Oil/ grease forms a thin layer between the two surfaces and reduces friction.

Q8: When a force of 40 N is applied on a body it moves with an acceleration of 5 ms2. Calculate the mass of the body.
Ans: 
Let m be the mass of the body.
Given : F = 40 N, a = 5 ms² 
From the relation F = m a, we have  
40 = m × 5
m = 40/5 = 8 kg

Q9: An object undergoes an acceleration of 8 ms–2 starting from rest. Find the distance travelled in 1 second.
Ans: Given,
Acceleration, a = 8 ms^–2 
Initial velocity, u = 0
Time interval, t = 1 s
Distance travelled, s = ?
Using the equation of motion,
s = ut + 1/2 at², one gets  
s = 0 × 1 + 1/2 × 8 × 1² = 4 m
The object travels a distance of 4 m.

Q10: Calculate the force required to impact to a car, a velocity of 30 ms–1 in 10 seconds. The mass of the car is 1,500 kg.
Ans:
Here
u = 0 ms^–1;
v = 30 ms^–1;
t = 10 s;
a = ?
Using v = u + at, we have  
30 = 0 + a (10) a
= 3 ms–2
Now
F = ma
= 1,500 × 3 or F = 4,500 NLong Question Answer: Force and Law of Motion

Q1: State all 3 Newton’s law of motion.
Ans: 

• Newton’s I law of motion : An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.
• Newton’s II law of motion : The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
• Newton’s III law of motion : To every action, there is an equal and opposite reaction and they act on two different bodies.

Q2: Explain inertia and momentum.
Ans :

• Inertia : The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia. For example : A book lying on a table will remain there until an external force is applied on it to remove or displace it from that position.
• Momentum : Momentum of body is the quantity of motion possessed by the body. It is equal to the product of the mass and velocity of the body and is denoted by p. p = mv Momentum is a vector quantity and its direction is same as the direction of velocity of the object. Its SI unit is kilogram metre per second (kg ms^–1).

Q3: Define force. What are different types forces?
Ans: 

• Force : It is a push or pull on an object that produces acceleration in the body on which it acts. The S.I. unit of force is Newton.
• Types of forces : Balanced force : When the forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.
• Unbalanced force : When two opposite forces acting on a body move a body in the direction of the greater force or change the state of rest, such forces are called as unbalanced forces.
• Frictional force : Force of friction is the force that always opposes the motion of object.

Q4: What is inertia? Explain different types of inertia.
Ans: 

• Inertia : The natural tendency of an object to resist change in their state of rest or of motion is called inertia. The mass of an object is a measure of its inertia. Its S.I. unit is kg.

Types of inertia :

• Inertia of rest : The object remain in rest unless acted upon by an external unbalanced force.
• Inertia of motion : The object in the state of uniform motion will continue to remain in motion with same speed and direction unless external force is not applied on it.

Q5: Give example to show the effects of force.
Ans: 
(i) Place a ball in the ground. Kick it with your foot. The ball starts moving. The ball moves because of the force applied to it.
(ii) If ball is coming towards you, you can kick it in any direction. The direction of motion of the ball changes because of the force applied to it.
(iii) Place a rubber on the ground. Press it with your foot. It is found that the ball is no longer round but takes the shape of an egg, i.e. it is oblong. The shape of the ball has changed because of the force applied on the ball.

Q6: (a) What is friction?
(b) What are the advantages of friction? or Why is friction necessary?
Ans: 
(a) The force which opposes the motion of one body over the surface of another is called friction or the force of friction.
(b) Friction is very desirable and an important force in our daily life.
Some advantages of friction are :

• The nails and screws hold the wooden boards together due to friction. 
• It is due to the friction between the ground/road and the soles of our shoes that we are able to walk. 
• The friction between the road and the surface of the tyres permits safe driving. 
• W e are able to write on paper because of the friction between the pen/pencil and the paper. That is why, it is very difficult to write on a glazed/waxed paper. 
• The application of brakes to stop a cycle, scooter or car, etc., is possible due to friction between the brake-lining and the rim of the wheel.

