Short Question Answer: Matter in Our Surroundings

Q1: The mass per unit volume of a substance is called density. (density = mass/volume). 
Arrange the following in order of increasing density – air, exhaust from chimneys, honey, water, chalk, cotton, and iron. 
Ans: The correct order of increasing densities of the substances is as follows:
Air < exhaust from chimneys< cotton< water< honey< chalk< iron.

Q2: Why does a desert cooler cool better on a hot dry day? 
Ans: A desert cooler works effectively on a hot, dry day due to the following reasons:

• The hot air that enters the cooler causes water to evaporate quickly.
• Higher temperatures increase the rate of evaporation, making the air cooler.
• On dry days, the air can hold more moisture, enhancing the cooling effect.

As a result, the cooler can lower the air temperature more efficiently than on humid days.

Q3: Why does our palm feel cold when we put some acetone or petrol or perfume on it? 
Ans: When substances like acetone, petrol, or perfume are applied to the palm, they feel cold due to the following reasons:

• The liquids are volatile, meaning they evaporate quickly.
• As they evaporate, they absorb heat from the skin.
• This heat absorption causes the palm to feel cool.

Essentially, the energy required for evaporation is taken from your palm, leading to a cooling sensation.

Q4: What type of clothes should we wear in summer? 
Ans: In summer, it is important to wear clothes that promote easy evaporation to help keep us cool. Here are some key points:

• Choose fabrics that allow sweat to evaporate quickly.
• Cotton is highly recommended as it absorbs sweat well.
• Cotton also facilitates evaporation, which cools the body.

Wearing cotton clothes can significantly enhance comfort during hot weather.

Q5: Convert the following temperatures to the Kelvin scale. 
(a) The temperature is 25 ⁰C
Ans: When we use: K = ⁰C + 273
= 25 + 273
= 298 K
(b)The temperature is 373 ⁰C 
Ans: When we use: K = ⁰C + 273
= 373 + 273
= 656 K

Q6: Give two reasons to justify – 
(a) Water at room temperature is a liquid. 
Ans: For a temperature of <0 ⁰Cwater is in solid-state, for 0⁰C → 100⁰C → water is in a liquid state and for temperature >100 ⁰C water is in a gaseous state. Since room temperature always lies between 0 ⁰C and 100 ⁰C and within this range the physical state of water is liquid so water is liquid at room temperature.
(b) An iron almirah is a solid at room temperature. 
Ans: Since the room temperature is very less than the melting point of iron hence an almirah made up of iron will be a solid at room temperature.

Q7: What produces more severe burns, boiling water or steam? 
Ans: Steam produces more severe burns than boiling water because:

• When steam condenses into water, it releases latent heat.
• This latent heat causes more damage to skin compared to the heat from boiling water alone.

Thus, steam can result in more severe burns.

Q8: What happens when we apply pressure to the particles of matter? 
Ans: Pressure is defined as the force applied per unit area. When we apply pressure to the particles of matter:

• The force pushes the particles closer together.
• This can lead to changes in the state of matter.
• In gases, increased pressure can compress the particles.

Overall, applying pressure affects the arrangement and movement of particles in different states of matter.

Q9: If the melting point of object A is high then what state do you expect it to be at room temperature? 
Ans: If an object has a high melting point, it will remain in a solid state at room temperature. This is because:

• The melting point is the temperature at which a solid turns into a liquid.
• At temperatures below the melting point, the substance stays solid.

Therefore, a high melting point indicates that the object will not melt at typical room temperatures.

Q10: When heat is being supplied to a solid, then what does the heat energy do to the particles of the solid? 
Ans: When heat is supplied to a solid:

• The kinetic energy of the particles increases.
• This increase allows the particles to overcome the forces of attraction holding them in place.
• As a result, the particles start to move more freely.
• Eventually, the solid melts and transforms into a liquid.

Q11: Why do we say that evaporation is a surface phenomenon? 
Ans: Evaporation is known as a surface phenomenon because:

• Only the particles at the surface of a liquid can absorb energy.
• These particles gain enough energy to break free from the liquid and turn into vapour.
• This process occurs without the liquid reaching its boiling point.

Q12: Which of the following matter? 
Chair, air, love, smell, hate, almonds, thought, cold, cold drink, the smell of perfume. 
Ans: As we can define matter as any ‘physical substance’, hence almonds,  air, chair, the smell of perfume, cold drink and smell can be considered as matter.

Q13: What is the physical state of water at:  
(a) A temperature of 250 ⁰C 
Ans: The boiling point of water is 100 ⁰C, hence the physical state of water at 250⁰C will be gaseous.
(b) A temperature of 100 ⁰C 
Ans: The boiling point of water is 100 ⁰C, hence at 100 ⁰C water is in the gaseous state.

Q14: Suggest a method to liquefy atmospheric gases. 
Ans: One method to liquefy atmospheric gases is to decrease the temperature and increase the pressure.

Q15: What is the physical state of water at- 
(a) A temperature of 25 ⁰C? 
Ans: The physical state of water at 25 ⁰C is liquid.
(b) A temperature of ⁰C? 
Ans: The physical state of water at ⁰C is solid.
(c) A temperature of 100⁰C? 
Ans: The physical state of water at 100⁰C is gas.

Q16: Which of the following statements is correct? 
(a) boiling is a bulk phenomenon and evaporation is a surface phenomenon 
(b) boiling is a surface phenomenon and evaporation is a bulk phenomenon 
(c) boiling and evaporation both are a surface phenomenon 
(d) boiling and surface both are bulk phenomenon 
Ans: The correct option is (a) boiling is a bulk phenomenon and evaporation is a  surface phenomenon.

Evaporation, on the other hand, is a surface phenomenon. It occurs only at the surface of the liquid, where molecules gain enough energy to escape into the gaseous phase.
So, boiling involves the conversion of a liquid into a gas throughout its bulk, while evaporation involves the conversion of liquid into a gas at its surface.

Q17: Which of the following has the least inter atomic spacing?
(a) solid
(b) liquid
(c) gases
(d) plasma  
Ans: The correct option is (a), solid.
In a solid, the atoms or molecules are closely packed together, which results in the least interatomic spacing compared to the other states of matter. In a liquid, the particles have more space between them than in a solid, while gases have even greater intermolecular spacing. Plasma is an ionized gas with very high temperatures, and it can have significant spacing between ions and electrons as well.

