Science for Class 8 ( Science Curiosity ) Solutions

Probe and Ponder

• What do you think Earth would look like if there were no life on it at all?
  Earth would look like a barren, rocky planet with vast empty lands, dry oceans, and no green forests or animals. There might be more dust storms, eroded mountains, and a thinner atmosphere without plants to produce oxygen or hold soil in place. It would be silent and lifeless, like Mars or the Moon.
• Life on Earth has survived for billions of years. What allows it to keep going despite major changes and disasters?
  Earth’s balance of air, water, sunlight, and soil, plus its position in the habitable zone, helps life adapt. Reproduction passes on traits that help survive changes, like disasters or climate shifts. Ecosystems recover through interactions between living things (like plants regrowing after a fire) and non-living parts (like nutrient recycling in soil).
• Why don’t dogs lay eggs? Or hens give birth to live chicks?
  Dogs are mammals, so they give birth to live young because their embryos develop inside the mother, getting nutrition directly. Hens are birds, so they lay eggs where the embryo grows outside with stored food. This is due to how each species evolved—mammals protect babies inside, while birds use eggs for safety in nests.
• If a spaceship carried soil and water to Mars, could plants start growing there?
  Maybe simple plants or microbes could grow if we add the right conditions, like protection from Mars’ thin atmosphere, extreme cold, and radiation. But most Earth plants would struggle without enough air pressure, liquid water stability, or Earth’s magnetic shield. Experiments show some bacteria survive, but full growth needs more, like a greenhouse dome.
• Share your questions
  Here are some fun questions from the chapter: Why is Earth called the Blue Planet? How does the ozone layer protect us? What would happen if Earth’s magnetic field disappeared? Could life exist on other planets in the habitable zone?

Keep the Curiosity Alive

1. What is one major reason Mars cannot currently support life like Earth? 
   (i) It has too many volcanoes. 
   (ii) It is too close to the Sun. 
   (iii) It lacks a thick atmosphere and liquid water. 
   (iv) Its magnetic field is too strong.
   Ans:
   (iii) It lacks a thick atmosphere and liquid water.
   Mars is at the edge of the habitable zone, but its thin atmosphere can’t trap heat or hold liquid water, making it too cold and dry for most life.
2. Which of these is an example of geodiversity? 
   (i) Variety of bird chirping in a forest. 
   (ii) Different landforms like mountains, valleys, and deserts. 
   (iii) Changing weather during monsoons. 
   (iv) Number of different types of fish in a pond.
   Ans: (ii) Different landforms like mountains, valleys, and deserts.
   Geodiversity is the variety of rocks, soils, and landforms that create unique habitats, supporting different plants and animals.
3. If the Earth were smaller with the same density, what might happen to its atmosphere? 
   (i) It would become thicker and hotter. 
   (ii) It would escape into space due to weaker gravity. 
   (iii) It would become frozen. 
   (iv) It would cause stronger winds.
   Ans: 
   (ii) It would escape into space due to weaker gravity.
   A smaller Earth couldn’t hold gases tightly, so the atmosphere would leak away, like on Mercury, leaving no air to breathe or protect life.
4. In sexual reproduction, why are offspring different from their parents? 
   (i) They grow in different climates. 
   (ii) They eat different food. 
   (iii) They acquire new instructions after birth. 
   (iv) They get mixed instructions (genes) from both parents.
   Ans: 
   Gametes from each parent combine, mixing genetic material, so kids have traits from both but are unique, like varying eye colors in a family.
5. You notice tiny green plants growing in cracks on your school wall after the monsoon. Where do you think the seeds came from? What conditions helped these plants grow there?
   Ans: Seeds likely came from wind, birds, or rain carrying them from nearby plants. Conditions like moisture from monsoon rain, trapped soil in cracks, and sunlight helped them germinate and grow roots—showing how plants adapt to tough spots.
6. A city has recently cut down a large patch of forest to build new roads and buildings. Discuss the possible effects this could have on the local climate and biodiversity? How might this affect water availability or quality in the area?
   Ans: Local climate could get hotter and drier without trees to provide shade and release moisture. Biodiversity drops as animals lose homes, leading to fewer species. Water availability might decrease (less rain from reduced evaporation), and quality could worsen from soil erosion causing muddy runoff into rivers.
7. A friend says, “The Earth has always had climate changes in the past, so today’s global warming is nothing new.” How would you respond using what you’ve learnt in this and other chapters of your science book?
   Ans: Past changes were natural and slow, like ice ages from Earth’s orbit shifts. Today’s warming is faster due to human actions like burning fossil fuels, releasing extra greenhouse gases. This causes extreme weather, rising seas, and biodiversity loss—unlike natural cycles. We need to act to reduce emissions.
8. Imagine Earth’s magnetic field suddenly disappeared. What kinds of problems could arise for life on Earth? Explain.
   Ans: Harmful solar wind and cosmic rays would hit Earth directly, damaging the atmosphere, reducing ozone, and letting in more UV rays. This could cause more cancers, harm plants, and disrupt electronics. Life might struggle, especially in exposed areas, without this protective shield.
9. You are tasked with designing a new settlement for humans on Mars. Name three things you would need to recreate from Earth to support human life there. Which of these do you think is the hardest to replicate, and why?
   Ans: Three things: Liquid water (for drinking and growing food), breathable atmosphere (with oxygen), and protection from radiation (like a magnetic field). Hardest is the atmosphere—Mars’ is thin and mostly carbon dioxide, so creating enough oxygen and pressure needs huge tech, unlike water which we could melt from ice.
10. In a village, the temperature has been increasing and rainfall has become unpredictable over the past few years. What could be causing this change? Suggest two ways the village could adapt to these new conditions.
    Ans: Causes: Climate change from global warming, deforestation reducing local rain, or pollution trapping heat. Adapt by planting trees to cool the area and improve rain, and using rainwater harvesting to store water for dry times.
11. If there were no atmosphere on the Earth, would it affect life, temperature, and water on the planet? Explain.
    Ans: Yes—without atmosphere, temperatures would swing wildly (hot days, freezing nights), water would evaporate or freeze easily, and no air means no breathing or wind. Harmful rays would reach the surface, killing most life. Atmosphere traps heat, holds water vapor, and shields us.
12. Discuss five examples of vegetative propagation.
    Ans: 
    ​Potato: Eyes on tubers grow into new plants. 
    Ginger: Rhizomes (underground stems) sprout roots and shoots.
    Money plant: Stem cuttings in water or soil grow roots.
    Onion: Bulbs divide and form new plants.
    Bryophyllum: Leaves with tiny plantlets fall and grow independently.

Discover, Design, and Debate

1. Design an ‘Earth Survival Kit’. Imagine you’re building a tiny model of Earth for another planet. What must it have to support life, and why?
Ans: My kit would include: 
1. Liquid water (essential for cells, plants, and drinking—keeps things hydrated). 
2. Breathable air with oxygen and carbon dioxide (for respiration and photosynthesis). 
3. Soil with nutrients (to grow plants and recycle waste). 
4. Sunlight simulator (for energy and warmth). 
5. Magnetic shield model (to block harmful rays). 
These recreate Earth’s balance—water and air sustain life, soil supports growth, sunlight provides energy, and the shield protects. Build a model with jars, plants, and magnets to show it!

2. India is planning for a challenging lunar mission, Chandrayaan-4, which will bring back samples of soil from the Moon. If the Moon had water, could plants grow in that soil? Think of some experiment that could help you explore whether plant growth is possible on the Moon. 

Ans: Plants might grow if we add water, but Moon soil (regolith) lacks nutrients and has sharp particles that could harm roots. Experiment: Mix Moon-like soil (use fine sand or volcanic ash) with Earth soil in pots. Add water and plant seeds like beans. Compare growth in pure “Moon” soil vs. mixed—track height, leaves, and roots over weeks. If nothing grows in pure regolith, it shows we need to add nutrients or protection for lunar farming!

3. Flowers are often brightly coloured and have a pleasant smell. How do you think these features help the plant reproduce?

Ans: Bright colors and sweet smells attract pollinators like bees, butterflies, or birds. They visit for nectar, picking up pollen (male gametes) and carrying it to other flowers for fertilization. This helps plants make seeds without moving—nature’s way of mixing genes for stronger offspring!