Q7: (i)  What are the causes of friction?
(ii) How can friction between any two surfaces can be reduced?  or  Describe some method for reducing friction.
Ans: The friction rises due to the following factors : 

• Due to the force of attraction between the molecules of the two surfaces in contact. This is called the force of adhesion. 
• Due to the interlocking of the surface irregularities. 
• Thus, friction is due to the roughness of the two surfaces in contact. 
• The friction between two surfaces can be reduced by following methods :
  • By polishing surfaces : Rough surfaces can be made smooth by polishing. Therefore, polishing reduces friction. 
  • By applying oil or grease on the surfaces : Oil/ grease forms a thin layer between the two surfaces and reduces friction.

Q8: When a force of 40 N is applied on a body it moves with an acceleration of 5 ms2. Calculate the mass of the body.
Ans: 
Let m be the mass of the body.
Given : F = 40 N, a = 5 ms² 
From the relation F = m a, we have  
40 = m × 5
m = 40/5 = 8 kg

Q9: An object undergoes an acceleration of 8 ms–2 starting from rest. Find the distance travelled in 1 second.
Ans: Given,
Acceleration, a = 8 ms^–2 
Initial velocity, u = 0
Time interval, t = 1 s
Distance travelled, s = ?
Using the equation of motion,
s = ut + 1/2 at², one gets  
s = 0 × 1 + 1/2 × 8 × 1² = 4 m
The object travels a distance of 4 m.

Q10: Calculate the force required to impact to a car, a velocity of 30 ms–1 in 10 seconds. The mass of the car is 1,500 kg.
Ans:
Here
u = 0 ms^–1;
v = 30 ms^–1;
t = 10 s;
a = ?
Using v = u + at, we have  
30 = 0 + a (10) a
= 3 ms–2
Now
F = ma
= 1,500 × 3 or F = 4,500 NLong Question Answer: Force and Law of Motion

Q1: State all 3 Newton’s law of motion.
Ans: 

• Newton’s I law of motion : An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.
• Newton’s II law of motion : The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
• Newton’s III law of motion : To every action, there is an equal and opposite reaction and they act on two different bodies.

Q2: Explain inertia and momentum.
Ans :

• Inertia : The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia. For example : A book lying on a table will remain there until an external force is applied on it to remove or displace it from that position.
• Momentum : Momentum of body is the quantity of motion possessed by the body. It is equal to the product of the mass and velocity of the body and is denoted by p. p = mv Momentum is a vector quantity and its direction is same as the direction of velocity of the object. Its SI unit is kilogram metre per second (kg ms^–1).

Q3: Define force. What are different types forces?
Ans: 

• Force : It is a push or pull on an object that produces acceleration in the body on which it acts. The S.I. unit of force is Newton.
• Types of forces : Balanced force : When the forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.
• Unbalanced force : When two opposite forces acting on a body move a body in the direction of the greater force or change the state of rest, such forces are called as unbalanced forces.
• Frictional force : Force of friction is the force that always opposes the motion of object.

Q4: What is inertia? Explain different types of inertia.
Ans: 

• Inertia : The natural tendency of an object to resist change in their state of rest or of motion is called inertia. The mass of an object is a measure of its inertia. Its S.I. unit is kg.

Types of inertia :

• Inertia of rest : The object remain in rest unless acted upon by an external unbalanced force.
• Inertia of motion : The object in the state of uniform motion will continue to remain in motion with same speed and direction unless external force is not applied on it.

Q5: Give example to show the effects of force.
Ans: 
(i) Place a ball in the ground. Kick it with your foot. The ball starts moving. The ball moves because of the force applied to it.
(ii) If ball is coming towards you, you can kick it in any direction. The direction of motion of the ball changes because of the force applied to it.
(iii) Place a rubber on the ground. Press it with your foot. It is found that the ball is no longer round but takes the shape of an egg, i.e. it is oblong. The shape of the ball has changed because of the force applied on the ball.