Q18: Give reasons for the following observation: 
The smell of hot sizzling food reaches you several meters away, but to get the smell from cold food you have to go close. 
Ans: The smell of hot sizzling food travels quickly because:

• The temperature of hot food is higher, causing particles to move faster.
• Higher temperatures increase the diffusion rate of particles, allowing the aroma to spread quickly.
• In contrast, cold food has a lower temperature, resulting in a slower diffusion rate.
• Thus, the smell of hot food can reach us from several metres away, while cold food requires proximity.

Q19: Liquids generally have a lower density as compared to solids. But you must have observed that ice floats on water. Find out why. 
Ans: Ice floats on the water since there is a large empty space inside the 3D structure of ice due to which it becomes less in weight as compared to water and can float on water.

Q20: How does the water kept in an earthen pot (matka) become cool during summer? 
Ans: The water in an earthen pot becomes cool during summer due to the process of evaporation. Here’s how it works:

• Earthen pots have small pores that allow water to seep through.
• As water evaporates from these pores, it absorbs heat from the surrounding air.
• This heat absorption creates a cooling effect, lowering the temperature of the water inside the pot.

Q21: Why are we able to sip hot tea or milk faster from a saucer rather than a cup? 
Ans: When sipping hot tea or milk from a saucer, the following occurs:

• The surface area for evaporation is larger compared to a cup.
• This increased area allows for faster evaporation of liquid particles.
• As evaporation occurs, the liquid cools down more quickly.
• This makes it easier to take a sip without burning your mouth.

Q22: Convert the following temperatures to the Celsius scale.  
(a) The temperature is 293 K 
Ans: When we use: ⁰C = K – 273
= 293 – 273
= 20 ⁰C
(b) The temperature is 470 K 
Ans: When we use: ⁰C = K – 273
= 470 – 273
= 197 ⁰C

Q23: Give a reason for the following observations.
(a) Naphthalene balls disappear with time without leaving any solid. 
Ans: Sublimation can be defined as the conversion between the solid and the gaseous phases of matter, with no intermediate liquid stage. Naphthalene balls will be having the property of sublimation because of which they directly vary from solid to gaseous state without a conversion into liquid. Hence, naphthalene balls will be vanishing with time leaving no solid.
(b)We can get the smell of perfume sitting several meters away. 
Ans: Volatile substances such as perfumes change from liquid state to gaseous state very fast which allows them to diffuse and mix up with the air particles to reach our nostrils. Therefore we get the smell of perfume sitting several metres away.

Q24: Why is ice at 273 K more effective in cooling than water at the same temperature? 
Ans: Ice will be producing a more intense cooling effect as compared to water at  273 K because at 273 K ice will be absorbing latent heat of melting from the surroundings and will be getting converted into water. Therefore ice at 273 K is more effective in cooling than water at the same temperature.

Q25: What is evaporation? What are the factors affecting it? 
Ans: Evaporation can be defined as the process of conversion of a substance from its liquid state to a gaseous state at any temperature below its boiling point.
Evaporation will be dependent on the factors below:

• Surface area: A larger surface area increases the rate of evaporation. For example, spreading out wet clothes helps them dry faster.
• Temperature: Higher temperatures provide more energy to particles, allowing more of them to escape into the gas phase.
• Humidity: Lower humidity levels enhance evaporation. When the air is already saturated with water vapour, evaporation slows down.
• Wind speed: Increased wind speed helps remove water vapour from the surface, promoting faster evaporation.

Q26: Define latent heat of vaporization and latent heat of fusion. 
Ans: The heat energy required to change 1 kg of a substance from its liquid state to a gaseous state at atmospheric pressure without changing its temperature is known as latent heat of vaporization.

Q27: What happens when the temperature of the solids increase? 
Ans: When we increase the temperature of the solid, we are giving energy to it.  That energy is utilized in increasing the kinetic energy of the particles and as a  result, the speed of the particles is increased and they vibrate more freely. Once the particles overcome the force of attraction between them they start moving more freely.

Q28: Why is it that on increasing the wind speed the rate of evaporation increases? 
Ans: If we increase the speed of the wind, then they will be blowing away with them.  The water vapours in the air are blown away when the speed of wind is increased,  making room for more water vapours and increasing the rate of evaporation.

Q29: What are the characteristics of the particles of matter?  
Ans: The particles of matter have the following characteristics:
(i) The particles of matter are in continuous motion.
(ii) There are gaps between the particles of matter.
(iii) There is a force of attraction between the particles of matter which keeps them together.

Q30: Give reasons 
(a) A gas fills completely the vessel in which it is kept. 
Ans: The particles of gas have negligible attraction force between them because of which the particles move freely in all directions filling the whole container the gas is kept in.
(b) A gas exerts pressure on the walls of the container. 
Ans: The particles of gas move freely due to which they collide with the container walls continuously and randomly. Therefore the collision of particles on the container walls exerts pressure on the walls.
(c) A wooden table should be called a solid. 
Ans: Solids have rigid and fixed particles and have a definite shape and clear boundaries. Since a wooden table possesses all the qualities of a solid, it should be called a solid.
(d) We can easily move our hand in the air but to do the same through a solid block of wood we need a karate expert. 
Ans: Since air is a gas and the forces of attraction between the particles of gas are very less which makes it easy to separate the particles with the help of an external force and hence we can easily move our hand in the air. Whereas in the case of solids the forces of attraction are very strong and we need a very high force to separate the particles of a solid and hence we need a karate expert for it.

Q31: Are the three states of matter inter-convertible? How can they interconnect?
Ans: Yes, the three states of matter can be converted into each other.

The three states of matter are interconvertible as shown below:
(a) By heating we can convert solids into liquids and by cooling we can convert liquids into solids.
(b) We can convert liquids into gases by vaporization and we can convert gases to liquids by condensation.
(c) Using sublimation we can convert solids into gases and vice versa and using condensation we can convert liquids into solids.

Q32: Why should we wear cotton clothes in summer? 
Ans: Since cotton is a good absorbent of water, it absorbs all the sweat from our body and allows easy and fast evaporation. The sweat absorbs heat from our body and evaporates which makes us feel cooler during a hot summer day. That is why we should wear cotton clothes in summer.