4. Why do animals like fish and frogs lay hundreds or even thousands of eggs at a time, while other animals lay only a few? What might be the advantages and disadvantages of laying so many eggs?

Ans: Fish and frogs lay many eggs because most get eaten by predators or don’t survive in water—it’s a numbers game to ensure some hatch. Advantages: Increases survival chances without parental care. Disadvantages: Wastes energy if most fail, and no protection means high loss. Birds lay fewer but care for them, improving each egg’s odds!

5. Birds like sparrows build nests and care for their eggs and chicks, while reptiles like snakes usually lay their eggs and leave them without protection. How might this difference in parental care affect the chances of survival for the young ones in each case?

Ans: Sparrow chicks have higher survival because parents feed, warm, and protect them from predators—leading to more reaching adulthood. Snake eggs are hidden but face risks like weather or animals eating them, so fewer survive. Bird care boosts success but needs more energy; snake strategy spreads risk but has higher loss.

Probe and Ponder

1. How might the loss of forest cover and changes in rainfall patterns lead to elephants entering human farms and villages?
   When forests shrink due to cutting trees or less rain, elephants lose their food and water. They wander into farms for crops like bananas or sugarcane, causing damage. This happens because their natural habitat can’t support them anymore, leading to conflicts with humans.
2. Imagine you are a tree in a dense forest. What kind of relationships would you have with water, sunlight, other animals, and other components of the forest?
   As a tree, I’d soak up water from soil for growth, use sunlight to make food (photosynthesis), provide shade and fruits for animals like birds and monkeys, and my roots would hold soil to prevent erosion. Animals might help by spreading my seeds, and I’d release oxygen for everyone—it’s all connected!
3. Do you think the Earth can thrive without humans? Can humans survive without the Earth?
   Yes, Earth can thrive without humans—nature balanced itself for millions of years before us with plants, animals, and weather cycles. But humans can’t survive without Earth; we need its air, water, food, and resources. We’re part of nature, not separate!
4. If two kinds of birds compete for the same fruit, how might their way of living change over time?
   Over time, one bird might adapt by eating different fruits or at different times to avoid fights. The other could evolve stronger beaks or move to new areas. This competition helps balance populations and encourages changes for survival.
5. Can human actions cause natural disasters?
   Yes, actions like deforestation can cause floods or landslides by removing trees that hold soil and absorb water. Pollution and climate change from burning fuels can lead to stronger storms or droughts. We’re not causing all disasters, but we can make them worse.
6. Share your questions
   Here are some fun questions from the chapter: How do decomposers help recycle waste? Why are mangroves important for coastal areas? What happens if predators disappear from a forest? How can we make farming more eco-friendly?

Keep the Curiosity Alive

1. Refer to the given diagram (Fig. 12.19) and select the wrong statement.
   (i) A community is larger than a population. 
   (ii) A community is smaller than an ecosystem. 
   (iii) An ecosystem is part of a community.
   Ans: (iii) An ecosystem is part of a community.
   Actually, a community (groups of populations) is part of an ecosystem, which includes both living things and non-living parts like soil and water. The other statements are correct: communities are larger than populations and smaller than ecosystems.
2. A population is part of a community. If all decomposers suddenly disappear from a forest ecosystem, what changes do you think would occur? Explain why decomposers are essential.
   Ans: Dead plants and animals would pile up without breaking down, leading to less nutrients in the soil. Plants might not grow well, affecting herbivores and the whole food chain. Decomposers like fungi and bacteria are essential because they recycle nutrients back into the soil, keeping the ecosystem healthy and balanced.
3. Selvam from Cuddalore district, Tamil Nadu, shared that his village was less affected by the 2004 Tsunami compared to nearby villages due to the presence of mangrove forests. This surprised Sarita, Shabnam, and Shijo. They wondered if mangroves were protecting the village. Can you help them understand this?
   Ans: Yes, mangroves act like natural barriers! Their roots slow down waves and winds during tsunamis or storms, reducing damage to villages. They also absorb carbon dioxide, prevent erosion, and provide homes for animals— that’s why Selvam’s village was safer.
4. Look at this food chain: Grass → Grasshopper → Frog → Snake. If frogs disappear from this ecosystem, what will happen to the population of grasshoppers and snakes? Why?
   Ans: Grasshoppers would increase because fewer frogs eat them, leading to overeating of grass. Snakes would decrease because they lose their main food (frogs). This shows how removing one link disrupts the balance—frogs control grasshoppers and feed snakes.
5. In a school garden, students noticed fewer butterflies the previous season. What could be the possible reasons? What steps can students take to have more butterflies on campus?
   Ans: 
   Reasons: Fewer flowers (food for butterflies), pesticide use killing them, or weather changes. 
   Steps: Plant nectar-rich flowers like marigolds, avoid chemicals, create shady spots, and add water sources. This attracts butterflies and helps pollination!
6. Why is it not possible to have an ecosystem with only producers and no consumers or decomposers?
   Ans: Producers like plants make food, but without consumers (animals that eat them), plant populations would explode and use up resources. No decomposers means no nutrient recycling—dead stuff piles up, and soil gets poor. All parts are needed for balance!
7. Observe two different places near your home or school (e.g., a park and a roadside). List the living and non-living components you see. How are the two ecosystems different?
   Ans: Park: Living—trees, birds, insects, grass; Non-living—soil, water pond, sunlight, air. Roadside: Living—few weeds, ants, stray dogs; Non-living—asphalt, rocks, polluted air, vehicles. Differences: Park has more biodiversity and clean air; roadside is disturbed by traffic and has less life.
8. ‘Human-made ecosystems like agricultural fields are necessary, but they must be made sustainable.’ Comment on the statement.
   Ans: Farms provide food, but overuse of chemicals harms soil and wildlife. Sustainable means using organic methods, crop rotation, and less water to keep soil healthy and reduce pollution. This way, farms last longer without damaging nature.
9. If the Indian hare population (Fig. 12.20) drops because of a disease, how would it affect the number of other organisms?
   Deer might increase (less competition for grass), but foxes and eagles would decrease (less food). Grass could grow more with fewer hares eating it. Overall, the food web shifts—predators suffer, plants thrive, showing hares are key for balance.

Discover, Design, and Debate

• Plan a clean-up day at school or a nearby park. Wearing gloves and using bags, collect the litter you find. Discuss the kinds of waste you found. Which was the most common? How can we reduce such waste? 
  Ans: Organize with friends—pick a Saturday, get gloves, bags, and posters saying “Keep Nature Clean!” Common waste: Plastic bottles, wrappers, paper. Plastics are most common because they’re not biodegradable. Reduce by using reusable bags, recycling, and teaching others—no littering! This helps ecosystems by preventing pollution that harms animals and plants.
• In Arunachal Pradesh, the Nyishi and Mishmi tribes treat the Tiger as sacred. In Chhattisgarh, the Baiga tribe worships Bagheshwar or Bagesur Dev and believes the Tiger is the protector of the forest. Find out about another Indian tribe that has a special bond with any animal. 
  Ans: The Bishnoi tribe in Rajasthan has a special bond with blackbucks (antelopes). They protect them as sacred, following Guru Jambheshwar’s teachings to not harm animals or trees. This helps conserve wildlife—Bishnois even risk their lives to save them! It’s a great example of how culture protects ecosystems.
• Pick a tree near your home or school. Observe it once a week for 4 weeks. Note any new leaves, flowers, fruits, or visiting birds and insects. Record your observations. You may even upload your findings to www.seasonwatch.in and become a young citizen scientist.
  Ans: I picked a mango tree. Week 1: New green leaves budding, ants crawling. Week 2: Small flowers blooming, bees visiting. Week 3: Tiny fruits forming, birds like sparrows perching. Week 4: Fruits growing bigger, squirrels eating. Upload to SeasonWatch to track changes with seasons—it’s fun and helps scientists study climate effects on trees!
• Interact with farmers and record indigenous practices followed by them for sustainable farming. Create a sustainable herbal garden/natural farm at home or at school. It could be a group activity with students from different grades.
  Ans:  Talked to a farmer who uses cow dung as natural fertilizer (compost) and plants marigolds to repel pests—instead of chemicals. 
  For our school garden: Plant herbs like tulsi and mint in pots with organic soil, water wisely, and rotate crops. Involve juniors to weed and seniors to label—it’s sustainable, provides fresh herbs, and teaches eco-farming!
• Look at Fig. 12.21 to understand the different farming practices adopted by farmers or you may also visit a nearby farm with an elderly person to observe the same. List a few suggestions in your notebook to improve farming practices by adopting eco-friendly and sustainable techniques. You can also make posters or model and display while participating in school functions, science fairs or Krishi Mela. The school may also invite agricultural scientists, farmers, and experts to discuss the prevalent farming practices with the students.
  Ans: From the figure/visit: Farmers use crop rotation, natural manure, and drip irrigation. Suggestions: Use compost instead of chemicals, plant diverse crops to boost soil health, harvest rainwater. Make posters like “Go Organic for Healthy Soil!” and a model farm with recycled materials. Invite experts for a talk—learn about Vrikshayurveda (ancient organic methods) and share in a school fair!