Q6: (a) What is friction?
(b) What are the advantages of friction? or Why is friction necessary?
Ans: 
(a) The force which opposes the motion of one body over the surface of another is called friction or the force of friction.
(b) Friction is very desirable and an important force in our daily life.
Some advantages of friction are :

• The nails and screws hold the wooden boards together due to friction. 
• It is due to the friction between the ground/road and the soles of our shoes that we are able to walk. 
• The friction between the road and the surface of the tyres permits safe driving. 
• W e are able to write on paper because of the friction between the pen/pencil and the paper. That is why, it is very difficult to write on a glazed/waxed paper. 
• The application of brakes to stop a cycle, scooter or car, etc., is possible due to friction between the brake-lining and the rim of the wheel.

Q7: (i)  What are the causes of friction?
(ii) How can friction between any two surfaces can be reduced?  or  Describe some method for reducing friction.
Ans: The friction rises due to the following factors : 

• Due to the force of attraction between the molecules of the two surfaces in contact. This is called the force of adhesion. 
• Due to the interlocking of the surface irregularities. 
• Thus, friction is due to the roughness of the two surfaces in contact. 
• The friction between two surfaces can be reduced by following methods :
  • By polishing surfaces : Rough surfaces can be made smooth by polishing. Therefore, polishing reduces friction. 
  • By applying oil or grease on the surfaces : Oil/ grease forms a thin layer between the two surfaces and reduces friction.

Q8: When a force of 40 N is applied on a body it moves with an acceleration of 5 ms2. Calculate the mass of the body.
Ans: 
Let m be the mass of the body.
Given : F = 40 N, a = 5 ms² 
From the relation F = m a, we have  
40 = m × 5
m = 40/5 = 8 kg

Q9: An object undergoes an acceleration of 8 ms–2 starting from rest. Find the distance travelled in 1 second.
Ans: Given,
Acceleration, a = 8 ms^–2 
Initial velocity, u = 0
Time interval, t = 1 s
Distance travelled, s = ?
Using the equation of motion,
s = ut + 1/2 at², one gets  
s = 0 × 1 + 1/2 × 8 × 1² = 4 m
The object travels a distance of 4 m.

Q10: Calculate the force required to impact to a car, a velocity of 30 ms–1 in 10 seconds. The mass of the car is 1,500 kg.
Ans:
Here
u = 0 ms^–1;
v = 30 ms^–1;
t = 10 s;
a = ?
Using v = u + at, we have  
30 = 0 + a (10) a
= 3 ms–2
Now
F = ma
= 1,500 × 3 or F = 4,500 N
Q8: When a force of 40 N is applied on a body it moves with an acceleration of 5 ms2. Calculate the mass of the body.
Ans: 
Let m be the mass of the body.
Given : F = 40 N, a = 5 ms² 
From the relation F = m a, we have  
40 = m × 5
m = 40/5 = 8 kg

Q9: An object undergoes an acceleration of 8 ms–2 starting from rest. Find the distance travelled in 1 second.
Ans: Given,
Acceleration, a = 8 ms^–2 
Initial velocity, u = 0
Time interval, t = 1 s
Distance travelled, s = ?
Using the equation of motion,
s = ut + 1/2 at², one gets  
s = 0 × 1 + 1/2 × 8 × 1² = 4 m
The object travels a distance of 4 m.

Q10: Calculate the force required to impact to a car, a velocity of 30 ms–1 in 10 seconds. The mass of the car is 1,500 kg.
Ans:
Here
u = 0 ms^–1;
v = 30 ms^–1;
t = 10 s;
a = ?
Using v = u + at, we have  
30 = 0 + a (10) a
= 3 ms–2
Now
F = ma
= 1,500 × 3 or F = 4,500 N