Q33: A solution of H₂SO₄ is labeled 40%. The density of the solution is  1.3 gm/l. What is the concentration of the solution in %(m/v)? 
Ans: The given concentration of the solution is 40%.
Therefore, 100 gm of the solution contains 40 g of H₂SO₄  
Density = mass / volume
1.3gm/l = 100g / volume
Volume of the solution = 100 / 1.3
= 100 / 1.3ml
So, = 100 / 1.3 of the solution contains 40g of H₂SO₄
Therefore, 100 ml of solution will contain 100 × 40 × 1.3 / 100 g of H₂SO₄
= 52 g of H₂SO₄
Therefore, the concentration is 52% (m/v).

Q34: Why is it that to smell cold food, we have to go close but the smell of hot food reaches us several meters away? 
Ans: When the particles are at higher temperature, their movements are fast and therefore they can travel up to several meters. Hence the hot food’s smell will be reaching us several meters away.
At lower temperatures, the movements of particles are not very fast and particles do not have enough kinetic energy to travel a distance of several meters.  Therefore we have to go close to smell cold food.

Q35: Give an experiment to show that ammonium chloride undergoes sublimation. 
Ans: Experiment for representing the sublimation of ammonium chloride (NHCl₄):
(a) Take a crystal of ammonium chloride (NHCl₄) inside a china dish and an inverted funnel.
(b) With the help of a burner, heat the ammonium chloride (NHCl₄) crystals.
(c) When the ammonium chloride (NHCl₄) crystal is heated, vapours of (NHCl₄) and the Ammonium chloride (NHCl₄) which is solidified along the walls at the beaker’s upper end is observable.
(d) This experiment shows that solid ammonium chloride (NHCl₄) undergoes solidification. It directly changes to vapour state from a solid state, it does not convert into liquid. 

Q36: What is distillation and fractional distillation? What is the basic property that separates the two methods? 
Ans: The process of distillation is used for separating the components of a  mixture containing two liquids, having different boiling points and both liquids boil without decomposition.
The process of fractional distillation is used for separating the components of a  mixture containing more than two liquids having a boiling point difference of less than 25 K.
The basic property that separates these two methods is: 
Using distillation we can separate only those components which have a  significant difference in their boiling points. While fraction distillation is used when the difference in boiling points is less.


Q37: What is the state of inter particle distance inside a solid, liquid, and gas? 
Ans: In a solid, the forces of attraction between the particles are very high and hence the particles of a solid will be very close to each other and the inter particle distance is the least. 

• In a liquid, the forces of attraction between the particles are very weak, and therefore the particles of a liquid will not be closely packed with each other and the inter-particle distance is large.  
• In a gas, the forces of attraction between the particles are almost negligible or extremely weak and therefore the particles of a gas are very loosely packed and are very far from each other and the inter particle distance is largest.

Q38: Differentiate between physical and chemical change?
Ans: The difference physical and chemical change is given below:

Q39: Name A, B, C, D, E, and F in the following diagram showing changes in its state. 

Ans:
A is fusion or heating or melting.  
B is vapourisation.
C is cooling or Condensation.
D is cooling or solidification.
E is sublimation.  
F is solidification.

Q40: (a) Tabulate the differences in the characteristics of states of matter. 
Ans: Tabular differences in characteristics of matters are given below:

(b) Comment upon the following: rigidity, compressibility, fluidity, filling a  gas container, shape, kinetic energy, and density. 
Ans: 

• Rigidity → The property of matter to maintain its shape when external forces are applied to it is known as rigidity. Solids have this property.
• Compressibility → The property of matter to allow compression when high pressure is applied to it is known as compressibility. Some Liquids and all gases have this property. 
• Fluidity → The property of matter to flow and change in its shape when external forces are applied to it is known as fluidity. Both liquids and gases have this property. 
• Filling a gas container → Gases are fluid in nature and are highly compressible which allows them to be filled within a vessel at high pressure. A large volume of gas can be filled in a container of less volume making it suitable and more cost-efficient for transportation. 
• Shape → Only solid objects have well-defined shapes while liquids can acquire any shape depending on the container they are kept in and gases don’t have any shape. 
• Kinetic energy → The particles of a matter are continuously in motion and thus have kinetic energy. As the particles in solids have the least movement, the kinetic energy of solids is the least. The particles of gases have the freest movements and hence they have the highest kinetic energy. The order of kinetic  energies for different types of matters is: solid < liquid < gas  
• Density → Density of any substance can be explained as Mass per unit volume i.e. density = mass/volume.

Q1: Name any two fodder crops.
Ans: Berseem and Sudan grass are raised as food for the livestock, called fodder crops.

Q2: What do you understand by photoperiod of sunlight?
Ans: Photoperiods are related to the duration of sunlight required for plant growth.

Q3: Name two kharif crops.
Ans: Paddy and soybean.

Q4: Name two rabi crops.
Ans: Wheat and gram.

Q5: Define hybridization.
Ans: Hybridization refers to crossing between genetically dissimilar plants to obtain a better variety of crops.

Q6: What are genetically modified crops?
Ans: A gene with required characters can be introduced into a crop for its improvement; this crop is called genetically modified crop.

Q7: “Shorter the duration of the crop from sowing to harvesting, the more economical is the variety.” Give reason for this.
Ans: Farmers can grow more crops in a year due to short duration of crop growth and reduce the cost of crop production.

Q8: Name different types of crop production practices involved in India.
Ans: They are (a) no cost production, (b) low cost production and (c) high cost production.

Q9: What is mariculture?
Ans: The culture of marine fish in sea water is called mariculture.

Q10: What are macro-nutrients?
Ans: The nutrients required by plants in larger quantity are called macro-nutrients. They are nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur.

Q11: Name the nutrients that plant obtains from air and water.
Ans: Air – Carbon and oxygen, Water – Hydrogen and oxygen.

Q12: State the difference between compost and vermicompost.
Ans: Compost is obtained by decomposition of organic waste naturally due to decomposition by bacteria. Vermi-compost is obtained by adding red-worms to organic matter to speed up decomposition.

Q13: How does deficiency of nutrients affect the crop?
Ans: Physiological processes can be affected by deficiency of any nutrient in plants including reproduction, growth and susceptibility to diseases.

Q14: Name two Indian cattle.
Ans: Bos indicus – cows, Bos bubalis – buffaloes.

Q15: How does Bos indicus differ from Bos bubalis?
Ans: Bos indicus is a cow while Bos bubalis is a buffalo.