Probe and Ponder

1. Can we make mirrors which can give enlarged or diminished images?
Ans: 

• Yes! Concave mirrors can make enlarged (bigger) images when objects are close, like a dentist’s mirror for seeing teeth better. 
• Convex mirrors always make diminished (smaller) images, like side-view mirrors on cars that show a wider view but make things look tinier.

2. On side-view mirrors of vehicles, there is a warning that says “Objects in mirror are closer than they appear”. Why is this warning written there?
Ans: 

• These are convex mirrors, which make images smaller and show a bigger area behind the vehicle. 
• But because things look smaller, they seem farther away than they really are. 
• The warning reminds drivers that cars or bikes might be closer to avoid accidents.

3. Why is there a curved line on some reading glasses?
Ans: 

• Reading glasses often use convex lenses, which are curved to help focus light and make small text look bigger. 
• The curve helps bend light rays to fix vision problems, like making far or near things clearer.

4. Share your questions

Ans: Here are some fun questions you might think of from the chapter: 

• How does a magnifying glass make things look bigger? 
• Why do spoons act like funny mirrors? 
• Can lenses in our eyes change shape? 
• What happens if you use a concave mirror to focus sunlight?

Keep the Curiosity Alive

1. A light ray is incident on a mirror and gets reflected by it (Fig. 10.21). The angle made by the incident ray with the normal to the mirror is 40°. What is the angle made by the reflected ray with the mirror?

Ans: (ii) 50°.
The angle of incidence is 40° (between incident ray and normal). By the law of reflection, the angle of reflection is also 40°. So, the angle between the reflected ray and the mirror is 90° – 40° = 50°.

2. Fig. 10.22 shows three different situations where a light ray falls on a mirror:
(i) The light ray falls along the normal
(ii) The mirror is tilted, but the light ray still falls along the normal to the tilted surface. 

(iii) The mirror is tilted, and the light ray falls at an angle of 20° from the normal. 

Draw the reflected ray in each case (Use a ruler and protractor for accurate drawing). What is the angle of reflection in each case?
Ans: (i) When the ray falls along the normal, the reflected ray goes back along the same path. Angle of reflection = 0°.
(ii) For the tilted mirror with ray along the normal, reflected ray goes back along the normal. Angle = 0°.
(iii) For the ray at 20° to the normal, reflected ray is also at 20° to the normal on the other side. Angle of reflection = 20°.

3. In Fig. 10.23, the cap of a sketch pen is placed in front of three types of mirrors. Match each image with the correct mirror.

Ans: 

• (i) Convex mirror (small and erect).
• (ii) Concave mirror (enlarged image).
• (iii) Plane mirror (same size and erect).

4. In Fig. 10.24 the cap of a sketch pen is placed behind a convex lens, a concave lens, and a flat transparent glass piece — all at the same distance. Match each image with the correct type of lens or glass.

Ans: 

(i) Convex lens (enlarged if close).

(ii) Concave lens (diminished and erect).

(iii) Flat transparent glass piece (same size, no change).

5. When the light is incident along the normal on the mirror, which of the following statements is true:
(i) Angle of incidence is 90° 
(ii) Angle of incidence is 0° 
(iii) Angle of reflection is 90°
(iv) No reflection of light takes place in this case
Ans: 
(ii) Angle of incidence is 0°.
The ray hits straight on, so incidence and reflection angles are both 0°, and it bounces straight back.

6. Three mirrors—plane, concave and convex are placed in Fig. 10.25. On the basis of the images of the graph sheet formed in the mirrors, identify the mirrors and write their names above the mirrors.

Ans: Left: Plane mirror (straight lines, same size).
Middle: Concave mirror (lines curve inward, can enlarge).
Right: Convex mirror (lines curve outward, diminished view).
7. In a museum, a woman walks towards a large convex mirror (Fig. 10.26). She will see that:
(i) her erect image keeps decreasing in size. 
(ii) her inverted image keeps decreasing in size. 
(iii) her inverted image keeps increasing in size and eventually it becomes erect and magnified. 
(iv) her erect image keeps increasing in size.

Ans: (i) her erect image keeps decreasing in size.
Convex mirrors always show erect, smaller images that get even tinier as you get closer.

8. Hold a magnifying glass over text and identify the distance where you can see the text bigger than they are written. Now move it away from the text. What do you notice? Which type of lens is a magnifying glass?
Ans: When close, the text looks bigger (enlarged and erect). As you move it away, the image might flip upside down and change size. A magnifying glass is a convex lens.

9. Match the entries in Column I with those in Column II.

Ans:
(i) Concave mirror – (a) Spherical mirror with a reflecting surface that curves inwards.
(ii) Convex mirror – (b) It forms an image which is always erect and diminished in size.
(iii) Convex lens – (c) Object placed behind it may appear inverted at some distance.
(iv) Concave lens – (d) Object placed behind it always appears diminished in size.

10. The following question is based on Assertion/Reason. 
Assertion: Convex mirrors are preferred for observing the traffic behind us. 
Reason: Convex mirrors provide a significantly larger view area than plane mirrors. 
Choose the correct option: 
(i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion. 
(ii) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion. 
(iii) Assertion is correct but Reason is incorrect. 
(iv) Both Assertion and Reason are incorrect.
Ans: (i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
Convex mirrors give a wider view, helping see more traffic, even if images are smaller.

11. In Fig. 10.27, note that O stands for object, M for mirror, and I for image. Which of the following statements is true?
(i) Figure (a) indicates a plane mirror and Figure (b) indicates a concave mirror. 
(ii) Figure (a) indicates a convex mirror and Figure (b) indicates a concave mirror. 
(iii) Figure (a) indicates a concave mirror and Figure (b) indicates a convex mirror. 
(iv) Figure (a) indicates a plane mirror and Figure (b) indicates a convex mirror.

Ans: Figure (a) indicates a convex mirror and Figure (b) indicates a concave mirror. 
Concave mirror shows enlarged image; convex shows smaller image.

12. Place a pencil behind a transparent glass tumbler (Fig. 10.28a). Now fill the tumbler halfway with water (Fig. 10.28b). How does the pencil appear when viewed through the water? Explain why its shape appears changed.

The pencil looks bent or broken at the water surface. This is because light bends (refracts) when going from water to air, making the part in water look shifted.

Discover, Design, and Debate

1. Visit a nearby hospital or the clinic of an ENT specialist, or a dentist, with your teacher or parents. Request the doctor to show you the mirrors used for examining ear, nose, throat, and teeth. Identify the kind of mirror used in these instruments.
Ans: Dentists use concave mirrors to get a bigger, clearer view of your teeth—it makes small areas look enlarged so they can spot problems easily. ENT doctors also use concave mirrors (sometimes on a headband) to focus light and see inside your ear, nose, or throat better. These mirrors curve inwards to magnify things when held close. It’s safe and helps doctors do their job without hurting you!

2. Harnessing sunlight is key to solving future energy challenges. In devices like solar cookers (Fig. 10.29), mirrors are used to converge sunlight and generate heat. In India, such designs are used in villages, thus saving electricity and reducing fossil fuel use. Think of a design for a solar cooker for your school or home and prepare a detailed proposal for it including the budget required.
Ans:  Solar cookers are great for using free sunlight to cook food without gas or electricity—perfect for saving energy in villages or at home!

My Design Idea: Let’s make a simple box-type solar cooker. It’s like a cardboard box lined with shiny materials to trap heat. Inside, place a big concave mirror at the bottom to focus sunlight onto a black cooking pot (black absorbs heat better). Cover the top with a clear glass lid to let light in but keep heat inside. Add aluminum foil on the inside walls for extra reflection. You can adjust the box to face the sun.