Q16: Name the two vitamins which are added in the poultry feed.
Ans: Vitamins A and K.

Q17: From where do plants acquire the following nutrients? 
(a) Nitrogen, 
(b) Hydrogen.
Ans:
(a) Nitrogen from soil,
(b) Hydrogen from water.

Q18: What is animal husbandry?
Ans: Animal husbandry is the management and care of farm animals by humans for profit.

Q19: Mention the components of food present in vegetables and fruits.
Ans: Vegetables, spices and fruits provide a range of vitamins and minerals in addition to small amounts of proteins, carbohydrates and fats.

Q20: What causes disease in plants?
Ans: It is caused by pathogens such as bacteria, fungi and viruses.

Q1: What does wave transfer – matter or energy?
Ans: Energy.

Q2: What is the intensity of sound?
Ans: Sound intensity is the sound energy passing per unit area per second, perpendicular to the direction of wave propagation.

Q3: Guess which sound has a higher pitch – guitar or car horn?
Ans: The guitar has a higher pitch because it has a higher frequency.

Q4: Name the waves used by bats while flying in the dark.
Ans: Bats use ultrasonic waves while flying in the dark.

Q5: Which of the following sound waves can we hear: 10 Hz, 500 Hz, 1500 Hz, 12000 Hz, 25000 Hz?
Ans:  We can hear sound waves of frequency: 500 Hz, 1500 Hz, 12000 Hz. These are within the audible range of 20 Hz to 20,000 Hz for a typical human ear.

Q6: What do you understand by the term ultrasonic vibrations?
Ans: Sounds having frequencies higher than 20,000 Hz are called ultrasonic vibrations.

Q7: What do you understand by the term echo?
Ans: The sound heard after reflection from a rigid obstacle is called an echo.

Q8: Name the two types of mechanical waves.
Ans: The two types of mechanical waves are: (i) Transverse wave and (ii) Longitudinal wave.

Q9: What is a wave?
Ans: A wave is a disturbance that travels through a medium, transferring energy from one point to another without the net movement of particles of the medium.

Q10: Define one hertz.
Ans: One hertz is one vibration per second. It is the SI unit of Frequency.

Q11: Define wavelength.
Ans: It is the distance between two nearest points in a wave which are in the same phase of vibration.

Q12: What is the audible range of the average human ear?
Ans: An average human ear can hear sound waves between frequencies 20 Hz to 20,000 Hz.

Q13: What is sound, and how is it produced?
Ans: Sound is a form of mechanical energy that produces a sensation of hearing. It is produced when an object vibrates.

Q14: Why is a sound wave called a longitudinal wave?
Ans: Sound is a form of energy that produces a sensation of hearing. It is generated when an object vibrates and these vibrations travel through a medium.

Q15: How are moths of certain families are able to escape capture?
Ans: Moths of certain families can hear the high-frequency sounds made by bats using sensitive hearing organs. This helps them detect the presence of bats and escape capture.

Q16: What is a transverse wave?
Ans: It is a wave in which the particles of the medium vibrate perpendicular to the direction of propagation of the wave.

Q17: What is a longitudinal wave?
Ans: It is a wave in which the particles of the medium vibrate in the direction of propagation of the wave.

Q18: What is a trough?
Ans: A trough is a depression (lowest point) in a wave, i.e., maximum displacement in the negative direction (below the mean position).

Q19: What do you understand by the term infrasonic vibrations?
Ans: Sounds having frequencies lower than 20 Hz are called infrasonic or subsonic vibrations.

Q20: Why do we see lightning before we hear thunder during a storm?
Ans: Because light travels much faster than sound in air.

Q1: A spring which has been kept compressed by tying its ends together is allowed to be dissolved in an acid. What happens to the potential energy of the spring?
Ans: The PE of the spring gets converted into KE of acid molecules whose temperature rises.

Q2: The work done in lifting a box on to a platform does not depend upon how fast it is lifted up. Explain your answer giving proper reasoning.
Ans: The work done (W) in lifting a box through a distance (s) against the gravitational force (F) is given by W = Fs. Hence, it is obvious that it is independent of the rate at which the box is lifted.

Q3: A body moves along a circular path. How much work is done in doing so? Explain.
Ans: In case of a body moving along a circular path, the force (centripetal) is always along the radius while displacement is tangential. Hence, work done W = FS cos 90° = 0 as angle between F and S is 90°.

Q4: What type of energy is stored in the spring of a watch?
Ans: When we wind a watch, the configuration of its spring is changed. The energy stored in the spring is obviously potential in nature (elastic potential energy to be more accurate).

Q5: A spring which is kept compressed by tying its ends together is allowed to be dissolved in an acid. What happens to the potential energy of the spring?
Ans: The potential energy of the spring gets converted into heat energy (kinetic energy of acid molecules). Due to this heat, the temperature of the acid rises.

Q6: (a) How much work is done when a force of 1 N moves a body through a distance of 1 m in its direction?
(b) Is it possible that a force is acting on a body but still the work done is zero? Explain giving one example.
Ans:
(a) 1 J of work is done.
(b) Yes, it is possible when force acts at right angles to the direction of motion of the body.
Example : Gravitational force of Earth acts on a satellite at right angles to its direction of motion.

Q7: A man rowing a boat upstream is at rest with respect to the shore. Is he doing work?
Ans: The man is doing work relative to the stream because he is applying force to produce relative motion between the boat and the stream. But he does zero work relative to the shore as the displacement relative to the shore is zero.

Q8: Is it possible that a body be in accelerated motion under a force acting on the body, yet no work is being done by the force? Explain your answer giving a suitable example.
Ans: Yes, it is possible, when the force is perpendicular to the direction of motion. The Moon revolving round the Earth under the centripetal force of attraction of the Earth, but Earth does no work on the motion.

Q9: Justify giving proper reasoning whether the work done in the following case is positive or negative – Work done by resistive force of air on a vibrating pendulum in bringing it to rest.
Ans: Work done is negative because the resistive force of air always acts opposite of the direction of motion of the vibrating pendulum.

Q10: Name the common forms of the mechanical energy.
Ans: The common forms of the mechanical energy are :
(i) Kinetic e nergy
(ii) Potential energy

Q11: A spring is compressed, what change is expected in the potential energy of the spring
Ans: When a spring is compressed, its potential energy is used up to changing its shape.

Q12: What type of energy is possessed by a flying bird and a flying aeroplane?
Ans: Both potential energy and kinetic energy.