How It Works: The concave mirror curves inwards to converge sunlight, making a hot spot on the pot to cook things like rice or veggies in 1-2 hours on a sunny day.

Materials and Budget (in ₹):

• Cardboard box (big enough for a pot): ₹100
• Aluminum foil (for lining and reflecting): ₹50
• Concave mirror (small one from a science kit): ₹150
• Glass sheet for lid: ₹200
• Black paint for the pot: ₹50
• Glue and tape: ₹50

Total Budget: ₹600. This is affordable and eco-friendly—try building it with friends and test cooking something simple!

3. Use online tools or animation to do virtual experiments with spherical mirrors and lenses. Move objects in the simulation and observe how the image changes.
Ans: 
​Activity: Use online tools to play with spherical mirrors and lenses. Move objects and see image changes.

Simple Answer: Go to free websites like PhET Interactive Simulations (search “PhET geometric optics” or “mirrors and lenses”). They’re like virtual labs!

What I Observed:

• Concave Mirror: Place an object close—it shows a big, upright (erect) image, great for magnifying. Move it farther—the image flips upside down (inverted) and gets smaller.
• Convex Mirror: Always shows a small, upright image that gives a wide view, like in car mirrors—things look tinier but you see more area.
• Convex Lens: Close object = big, upright image (like a magnifying glass). Far object = inverted and smaller.
• Concave Lens: Always small, upright images that spread out light—used in glasses to fix vision.

It’s super fun—change distances and see how images flip or resize. This helps understand why dentists use concave mirrors or why cameras have lenses!

Probe and Ponder

1. Can we make mirrors which can give enlarged or diminished images?
Ans: 

• Yes! Concave mirrors can make enlarged (bigger) images when objects are close, like a dentist’s mirror for seeing teeth better. 
• Convex mirrors always make diminished (smaller) images, like side-view mirrors on cars that show a wider view but make things look tinier.

2. On side-view mirrors of vehicles, there is a warning that says “Objects in mirror are closer than they appear”. Why is this warning written there?
Ans: 

• These are convex mirrors, which make images smaller and show a bigger area behind the vehicle. 
• But because things look smaller, they seem farther away than they really are. 
• The warning reminds drivers that cars or bikes might be closer to avoid accidents.

3. Why is there a curved line on some reading glasses?
Ans: 

• Reading glasses often use convex lenses, which are curved to help focus light and make small text look bigger. 
• The curve helps bend light rays to fix vision problems, like making far or near things clearer.

4. Share your questions

Ans: Here are some fun questions you might think of from the chapter: 

• How does a magnifying glass make things look bigger? 
• Why do spoons act like funny mirrors? 
• Can lenses in our eyes change shape? 
• What happens if you use a concave mirror to focus sunlight?

Keep the Curiosity Alive

1. A light ray is incident on a mirror and gets reflected by it (Fig. 10.21). The angle made by the incident ray with the normal to the mirror is 40°. What is the angle made by the reflected ray with the mirror?

Ans: (ii) 50°.
The angle of incidence is 40° (between incident ray and normal). By the law of reflection, the angle of reflection is also 40°. So, the angle between the reflected ray and the mirror is 90° – 40° = 50°.

2. Fig. 10.22 shows three different situations where a light ray falls on a mirror:
(i) The light ray falls along the normal
(ii) The mirror is tilted, but the light ray still falls along the normal to the tilted surface. 

(iii) The mirror is tilted, and the light ray falls at an angle of 20° from the normal. 

Draw the reflected ray in each case (Use a ruler and protractor for accurate drawing). What is the angle of reflection in each case?
Ans: (i) When the ray falls along the normal, the reflected ray goes back along the same path. Angle of reflection = 0°.
(ii) For the tilted mirror with ray along the normal, reflected ray goes back along the normal. Angle = 0°.
(iii) For the ray at 20° to the normal, reflected ray is also at 20° to the normal on the other side. Angle of reflection = 20°.

3. In Fig. 10.23, the cap of a sketch pen is placed in front of three types of mirrors. Match each image with the correct mirror.

Ans: 

• (i) Convex mirror (small and erect).
• (ii) Concave mirror (enlarged image).
• (iii) Plane mirror (same size and erect).

4. In Fig. 10.24 the cap of a sketch pen is placed behind a convex lens, a concave lens, and a flat transparent glass piece — all at the same distance. Match each image with the correct type of lens or glass.

Ans: 

(i) Convex lens (enlarged if close).

(ii) Concave lens (diminished and erect).

(iii) Flat transparent glass piece (same size, no change).

5. When the light is incident along the normal on the mirror, which of the following statements is true:
(i) Angle of incidence is 90° 
(ii) Angle of incidence is 0° 
(iii) Angle of reflection is 90°
(iv) No reflection of light takes place in this case
Ans: 
(ii) Angle of incidence is 0°.
The ray hits straight on, so incidence and reflection angles are both 0°, and it bounces straight back.

6. Three mirrors—plane, concave and convex are placed in Fig. 10.25. On the basis of the images of the graph sheet formed in the mirrors, identify the mirrors and write their names above the mirrors.

Ans: Left: Plane mirror (straight lines, same size).
Middle: Concave mirror (lines curve inward, can enlarge).
Right: Convex mirror (lines curve outward, diminished view).
7. In a museum, a woman walks towards a large convex mirror (Fig. 10.26). She will see that:
(i) her erect image keeps decreasing in size. 
(ii) her inverted image keeps decreasing in size. 
(iii) her inverted image keeps increasing in size and eventually it becomes erect and magnified. 
(iv) her erect image keeps increasing in size.

Ans: (i) her erect image keeps decreasing in size.
Convex mirrors always show erect, smaller images that get even tinier as you get closer.

8. Hold a magnifying glass over text and identify the distance where you can see the text bigger than they are written. Now move it away from the text. What do you notice? Which type of lens is a magnifying glass?
Ans: When close, the text looks bigger (enlarged and erect). As you move it away, the image might flip upside down and change size. A magnifying glass is a convex lens.

9. Match the entries in Column I with those in Column II.

Ans:
(i) Concave mirror – (a) Spherical mirror with a reflecting surface that curves inwards.
(ii) Convex mirror – (b) It forms an image which is always erect and diminished in size.
(iii) Convex lens – (c) Object placed behind it may appear inverted at some distance.
(iv) Concave lens – (d) Object placed behind it always appears diminished in size.

10. The following question is based on Assertion/Reason. 
Assertion: Convex mirrors are preferred for observing the traffic behind us. 
Reason: Convex mirrors provide a significantly larger view area than plane mirrors. 
Choose the correct option: 
(i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion. 
(ii) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion. 
(iii) Assertion is correct but Reason is incorrect. 
(iv) Both Assertion and Reason are incorrect.
Ans: (i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
Convex mirrors give a wider view, helping see more traffic, even if images are smaller.

11. In Fig. 10.27, note that O stands for object, M for mirror, and I for image. Which of the following statements is true?
(i) Figure (a) indicates a plane mirror and Figure (b) indicates a concave mirror. 
(ii) Figure (a) indicates a convex mirror and Figure (b) indicates a concave mirror. 
(iii) Figure (a) indicates a concave mirror and Figure (b) indicates a convex mirror. 
(iv) Figure (a) indicates a plane mirror and Figure (b) indicates a convex mirror.

Ans: Figure (a) indicates a convex mirror and Figure (b) indicates a concave mirror. 
Concave mirror shows enlarged image; convex shows smaller image.

12. Place a pencil behind a transparent glass tumbler (Fig. 10.28a). Now fill the tumbler halfway with water (Fig. 10.28b). How does the pencil appear when viewed through the water? Explain why its shape appears changed.

The pencil looks bent or broken at the water surface. This is because light bends (refracts) when going from water to air, making the part in water look shifted.

Discover, Design, and Debate

1. Visit a nearby hospital or the clinic of an ENT specialist, or a dentist, with your teacher or parents. Request the doctor to show you the mirrors used for examining ear, nose, throat, and teeth. Identify the kind of mirror used in these instruments.
Ans: Dentists use concave mirrors to get a bigger, clearer view of your teeth—it makes small areas look enlarged so they can spot problems easily. ENT doctors also use concave mirrors (sometimes on a headband) to focus light and see inside your ear, nose, or throat better. These mirrors curve inwards to magnify things when held close. It’s safe and helps doctors do their job without hurting you!