Q13: Give one example of potential energy due to position.
Ans: Water stored in the reservoir of a dam has potential energy.

Q14: Which type of energy is present in a battery?
Ans: Chemical energy.

Q15: At what rate is electrical energy consumed by a bulb of 60 watt?
Ans: A 60 watt bulb consume electrical energy at the ratio of 60 joule per second.

Q16: How much energy will an electric motor of 1 horsepower consume in one second?
Ans: An electric motor will consume 746 joule of energy per second.

Q17: What is meant by the term horsepower (hp)?
Ans: Horsepower is another commercial unit power 1 hp
= 746 W  
1W = 1 / 746 hp
= 0.0013 hp

Q18: A car and a truck are moving with the same velocity of 60 km/hr^–1, which one has more kinetic energy?
Ans: Truck has more kinetic energy as kinetic energy is directly proportional to the mass.

Q19: What kind of energy transformation takes place when a body is dropped from a certain height?
Ans: When a body falls, its potential energy gradually gets converted into kinetic energy. On reaching the ground, the whole of the potential energy of the body gets converted into kinetic energy.

Q20: Give one example each of potential energy (i) due to position (ii) due to shape.
Ans:
(i) Potential energy due to position : Water stored in dam has potential energy.
(ii) Potential energy due to shape : In a toy car, the wound spring possesses potential energy and as the spring is released, its potential energy changes into kinetic energy due to which the car moves.

Q21: Can any object have mechanical energy even if its momentum is zero? Explain.
Ans: Yes, mechanical energy comprises of both potential energy and kinetic energy. Zero momentum means that velocity is zero. Hence, there is no kinetic energy but the object may possess potential energy.

Q22: How much work is done by a force of 10 N in moving an object through a distance of 4 m in the direction of the force?
Ans:
 Work done = Force × Displacement
= F × s  
= (10 N) × (4 m)  
= 40 joule or 40 J.

Q23: When an arrow is shot from its bow, it has kinetic energy. From where does it get the kinetic energy?
Ans: A stretched bow possesses potential energy on account of a change in its shape. To shoot an arrow; the bow is released. The potential energy of the bow is converted into the kinetic energy of the arrow.

Q24: What do you mean by transformation of energy?
Ans: It is the change of energy from one form of energy into another form of energy.

Q25: A body performs no work. Does it imply that the body possesses no energy?
Ans: When a body does not perform any work, it never implies that the body has no energy. The body may have energy but still does not perform any work, e.g., a book placed on a table has potential energy but is not performing any work.

Q1: Write SI unit of G.
Ans:
We know,

or

In SI system, force F is measured in N, distance r in m and masses m₁ and m₂ in kg, therefore

Using G = Fr² / (m₁m₂), the SI unit of G becomes N m² kg^-2.

Q2: Why should we be sent flying in space if the force of gravity somehow vanishes today? 
Ans: Earth’s gravity provides the inward (centripetal) force that keeps us moving in a circular path as the Earth rotates. If gravity vanished, that centripetal force would disappear and objects on the surface would continue to move in a straight line tangent to the Earth’s surface. As a result, we would move away from the Earth and be sent off into space along that tangent.

Q3: A ball moving on a table reaches the edge and falls. Sketch the path it will follow while falling.
Ans: As the ball leaves the table it has a horizontal velocity and, at the same time, a vertical downward acceleration due to gravity. The horizontal velocity remains nearly constant while the vertical speed increases. The combined motion makes the path a parabola.

Q4: Is value of “g” same at all places on the Earth? Give reason for your answer.
Ans: No. The value of g is slightly larger at the poles and slightly smaller at the equator. This is because the Earth is not a perfect sphere (it is slightly flattened at the poles) and because Earth’s rotation produces a small centrifugal effect that reduces the effective gravitational acceleration at the equator.

Q5: What is the relation between gravitational force of the Moon with the Earth.
Ans: The surface gravitational acceleration on the Moon is about one-sixth of that on the Earth. Therefore the gravitational force (weight) on an object at the Moon’s surface is roughly 1/6 of its weight on Earth.

Q6: Why does a mug full of water feel lighter inside water?
Ans: A mug immersed in water experiences an upward buoyant force from the water. This upward force opposes part of the mug’s weight, so the apparent weight (how heavy it feels) is reduced when it is inside the water.

Q7: Name the force which accelerates a body in free fall.
Ans: Gravitational force of the Earth (weight) is the force that accelerates a body in free fall.

Q8: Why value of “g” more or less constant on or near the Earth?
Ans: Because the distance from the surface to the centre of the Earth changes very little over the Earth’s surface, the value of g remains nearly the same on or near the Earth. Small variations occur due to Earth’s shape and rotation, but they are minor.

Q9: What is the unit of “g”?
Ans: The unit of g is m s^-2 (metre per second squared).

Q10: What is the importance of universal law of gravitation? 
or

Write four phenomenons which were successfully explained using universal law of gravitation.
Ans: The universal law of gravitation explains how masses attract each other and helps us understand many natural phenomena. Some important examples are:
(i) It explains why objects remain bound to Earth (why we stay on the ground).
(ii) It explains the Moon’s motion around the Earth.
(iii) It explains the motion of planets around the Sun (planetary orbits).
(iv) It explains tides caused by the gravitational pull of the Moon and the Sun.

Q11: Name the scientist in whose honor the SI unit of pressure is named.
Ans: The SI unit of pressure, the pascal, is named after the scientist Blaise Pascal.

Q12: Define the weight of an object on Moon.
Ans: The weight of an object on the Moon is the gravitational force with which the Moon attracts that object. It is measured in newtons and is about one-sixth of the object’s weight on Earth.

Q13: What is weightlessness?
Ans: A body is in a state of weightlessness when the normal reaction force from a supporting surface becomes zero, so its apparent weight is zero. This happens, for example, during free fall or for astronauts orbiting the Earth, where they and their spacecraft fall freely together.

Q14: Give difference between `g’ and ‘G’ in a tabular form.
Ans:

Q15: Why is G called ‘a universal gravitational constant’?
Ans: The numerical value of G is the same for any pair of objects anywhere in the universe and does not depend on the nature of the medium between them. For these reasons G is called the universal gravitational constant.

Q1: Name two factors which determine the momentum of a body.
Ans: The two factors that determine the momentum of a body are its mass and velocity.

Q2: If a man jumps out of a boat, the boat moves backwards. Why?
Ans: When a man jumps forward from a boat, he exerts a force on the boat in the backwards direction. As a reaction, the boat moves backwards. This is the example and application of Newton’s third law of motion.

Q3: What is the direction of momentum?
Ans: The direction of momentum is the same as that of velocity.

Q4: Why are road accidents at high speeds very much worse than accidents at low speeds?
Ans: At high speeds, the momentum is large and the time of impact is small, resulting in a greater force during collision. 

Q5: State Newton’s first law of motion.
Ans: An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.

Q6: Which type of force brings an object into motion?
Ans: Unbalanced force

Q7: Name the scientist who introduced the property of momentum.
Ans:  Sir Isaac Newton introduced the concept of momentum as part of his laws of motion.

Q8: Why do athletes run some distance before jumping?
Ans: To gain momentum for a longer jump.

Q9: What decides the rate of change of momentum of an object?
Ans: he rate of change of momentum of an object depends on the unbalanced force applied to it. (Newton’s second law).

Q10: The fruits fall off the branches when a strong wind blows. Give a reason.
Ans: Fruits fall off branches during strong winds due to the principle of inertia. 

Q11: Define force.
Ans: Force is a push or pull that changes or tends to change the state of motion of an object.  Mathematically, according to Newton’s second law of motion, F = ma.

Q12: If a ball is thrown up in a moving train, it comes back into the person’s hands. Why?
Ans: If a ball is thrown up in a moving train, it comes back into the person’s hands because it retains the forward motion of the train due to inertia, so it comes back to the thrower’s hand

Q13: State Newton’s third law of motion.
Ans: Newton’s third law of motion states that, for every action, there is an equal and opposite reaction.

Q14: What is a frictional force?
Ans: The frictional force is the force that opposes the motion of an object. It acts between two surfaces in contact.

Q15: Define the SI unit of force. Or what is one Newton force?
Ans: A Newton is the force which acts on a body of mass 1 kg to produce an acceleration of 1 m/s² in it.

Q16: Name the physical quantity which is determined by the rate of change of linear momentum.
Ans: Force is the physical quantity that results from the rate of change of linear momentum.

Q17: It is easier to push an empty box than to push a box full of books. Give a reason.
Ans: It is easier to push an empty box than to push a box full of books because an empty box has less mass, it has less inertia, so it’s easier to push.

Q18: Is force a scalar quantity or a vector quantity?
Ans: Force is a vector quantity. 

Q19: What is inertia?
Ans: The natural tendency of an object to resist a change in its state of rest or uniform motion is known as inertia. 

Q20: There are three solid balls, made up of aluminium, steel, and wood of the same shape and volume. Which of them would have the highest inertia? Why?
Ans: The ball made of steel has the highest inertia among the three materials because steel has the highest density, so it has the greatest mass and therefore the highest inertia.

Q21: No force is required to move an object with constant velocity. Why?
Ans:  According to Newton’s first law, no force is needed to keep an object in motion with constant velocity in the absence of external forces like friction.

Q22: Explain our walking in terms of Newton’s third law of motion.
Ans: When we walk on the ground, our foot pushes the ground backwards, and in return, the ground pushes our foot forward. The forward reaction exerted by the ground on our foot makes us walk forward.
Q23: Which will have more inertia, a body of mass 10 kg or a body of mass 20 kg?
Ans: A body with a mass of 20 kg will have more inertia than a body with a mass of 10 kg.

Q24: Which type of force does not change the state of rest or of motion of an object?
Ans: Balanced force

Q25: State Newton’s second law of motion.
Ans: The second law of motion states that the rate of change of momentum of an object is proportional to the unbalanced force applied. This change occurs in the direction of the applied force.

Q26: What is momentum?
Ans: Momentum is the product of mass and velocity. It is a vector quantity and is measured in kg·m/s.

Q27: Why do action and reaction not cancel each other?
Ans: Action and reaction do not cancel each other because they act on different objects.

Q28: What is a balanced force?
Ans: Balanced forces are forces that act on an object from opposite directions and do not change its state of rest or motion. 

Q29: What is the momentum of a body at rest?
Ans: Zero.

Q1: Define the term velocity. What is its unit? Is it a scalar or vector quantity?
Ans: Velocity is a physical quantity that gives the speed and direction of motion of the body  
Velocity = Displacement  / Time
The S.I. unit of velocity is ms^–1. Velocity is a vector quantity because its description requires both magnitude and direction.

Q2: Define acceleration. Is it a scalar or a vector quantity?
Ans: In non-uniform motion, the velocity of a body changes with time. It has different velocities at different instants of time and at different points of its path. In such a situation, we define a physical quantity called acceleration which is a measure of the change in the velocity of a body per unit time. It is a vector quantity.

Q3: What is the acceleration of a body moving uniform velocity?
Ans: The acceleration of a body moving with uniform velocity is zero.

Q4: “The direction in which an object moves is given by the direction of velocity of the object and not the direction of acceleration.” Give an example to justify this statement.
Ans: When an object is thrown upward, its velocity is directed upward while its acceleration is directed downwards. Thus, the velocity determines the direction of motion.

Q5: Name the different types of graph.
Ans: Bar graphs, straight line graphs, histograms, etc.

Q6: Name the two types of physical quantities.
Ans: The two types of physical quantities are scalar quantity and vector quantity.

Q7: Define a scalar quantity.
Ans: A physical quantity which has only magnitude and no direction is called a scalar quantity.

Q8: Define vector quantity.

Ans: A physical quantity which has both magnitude and direction is called a vector quantity.

Q9: Give two examples of a vector quantity?
Ans: Two examples of a vector quantity are displacement and velocity.

Q10: Give the simplest type of motion.
Ans: The simplest type of motion is the motion along a straight line.

Q11: Give two examples of a scalar quantity.
Ans: Two examples of a scalar quantity are distance and speed.

Q12: What is magnitude?
Ans: The numerical value of a physical quantity is called its magnitude.

Q13: Which type of graph is used to describe motion?
Ans: Line graph are used to describe the motion of an object.

Q14: When is the acceleration of a body positive?
Ans: The acceleration of a body is taken to be positive if it is in the direction of velocity.

Q15: Differentiate acceleration from velocity.
Ans: Velocity is the change in position of a body per unit time in a given direction and acceleration is the change in velocity of a body per unit time.