2. Harnessing sunlight is key to solving future energy challenges. In devices like solar cookers (Fig. 10.29), mirrors are used to converge sunlight and generate heat. In India, such designs are used in villages, thus saving electricity and reducing fossil fuel use. Think of a design for a solar cooker for your school or home and prepare a detailed proposal for it including the budget required.
Ans:  Solar cookers are great for using free sunlight to cook food without gas or electricity—perfect for saving energy in villages or at home!

My Design Idea: Let’s make a simple box-type solar cooker. It’s like a cardboard box lined with shiny materials to trap heat. Inside, place a big concave mirror at the bottom to focus sunlight onto a black cooking pot (black absorbs heat better). Cover the top with a clear glass lid to let light in but keep heat inside. Add aluminum foil on the inside walls for extra reflection. You can adjust the box to face the sun.

How It Works: The concave mirror curves inwards to converge sunlight, making a hot spot on the pot to cook things like rice or veggies in 1-2 hours on a sunny day.

Materials and Budget (in ₹):

• Cardboard box (big enough for a pot): ₹100
• Aluminum foil (for lining and reflecting): ₹50
• Concave mirror (small one from a science kit): ₹150
• Glass sheet for lid: ₹200
• Black paint for the pot: ₹50
• Glue and tape: ₹50

Total Budget: ₹600. This is affordable and eco-friendly—try building it with friends and test cooking something simple!

3. Use online tools or animation to do virtual experiments with spherical mirrors and lenses. Move objects in the simulation and observe how the image changes.
Ans: 
​Activity: Use online tools to play with spherical mirrors and lenses. Move objects and see image changes.

Simple Answer: Go to free websites like PhET Interactive Simulations (search “PhET geometric optics” or “mirrors and lenses”). They’re like virtual labs!

What I Observed:

• Concave Mirror: Place an object close—it shows a big, upright (erect) image, great for magnifying. Move it farther—the image flips upside down (inverted) and gets smaller.
• Convex Mirror: Always shows a small, upright image that gives a wide view, like in car mirrors—things look tinier but you see more area.
• Convex Lens: Close object = big, upright image (like a magnifying glass). Far object = inverted and smaller.
• Concave Lens: Always small, upright images that spread out light—used in glasses to fix vision.

It’s super fun—change distances and see how images flip or resize. This helps understand why dentists use concave mirrors or why cameras have lenses!

Probe and Ponder

1. What do you think is happening in the picture above?

• The picture shows Mahatma Gandhi obtaining salt from the sea during the Salt March, accompanied by followers. 
• This illustrates the historical process of extracting salt from seawater through evaporation, where seawater acts as a natural solution with salt as the solute and water as the solvent, highlighting concepts of solubility and traditional salt production.

2. What happens when you add too much sugar to your tea and it stops dissolving? How can you solve this problem?
Ans: 

• When too much sugar is added, the tea becomes a saturated solution, and excess sugar settles at the bottom as it can no longer dissolve at that temperature. 
• To solve this, heat the tea to increase the solubility of sugar, allowing more to dissolve, as solubility generally increases with temperature for solids in liquids.

3. Why do sugar and salt dissolve in water but not in oil? Why is water considered a good solvent?
Ans: 

• Sugar and salt dissolve in water because water is a polar solvent that can break apart ionic bonds in salt or form hydrogen bonds with sugar molecules. 
• Oil is non-polar, so it does not interact with these polar or ionic solutes. 
• Water is considered a good solvent due to its polarity, allowing it to dissolve a wide range of substances, earning it the title “universal solvent”.

4. Why are water bottles usually tall and cylindrical in shape instead of spherical?
Ans: 

• Tall, cylindrical shapes are more efficient for storage, stacking, and manufacturing. 
• They provide better grip, stability, and use less material for the same volume compared to spheres, which would roll and be harder to handle.

5. Share your questions
Ans: Based on the chapter, potential questions could include: 

• How does temperature affect the solubility of gases differently from solids? 
• Why does ice float on water despite being a solid? 
• What role does density play in everyday phenomena like hot air balloons?

Keep the Curiosity Alive

1. State whether the statements given below are True [T] or False [F]. Correct the false statement(s).
(i) Oxygen gas is more soluble in hot water rather than in cold water. 
(ii) A mixture of sand and water is a solution. 
(iii) The amount of space occupied by any object is called its mass. 
(iv) An unsaturated solution has more solute dissolved than a saturated solution. 
(v) The mixture of different gases in the atmosphere is also a solution.
Ans: 
(i) False. Oxygen gas is more soluble in cold water rather than in hot water, as solubility of gases decreases with increasing temperature.
(ii) False. A mixture of sand and water is a non-uniform mixture, not a solution.
(iii) False. The amount of space occupied by any object is called its volume, not mass.
(iv) False. An unsaturated solution has less solute dissolved than a saturated solution; a saturated solution cannot dissolve more solute at that temperature.
(v) True. The mixture of different gases in the atmosphere is a solution, with nitrogen as the solvent.

2. Fill in the blanks.
(i) The volume of a solid can be measured by the method of displacement, where the solid is __________ in water and the ____________ in water level is measured. 
Ans: (i) The volume of a solid can be measured by the method of displacement, where the solid is immersed in water and the rise in water level is measured.
(ii) The maximum amount of _______________ dissolved in _______________ at a particular temperature is called solubility at that temperature. 
Ans: The maximum amount of solute dissolved in solvent at a particular temperature is called solubility at that temperature.
(iii) Generally, the density ____________ with increase in temperature. 
Ans: Generally, the density decreases with increase in temperature.
(iv) The solution in which glucose has completely dissolved in water, and no more glucose can dissolve at a give
Ans: The solution in which glucose has completely dissolved in water, and no more glucose can dissolve at a given temperature, is called a saturated solution of glucose.

3. You pour oil into a glass containing some water. The oil floats on top. What does this tell you? 
(i) Oil is denser than water 
(ii) Water is denser than oil 
(iii) Oil and water have the same density 
(iv) Oil dissolves in water
Ans: (ii) Water is denser than oil. 
Oil floats because its density is lower than water’s, causing less dense substances to float on denser ones.

4. A stone sculpture weighs 225 g and has a volume of 90 cm³. Calculate its density and predict whether it will float or sink in water.
Ans: Density = Mass / Volume = 225 g / 90 cm³ = 2.5 g/cm³. Since this is greater than water’s density (1 g/cm³), the sculpture will sink in water.

5. Which one of the following is the most appropriate statement, and why are the other statements not appropriate?
(i) A saturated solution can still dissolve more solute at a given temperature. 
(ii) An unsaturated solution has dissolved the maximum amount of solute possible at a given temperature. 
(iii) No more solute can be dissolved into the saturated solution at that temperature. 
(iv) A saturated solution forms only at high temperatures.
Ans: 

• ​(iii) No more solute can be dissolved into the saturated solution at that temperature. This is correct as a saturated solution has reached its solubility limit. 
• (i) is incorrect because a saturated solution cannot dissolve more. 
• (ii) describes a saturated, not unsaturated, solution. 
• (iv) is wrong as saturation can occur at any temperature.

6. You have a bottle with a volume of 2 litres. You pour 500 mL of water into it. How much more water can the bottle hold?
Ans: The bottle can hold an additional 1,500 mL (or 1.5 litres) of water, as 2 litres – 0.5 litres = 1.5 litres.

7. An object has a mass of 400 g and a volume of 40 cm³. What is its density?
Ans: Density = 400 g / 40 cm³ = 10 g/cm³.

8. Analyse Fig. 9.25a and 9.25b. Why does the unpeeled orange float, while the peeled one sinks? Explain.
Ans: The unpeeled orange floats due to air pockets in the peel, reducing its overall density below that of water. The peeled orange sinks because, without the peel, its density is higher than water’s.

9. Object A has a mass of 200 g and a volume of 40 cm³. Object B has a mass of 240 g and a volume of 60 cm³. Which object is denser?
Ans: Density of A: 200 g / 40 cm³ = 5 g/cm³. Density of B: 240 g / 60 cm³ = 4 g/cm³. Object A is denser.

10. Reema has a piece of modeling clay that weighs 120 g. She first moulds it into a compact cube that has a volume of 60 cm³. Later, she flattens it into a thin sheet. Predict what will happen to its density.
Ans: The density remains the same (120 g / 60 cm³ = 2 g/cm³ initially) because mass is constant, and changing shape does not affect density, only volume and arrangement.

11. A block of iron has a mass of 600 g and a density of 7.9 g/cm³. What is its volume?
Ans: Volume = Mass / Density = 600 g / 7.9 g/cm³ ≈ 75.95 cm³.

12. You are provided with an experimental setup as shown in Fig. 9.26a and 9.26b. On keeping the test tube (Fig 9.26b) in a beaker containing hot water (~70 °C), the water level in the glass tube rises. How does it affect the density?
Ans: Heating causes the air in the tube to expand, increasing volume and decreasing density, which makes the water level rise as less dense air pushes up.

Discover, Design, and Debate

1. Research project on Dead Sea: Why is there no aquatic life in the Dead Sea? Try to find out if there are any other similar water bodies.
Ans: The Dead Sea is super salty—about 34% salt! That’s way too much for most fish, plants, or animals to live there because the salt pulls water out of their bodies, stressing them out. Only tough bacteria can survive. This happens because the water evaporates a lot, leaving salt behind, and there’s no river to wash it away. Other places like this are the Great Salt Lake in Utah, USA (too salty for most life), and Lake Assal in Djibouti (one of the saltiest spots on Earth with very few living things).

2. Investigate how well common salt dissolves in different solvents, such as water, vinegar, and oil. Compare the solubility of salt in each solvent and record your observations.
Ans: Try this experiment! In water (a polar liquid that attracts salt), the salt mixes in completely and makes a clear liquid. In vinegar (also polar but a bit weaker), some salt dissolves, but not as much as in water—it might look a little cloudy. In oil (non-polar, like grease), the salt doesn’t dissolve at all and just sits at the bottom. What I observed: Water is best because its molecules pull salt apart easily. Oil doesn’t work because it doesn’t mix with salt’s particles. This shows why some things dissolve and others don’t!

3. Debate in class—Is water truly the most versatile solvent?
Ans:  Let’s debate! 
​Yes side: Water is awesome because it’s polar, so it can dissolve salts, sugars, gases, and more. It helps in our blood, oceans, and even rain—keeping life going! 
​No side: Water isn’t perfect—it can’t dissolve oily stuff like fats or petrol. Other solvents like alcohol (ethanol) can handle both watery and oily things better in some cases, like in medicines or cleaning. 
​Conclusion: Water is super useful for many things, but it’s not the only star—it depends on what you’re trying to dissolve. Have fun arguing with your classmates!

Probe and Ponder

1. Which of the entities in the picture above consist of matter, and which of them do not?
Ans: 

• Entities consisting of matter include physical objects like staircases, air, water, food, clothes, shoes, books, trees, balls, and sticks, as these have mass and occupy space. 
• They are made of tiny particles. Entities that do not consist of matter include light, heat, electricity, thoughts, and emotions, as they lack mass and do not occupy space.

2. How can elements be combined to form a compound?

Ans: 

• Elements combine chemically in fixed ratios to form compounds. 
• For example, hydrogen and oxygen combine in a 2:1 ratio to form water, where the atoms bond tightly, creating a new substance with properties different from the original elements. 
• This requires a chemical reaction, not just physical mixing.

3. How could the discovery of a compound that absorbs carbon dioxide from the air contribute to solving environmental challenges?
Ans: 

• Such a compound could reduce atmospheric carbon dioxide levels, mitigating global warming and climate change. 
• For instance, it might be used in technologies to capture emissions from industries or vehicles, similar to how calcium hydroxide reacts with carbon dioxide to form calcium carbonate.
•  This could help address air pollution and support environmental cleanup efforts.

4. Share your questions
Ans: Based on the chapter, potential questions could include: 

• What happens when elements like iron and sulfur are heated together?
• Why does water extinguish fire while its components (hydrogen and oxygen) support combustion? 
• How do alloys like stainless steel improve everyday materials?

Keep the Curiosity Alive

Q1. Consider the following reaction where two substances, A and B, combine to form a product C: A + B → C. Assume that A and B cannot be broken down into simpler substances by chemical reactions. Based on this information, which of the following statements is correct?
(i) A, B, and C are all compounds and only C has a fixed composition. 
(ii) C is a compound, and A and B have a fixed composition. 
(iii) A and B are compounds, and C has a fixed composition. 
(iv) A and B are elements, C is a compound, and has a fixed composition.
Ans: (iv) A and B are elements, C is a compound, and has a fixed composition.
This is because elements are simplest substances that cannot be broken down further, and they combine chemically in fixed ratios to form compounds with new properties.

Q2. Assertion: Air is a mixture. 
Reason: A mixture is formed when two or more substances are mixed, without undergoing any chemical change. 
(i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion. 
(ii) Both Assertion and Reason are true, but Reason is not the correct explanation for Assertion. 
(iii) Assertion is true, but Reason is false. 
(iv) Assertion is false, but Reason is true
Ans: (i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
Air consists of gases like nitrogen, oxygen, argon, carbon dioxide, and water vapor mixed without chemical reaction, retaining their individual properties.

Q3. Water, a compound, has different properties compared to those of the elements oxygen and hydrogen from which it is formed. Justify this statement.
Ans: 

• Water is formed by hydrogen and oxygen combining in a 2:1 ratio through a chemical reaction. 
• Hydrogen is a flammable gas, oxygen supports combustion, but water is a liquid that extinguishes fire. 
• This shows compounds have properties distinct from their constituent elements,  where water decomposes into hydrogen and oxygen via electrolysis.

Q4. In which of the following cases are all the examples correctly matched? Give reasons in support of your answers.
(i) Elements — water, nitrogen, iron, air.
(ii) Uniform mixtures— minerals, seawater, bronze, air. 
(iii) Pure substances— carbon dioxide, iron, oxygen, sugar. 
(iv) Non-uniform mixtures — air, sand, brass, muddy water.
Ans: (iii) Pure substances— carbon dioxide, iron, oxygen, sugar.

• Correct option: Option (iii)  iron and oxygen are elements (cannot be broken down), while carbon dioxide and sugar are compounds (made of elements in fixed ratios). 
• Option (i) incorrectly includes water (compound) and air (mixture); 
• Option (ii) lists minerals (compounds/elements), seawater and air (mixtures), but bronze is uniform; 
• Option (iv) has air (uniform mixture) and brass (uniform alloy), not all non-uniform.

Q5. Iron reacts with moist air to form iron oxide, and magnesium burns in oxygen to form magnesium oxide. Classify all the substances involved in the above reactions as elements, compounds or mixtures, with justification.

Ans:

• Iron: Element (pure metal, cannot be broken down).
• Moist air: Mixture (air gases plus water vapor, components retain properties).
• Iron oxide: Compound (iron and oxygen combined chemically).
• Magnesium: Element (pure metal).
• Oxygen: Element (pure gas).
• Magnesium oxide: Compound (magnesium and oxygen in fixed ratio).
• Justification: Elements are simplest substances; compounds form from elements via chemical reactions with new properties; mixtures do not involve chemical bonding.

Q6. Classify the following as elements, compounds, or mixtures in Table 8.3. 
Carbon dioxide, sand, seawater, magnesium oxide, muddy water, aluminium, gold, oxygen, rust, iron sulfide, glucose, air, water, fruit juice, nitrogen, sodium chloride, sulfur, hydrogen, baking soda.
Identify pure substances amongst these and list them below

Ans:

Pure substances: Carbon dioxide, magnesium oxide, rusty (iron oxide), iron sulfide, glucose, water, sodium chloride, baking soda, aluminium, gold, oxygen, nitrogen, sulfur, hydrogen. (These are either elements or compounds with uniform particles).

Q7. What new substance is formed when a mixture of iron filings and sulfur powder is heated, and how is it different from the original mixture? Also, write the word equation for the reaction.
Ans: Iron sulfide (a black compound) is formed. It differs as it has uniform texture, no magnetic attraction (unlike iron in the mixture), and reacts with hydrochloric acid to produce hydrogen sulfide (rotten egg odor), not hydrogen gas like the mixture. Word equation: Iron + Sulfur → Iron sulfide.