Q16: What is the Si unit of acceleration?
Ans: The SI unit of acceleration is m/s².

Q17: What is the relationship between distance travelled and the time elapsed for the motion with uniform velocity?
Ans: Distance is directly proportional to time used.
In fact, distance travelled  
= Uniform velocity × Time used.

Q1: What is the function of epidermis in plants?
Ans: Epidermal tissue forms a protective layer for plants and helps to protect the internal parts of plants which prevent the plant from loss of water, attack by parasitic fungi and outer injury.

Q2: What is the structure of a neuron?
Ans: Building block of a nervous tissue is a neuron. It has a thread-like structure with axon and cell body.

Q3: Where can we find apical meristem?
Ans: It is found in the growing tips of roots and stem. The length of roots and stem is increased due to the presence of apical meristem.

Q4: What do you mean by aerenchyma?
Ans: Aerenchyma is the cell with large air-filled cavities of parenchyma. It helps aquatic plants to float.

Q5: Name two types of plant tissues.
Ans:
(i) Permanent tissue
(ii) Meristematic tissue

Q6: Give the types of meristematic tissues.
Ans:
(i) Intercalary tissue — nodes
(ii) Apical tissue — tips of shoot and roots
(iii) Lateral tissue — stem sides

Q7: Name the tissues which are present in the plants at the nodes.
Ans: Intercalary tissues.

Q8: Define tracheids.
Ans: Tracheids have tapering ends and are basically elongated cells.

Q9: What is the use of tissues in multicellular organisms?
Ans: Tissue provides structural and mechanical strength as well as to allow division of labour in multicellular organisms.

Q10: Where is parenchyma tissue present?
Ans: Parenchyma tissue is present below the epidermis.

Q11: Name the tissue which is responsible for the movement of our body?
Ans: A combination of following two tissues is responsible for the movement of our body :
(i) Muscular t issue
(ii) Nervous tissue

Q12: Write the various functions of all types of epithelial tissues?
Ans: Epithelial tissues help in protection, absorption, excretion, exchange of respiratory gases and secretion.

Q13: How is ligament different from tendons?
Ans: Ligaments are elastic connective tissue which attach bone to bone to keep them in their place. Tendons are less elastic connective tissues which attach muscles to a bone.

Q14: What is the function of connective tissue?
Ans: Connective tissue connects different tissues and organs. It provides support to different parts of the body by forming packing around different organs of the body.

Q15: What will happen if ligaments are over stretched?
Ans: Sprain will occur.

Q16: What are blood platelets?
Ans: Blood platelets are minute (about 2 to 4 μm in diameter), anucleated, disc like bodies. The main function of platelets is to help in clotting of blood.

Q17: What is basement membrane?
Ans: It is very thin non-cellular membrane on which cells of epithelial tissue rest. It also separates the epithelial tissue from the underlying tissues.

Q18: Write the main functions of collenchyma?
Ans: The main functions of collenchyma are to provide mechanical support, tensile strength, elasticity and flexibility to stem, leaf stalks and leaves.

Q19: Where is chlorenchyma tissue present?
Ans: It is present in the centre of stems and roots. If it has chlorophyll then it is called chlorenchyma. It is present in green leaves.

Q20: Mention the functions of cuboidal epithelium.
Ans:
(i) Absorption
(ii) Excretion
(iii) Secretion
(iv) Mechanical support

Q21: Mention the different types of blood cells.
Ans:
(i) WBCs
(ii) RBCs
(iii) Platelets

Q22: Why does the growth of plants occur only in certain specific regions?
Ans: Growth of plants occurs only in certain specific region because the meristematic tissue is located only at these points.

Q23: Define differentiation.
Ans: The process of taking up permanent size, shape and function of cell is called differentiation.

Q24: Name the two types of tissues.
Ans:
(i) Animal tissues
(ii) Plant tissues

Q25: What is the location of stratified squamous epithelium? Also, mention its functions.
Ans: Stratified squamous epitheliums are found in the skin. They are present in layers to prevent wear and tear.

Q1: Name the smallest cell and the longest cell in human body.
Ans: The smallest cell in the human body is the red blood cell or the sperm cell in males. The longest cell is the nerve cell.

Q2: What is osmosis?
Ans: The movement of water molecules through a selectively permeable membrane is known as osmosis. This process occurs when water moves from an area of higher concentration to an area of lower concentration.

Q3: Why does mitochondria have largely folded inner membrane?
Ans: Mitochondria have largely folded inner membrane which provides the increased surface area for ATPgenerating chemical reactions. Mitochondria is the site for cellular respiration and provides energy to the cell.

Q4: Which organelle makes the digestive enzyme of lysosome?
Ans: The rough endoplasmic reticulum (RER) is responsible for producing the digestive enzymes found in lysosomes.

Q5: Name the cell organelles that have their own DNA and ribosomes. Name the autonomous organelles in the cell.
Ans: The cell organelles with their own DNA and ribosomes are mitochondria and plastids. Chloroplast and mitochondria are the autonomous organelles in the cells.

Q6: What is the energy currency of the cell?
Ans: ATP—Adenosine Triphosphate.

Q7: Where are genes located in the cell?
Ans: Genes are located in the chromosomes in the nucleus of the cell.

Q8: What is the cell wall composed of ?
Ans: The cell wall is primarily made of cellulose, which is a complex substance. This composition provides structural strength to plants.

Q9: Where are proteins synthesized inside the cell?
Ans: Proteins are synthesised in the ribosomes, which are often referred to as the cell’s protein factories.

Q10: What would happen if the plasma membrane breaks down?
Ans: If plasma membrane breaks down then molecules of some substances will freely move in and out.

Q11: State two conditions required for osmosis.
Ans: Conditions required for osmosis:

• Concentration difference: There must be a difference in the concentration of water, with one side having a higher concentration than the other.
• Semi-permeable membrane: A semi-permeable membrane is necessary to allow the flow of water.

Q12: What is the function of vacuoles?
Ans: Vacuoles are the 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.

Q13: Why are plasma membrane called selectively permeable membrane?
Ans: Plasma membrane allows to go in and out some materials to and from the cell. It also prevents movement of some other materials. So, it is called selectively permeable membrane.

Q14: Why are plasma membrane called selectively permeable membrane?
Ans: Plasma membrane allows to go in and out some materials to and from the cell. It also prevents movement of some other materials. So, it is called selectively permeable membrane.