Q8. Is it possible for a substance to be classified as both an element and a compound? Explain why or why not.
Ans: No, because elements are simplest substances made of identical atoms and cannot be broken down, while compounds are made of two or more elements chemically combined in fixed ratios with different properties. A substance cannot be both simplest and composed of multiple elements.

Q9. How would our daily lives be changed if water were not a compound but a mixture of hydrogen and oxygen?
Ans: Water as a mixture would separate easily into flammable hydrogen and combustion-supporting oxygen, making it explosive and unusable for drinking, cooking, or extinguishing fires. It would lack stability, affecting life processes, weather, and ecosystems reliant on water’s unique properties as a compound.

Q10. Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction.

Ans: Gas A is hydrogen (produced when iron reacts with acid, burning with a pop sound). Word equation: Iron + Dilute hydrochloric acid → Iron chloride + Hydrogen.

Q11. Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.
Ans: 

• Carbon dioxide: Used in fire extinguishers (absorbs heat, displaces oxygen) and aerated drinks (provides fizz).
• Water: Used for hydration (essential for life) and cleaning (dissolves substances).

Q12. How can gold be classified as both a mineral and a metal?
Ans: Gold is a native mineral (pure element found naturally in rocks) and a metal (shiny, malleable, conductive element). It fits both as it’s an element in pure form, not a compound, unlike most minerals.

Discover, design, and debate

Q1. Design and create comic strips from real-life examples to differentiate between elements, compounds, and mixtures with diagrams and illustrate their properties and uses.
Ans: 
Comic Strip Idea: 
Panel 1: Element (iron) – A superhero “Iron Atom” stands alone, unbreakable. 
Panel 2: Compound (water) – Hydrogen and oxygen “team up” chemically to form water, extinguishing a fire (unlike their flammable selves). Panel 3: Mixture (air) – Gases like nitrogen and oxygen mix loosely, shown as friends hanging out without changing. Include diagrams of atoms/molecules and uses (e.g., iron in bridges, water for life, air for breathing).

Q2. Search for discoveries of some elements (such as phosphorus, sodium), compounds (such as penicillin) and mixtures (such as brass, bronze, stainless steel). Present your findings in the class.
Ans: 

• Phosphorus: Discovered in 1669 by Hennig Brand from urine; used in matches and fertilizers.
• Sodium: Isolated in 1807 by Humphry Davy via electrolysis; used in soaps and streetlights.
• Penicillin: Discovered in 1928 by Alexander Fleming from mold; antibiotic compound saving lives from infections.
• Brass: Ancient mixture of copper and zinc; used in musical instruments for durability.
• Bronze: Early alloy of copper and tin; for statues and tools.
• Stainless steel: Modern mixture with iron, chromium; rust-resistant for utensils. Present as a slideshow with timelines.

Q3. Let us search: Read labels on items like detergents or snacks, and try to list the mixtures and compounds they contain.
Ans: 

• ​Detergent label: Mixtures – surfactants with water; Compounds – sodium carbonate (cleaning agent), sodium sulfate (filler). 
• Snacks (chips): Mixtures – potato with oil and spices; Compounds – sodium chloride (salt), citric acid (flavoring).

Q4. Work in groups: Each group will pretend to be in the role of either an element, a compound, or a mixture. Debate which category among them is the most important.

Ans: Debate Structure: Group 1 (Elements) argues they are building blocks (e.g., oxygen for life). 
Group 2 (Compounds) claims innovation (e.g., water sustains ecosystems). 
Group 3 (Mixtures) highlights versatility (e.g., air for breathing, alloys for tools). Conclude all are essential, as matter relies on their interplay.

Probe and Ponder

1. Why is it possible to pile up stones or sand, but not a liquid like water?
Ans: 

• ​Stones and sand are solids, where particles are tightly packed with strong interparticle attractions, giving them a fixed shape and allowing them to be piled up without flowing. 
• Water is a liquid, with weaker interparticle attractions, so its particles can move freely and flow, taking the shape of the container or spreading out, making piling impossible.

2. Why does water take the shape of folded hands but lose that shape when released?
Ans: 

• Water is a liquid, so its particles have enough freedom to move and adapt to the shape of the container (like folded hands). 
• When released, the particles flow due to gravity and weaker interparticle forces, losing the shape because liquids do not have a fixed form—they only have a fixed volume.

3. We cannot see air, so how does it add weight to an inflated balloon?
Ans: 

• ​Air is a gas made of tiny, invisible particles (like nitrogen and oxygen molecules) that are far apart and in constant motion. 
• When a balloon is inflated, these particles occupy space inside, adding mass (and thus weight) to the balloon, even though the particles themselves are too small to see.

4. Is the air we breathe today the same that existed thousands of years ago?
Ans: 

• ​Yes, in terms of composition, the air we breathe is largely the same mixture of gases (nitrogen, oxygen, etc.) that has existed for thousands of years, as matter is conserved and particles cycle through natural processes like respiration and photosynthesis. 
• However, human activities have slightly altered its composition, adding pollutants.

5. Share your questions?
Ans: 

• How small are the tiniest particles of matter, and can we ever see them?
• Why do some solids melt easily while others need very high temperatures?
• If gases have no fixed volume, how do they stay contained in the atmosphere?
• What happens to the particles when a substance changes from solid to liquid?
• Why don’t all solids dissolve in water like sugar does?

Keep the Curiosity Alive

1. Choose the correct option.
The primary difference between solids and liquids is that the constituent particles are: 
(i) closely packed in solids, while they are stationary in liquids. 
(ii) far apart in solids and have fixed position in liquids. 
(iii) always moving in solids and have fixed position in liquids. 
(iv) closely packed in solids and move past each other in liquids.
Ans: (iv) closely packed in solids and move past each other in liquids.
In solids, particles are fixed in position due to strong attractions, while in liquids, particles can slide past one another, allowing flow.

2. Which of the following statements are true? Correct the false statements.

(i) Melting ice into water is an example of the transformation of a solid into a liquid. 
Ans: True
(ii) Melting process involves a decrease in interparticle attractions during the transformation. 
Ans: False
Melting involves overcoming interparticle attractions with heat energy, but the attractions weaken as particles move farther apart.
(iii) Solids have a fixed shape and a fixed volume. 
Ans: True 
(iv) The interparticle interactions in solids are very strong, and the interparticle spaces are very small.
Ans: True

(v) When we heat camphor in one corner of a room, the fragrance reaches all corners of the room. 
Ans: True

(vi) On heating, we are adding energy to the camphor, and the energy is released as a smell.
Ans: False
Heating adds energy to camphor particles, causing sublimation (solid to gas), and the gas particles spread due to constant motion, carrying the fragrance.

3. Choose the correct answer with justification. If we could remove all the constituent particles from a chair, what would happen?
(i) Nothing will change. (ii) The chair will weigh less due to lost particles. (iii) Nothing of the chair will remain.
Ans: A chair is made entirely of constituent particles (atoms and molecules of wood or other materials). Removing all particles would leave no matter behind, as matter is composed of these tiny units—there would be no structure, mass, or volume left.

4. Why do gases mix easily, while solids do not?

• ​Gases mix easily because their particles have negligible interparticle attractions, maximum spacing, and constant random motion, allowing them to spread and intermingle freely in all directions. 
• Solids do not mix easily as their particles are tightly packed with strong attractions, fixed positions, and only vibrational motion, preventing them from blending without external force like grinding.

5. When spilled on the table, milk in a glass tumbler flows and spreads out, but the glass tumbler stays in the same shape. Justify this statement.
Ans: 

• ​Milk is a liquid, with particles that have weaker interparticle attractions and can move past each other, allowing it to flow and spread under gravity. 
• The glass tumbler is a solid, with particles tightly packed and held by strong attractions in fixed positions, maintaining its rigid shape and volume regardless of external forces like spilling.

6. Represent diagrammatically the changes in the arrangement of particles as ice melts and transforms into water vapour.
Ans: 

• Ice (Solid): Particles closely packed in a fixed lattice, vibrating in place. (Imagine a grid of dots tightly together.)
• Water (Liquid): Particles slightly farther apart, sliding past each other but still close. (Dots loosely arranged, with some movement arrows.)
• Water Vapour (Gas): Particles far apart, moving freely in all directions. (Scattered dots with long arrows showing random motion.)
  The transformation: Heat weakens attractions, increasing particle spacing and motion from solid to liquid (melting) and liquid to gas (boiling/evaporation).