Q15: What are genes?
Ans: Gene is a part of DNA. They are located on chromosomes in linear fashions. One gene may perform one or more function. Genes are carrier of genetic character.

Q16: Do vacuoles store material? If so, name them.
Ans: Yes, vacuoles store some important substances required in life of the plant cell. These are amino acids, sugars, various organic acids and also some proteins.
Example : Amoeba, vacuoles also store food.

Q17: Which cell organelle is known as the ‘suicidal bags’ of a cell? Why?
Ans: Lysosomes are known as suicidal bag of cell. During the disturbance in cellular metabolism, lysosome may burst and the enzymes digest their own cell. Hence, lysosome are called ‘suicide bags’ of a cell.

Q18: What are chromosomes? What are they made of?
Ans: The compact rod-like bodies inside the nucleus are called chromosomes. These are seen at the time of cell division. They are made up of deoxyribonucleic acid or DNA and proteins.

Q19: How are new cells reproduced?
Ans: Cells multiply by dividing themselves again and again. Cells divide to produce cells of their own kind.

Q20: What is a prokaryotic cell?
Ans: A cell in which there is absence of a defined nuclear region and a nuclear membrane is called a prokaryotic cell. Prokaryotic cells are generally small in size. They lack membrane bound cell organelles. They have a single chromosome.

Q21: How does a living cell perform basic functions?
Ans: A living cell perform basic functions by division of labour among specific components within it is known as cell organelles.

Q22: Write a function of cell membrane.
Ans: Its major function is to hold cellular contents and control passage of materials in and out of the cell.

Q23: Give example of working of chromoplasts.
Ans: Spinach looks green due to the presence of chloroplasts, papaya is yellow and edible part of watermelon is red due to the presence of chromoplasts.

Q24: Which cell organelle transmits the heredity information from parents to offspring?
Ans: Genes are the functional units of chromosomes which transmit the heredity information from parents to offspring. These are located on chromosomes.

Q25: Where do lipids and proteins get synthesized?
Ans: Lipids get synthesized in the smooth endoplasmic reticulum and proteins get synthesised in the ribosome and rough endoplasmic reticulum.

Q1: Why did Rutherford select a gold foil in his alpha-ray scattering experiment?
Ans: Rutherford selected a gold foil in his alpha-ray scattering experiment because gold has high malleability and can be hammered into a thin sheet.

Q2: What is an orbit?
Ans: An orbit is the fixed path followed by an electron around the nucleus according to the Bohr model of the atom. It represents a definite energy level in which an electron can remain without radiating energy.

Q3: What are nucleons?
Ans: Protons and neutrons that are present in the nucleus of an atom are collectively called nucleons.

Q4: Why do noble gases show the least reactivity?
Ans: Noble gases have completely filled outer electron shells (a full duplet in helium and an octet in other noble gases). This stable electronic configuration makes them unlikely to gain, lose or share electrons, so they show very little chemical reactivity.

Q5: Electrons have a negative charge, protons have a positive charge. An atom has both, but why is there no charge?
Ans: An atom is electrically neutral because it contains equal numbers of protons (positive) and electrons (negative). The positive and negative charges cancel each other, giving no net charge.

Q6: What is the charge and mass of an alpha particle?
Ans: Charge = +2 units; Mass = 4 u. An alpha particle is the nucleus of a helium atom, containing two protons and two neutrons.

Q7: A Helium atom has two electrons in its valence shell, but its valency is not two. Why?
Ans: Helium’s outermost shell is completely filled with two electrons, which is a stable duplet. Because its valence shell is full, helium does not need to gain or lose electrons to become stable, so its valency is zero, not two.

Q8: If an atom contains one electron and one proton, will it carry any charge or not?
Ans: It will not carry any net charge because the single positive charge of the proton is exactly balanced by the single negative charge of the electron; the atom is electrically neutral.

Q9: Give two uses of isotopes in the field of medicine.
Ans:
(i) An isotope of cobalt (Co-60) is used in radiotherapy to treat certain types of cancer.
(ii) An isotope of iodine (I-131) is used in the diagnosis and treatment of thyroid disorders such as goitre.

Q10: What is meant by the electronic configuration of elements?
Ans: The electronic configuration of an element is the systematic arrangement of its electrons in different shells or orbits around the nucleus, showing how many electrons occupy each energy level.

Q11: Why do some elements possess fractional atomic mass?
Ans: Some elements possess fractional atomic mass because they occur in nature in different isotopic forms. So, accordingly, their average mass is calculated. For example, Chlorine has two isotopes Cl ₃₅ and Cl _37, and its average atomic mass is 35.5.

Q12: Are noble gases inert?
Ans: Yes. Noble gases are largely inert because their outermost electron shells are full, so they have little tendency to participate in chemical reactions.

Q13: How can an atom become stable by losing or gaining electrons?
Ans: For the stability of an atom, it must have either 2 or 8 electrons in the outermost orbit. So, by losing or gaining electrons, it reaches the electronic configuration of the nearest noble gas.

Q14: If Z = 3, what would be the valency of the element? Also, name the element.
Ans:  
Atomic number Z = 3
Electronic configuration = 2 (K), 1 (L)
The valence shell has 1 electron, so the valency of the element is 1. The element is lithium.

Q15: How are the canal rays different from electrons in terms of charge and mass?
Ans:  Canal rays consist of positively charged particles, whereas electrons are negatively charged. The mass of a typical canal-ray particle (for example a proton) is much larger than that of an electron – roughly 2000 times greater.

Q16: What is the limitation of J.J. Thomson’s model of an atom?
Ans: J.J. Thomson’s atomic model, often called the “plum pudding” model, had several limitations. It couldn’t explain the stability of the atom, particularly how the positive charge held the negative electrons. It also failed to account for the nucleus of an atom and couldn’t explain the results of Rutherford’s alpha-particle scattering experiment, where most particles passed through a gold foil but some were deflected at different angles. 

Q17: Do isotopes of an element have similar chemical properties?
Ans: Yes, Isotopes of an element have the same atomic number and the same arrangement of valence electrons, so they exhibit very similar chemical behaviour.

Q18: Why is the valency of Na is 1 and not 7?
Ans: A sodium atom can attain a stable electronic configuration more easily by losing one electron than by gaining seven. Losing one electron gives it the noble-gas configuration, so its valency is 1 and not 7.