7. Draw a picture representing particles present in the following:
(i) Aluminium foil (Solid): Closely packed particles in a regular pattern, with minimal spacing and only vibrations.
(ii) Glycerin (Liquid): Particles close but irregular, with some sliding motion and small spaces.
(iii) Methane gas (Gas): Widely spaced particles moving randomly in all directions, with large empty spaces.

8. Observe Fig. 7.16a which shows the image of a candle that was just extinguished after burning for some time. Identify the different states of wax in the figure and match them with Fig. 7.16b showing the arrangement of particles.

Ans:

• Solid wax (at base): Rigid, unmelted portion—matches tightly packed particles.
• Liquid wax (melted pool): Flowing around wick—matches loosely arranged particles with movement.
• Gaseous wax (vapour/smoke): Rising as fumes—matches widely spaced, freely moving particles.
  The figure shows transitions: solid to liquid (melting) and liquid to gas (evaporation), with particle arrangements changing from fixed to mobile.

9. Why does the water in the ocean taste salty, even though the salt is not visible? Explain.
Ans: 

• Ocean water tastes salty because salt (sodium chloride) dissolves into tiny particles that mix uniformly with water particles, occupying interparticle spaces. 
• The salt particles are too small to see but are detected by taste buds. 
• This is a solution where solute (salt) particles are distributed evenly in the solvent (water) without changing visibility.

10. Grains of rice and rice flour take the shape of the container when placed in different jars. Are they solids or liquids? Explain.
Ans: 

• ​They are solids. Both rice grains and flour consist of solid particles with strong interparticle attractions and fixed shapes individually.
• However, as a collection of many small particles, they can flow and adapt to the container’s shape due to gravity, similar to liquids, but they remain solids because their particles do not truly flow past each other like liquid particles.

Discover, Design, and Debate

1. Fix a balloon over the neck of a bottle and put the bottle in hot water. Explore what will happen?
Ans: The balloon inflates. Hot water heats the air inside the bottle, increasing the kinetic energy of air particles, causing them to move faster and spread out (expand). This raises the pressure inside, pushing air into the balloon. It demonstrates gas particles’ response to heat, with increased motion and spacing.

2. Design and create simple models to represent particles of solids, liquids, and gases showing interparticle spacing using clay balls, beads, etc.

• Solid: Arrange clay balls in a tight grid (e.g., in a box), touching each other to show minimal spacing and fixed positions.
• Liquid: Place beads in a shallow tray, close but movable when shaken, illustrating slight spacing and flow.
• Gas: Scatter beads loosely in a large container, shaking to show random motion and maximum spacing.
  These models visualize how interparticle spacing increases from solids to gases, affecting properties like shape and volume.

3. Pretend to be particles of solids, liquids, and gases, at different temperatures—create and perform a role-play/dance showing particles in motion.
Ans: 

• Solid (low temperature): Students stand close, holding hands tightly, only vibrating in place (small shakes).
• Liquid (medium temperature): Loosen grips, move arms while sliding around in a group, showing flow but staying somewhat together.
• Gas (high temperature): Break apart, run freely in all directions with energetic jumps, simulating random motion.
  Increase “temperature” by speeding up movements to show phase changes, like “melting” from solid to liquid.

4. Debate in the class — ‘Gases can spread and fill all the available space’. Is this property of gases beneficial or harmful?
Ans: 
Beneficial side: Gases spreading enables essential processes like oxygen diffusion in breathing, fragrance dispersal in perfumes, and even weather patterns (wind). It aids in cooking (gas stoves) and inflation (tires/balloons), making life convenient and supporting ecosystems.
Harmful side: It can spread pollutants or toxic gases quickly (e.g., air pollution, gas leaks causing accidents), leading to health risks or environmental damage like greenhouse gases contributing to climate change.
Overall, the property is mostly beneficial when controlled but harmful if unmanaged—debate could conclude with the need for safety measures like ventilation.

Probe and ponder

1. Why are winds stronger on some days than on others? 
Ans: 

• ​Winds are stronger on some days due to greater differences in air pressure (pressure gradients) between different locations. 
• When a weather system, such as a storm or a cyclone, creates steep pressure differences, air moves rapidly from high-pressure areas to low-pressure areas, resulting in strong winds.
• Temperature contrasts (such as between land and sea, or during weather fronts), and local topography can intensify these differences and wind speeds.

2. Why are water tanks usually placed at a height?
Ans: 

• ​Water tanks are elevated to use gravity to increase water pressure for distribution. 
• Height ensures that water flows with enough pressure through pipes to reach all taps—even those on upper floors—without requiring electric pumps. 
• The formula P=hρg shows water pressure depends on the height (h) above the outlet.

3. Can air pressure really crush us?
Ans: 

• ​Atmospheric pressure is indeed very strong (about 101,325 Pa at sea level, or approximately 10 tons on your body surface!). 
• However, the pressure inside our bodies matches the outside pressure, balancing the force. 
• Without this balance (for example, if strong pressure differentials appear suddenly), air pressure can cause serious damage—as seen when containers or objects are evacuated of air and crushed by outside air.

4. What causes storms and cyclones? If the Earth stopped rotating, would cyclones still form?

Ans: 

• Storms and cyclones are caused by warm, moist air rising rapidly. As this air cools, water vapor condenses, releasing heat and causing strong winds and clouds. Cyclones specifically form over warm oceans, with rotation guided by Earth’s spin (the Coriolis effect).
• If the Earth stopped rotating, cyclones as we know them wouldn’t form. The rotation (Coriolis force) is what makes winds spiral, creating the cyclonic structure. Without it, storms could still occur, but organized, rotating cyclones would not.

5. Share your questions

Ans: Possible questions you might ask after exploring these ideas:

• Why don’t we feel atmospheric pressure even though it is so high?
• How do buildings and bridges withstand strong winds during storms?
• Do animals sense changes in air pressure before storms?
• What scientific instruments are used to measure wind speed and pressure?
• How do disaster warning systems work for cyclones?

Keep the curiosity alive

1. Choose the correct statement. 
(i) Look at Fig. 6.21 carefully. Vessel R is filled with water. When pouring of water is stopped, the level of water will be ____________________. 
(a) the highest in vessel P 
(b) the highest in vessel Q 
(c) the highest in vessel R 
(d) equal in all three vessels 
Ans: (d) equal in all three vessels
The level of water in connected vessels (communicating vessels) will be the same regardless of shape, as liquid pressure depends on the height of the column, not the vessel’s shape or width

(ii) A rubber sucker (M) is pressed on a flat smooth surface and an identical sucker (N) is pressed on a rough surface: 
(a) Both M and N will stick to their surfaces. 
(b) Both M and N will not stick to their surfaces. 
(c) M will stick but N will not stick. 
(d) M will not stick but N will stick.
Ans: (c) M will stick but N will not stick
The rubber sucker sticks due to atmospheric pressure creating a vacuum on a smooth surface. On a rough surface, air leaks in, preventing the vacuum and thus the sticking.

(iii) A water tank is placed on the roof of a building at a height ‘H’. To get water with more pressure on the ground floor, one has to 
(a) increase the height ‘H’ at which the tank is placed. 
(b) decrease the height ‘H’ at which the tank is placed. 
(c) replace the tank with another tank of the same height that can hold more water. 
(d) replace the tank with another tank of the same height that can hold less water. 
Ans: (a) increase the height ‘H’ at which the tank is placed
Liquid pressure increases with the height of the water column. Raising the tank increases the pressure, resulting in a stronger stream of water.

(iv) Two vessels, A and B contain water up to the same level as shown in Fig. 6.22. P_A and P_B is the pressure at the bottom of the vessels. F_A and F_B is the force exerted by the water at the bottom of the vessels A and B. 

(a) P_A = P_B, F_A = F_B 
(b) P_A = P_B, F_A < F_B 
(c) P_A < P_B, F_A = F_B
(d) P_A > P_B, F_A > F_{B
Ans: Pressure at the bottom depends on the height of the water, which is the same in both vessels, so PA = PB. Force equals pressure times area; since vessel A is narrower (smaller area), FA < FB.}

_{2. State whether the following statements are True [T] or False [F]. 
(i) Air flows from a region of higher pressure to a region of lower pressure. [ ] 
(ii) Liquids exert pressure only at the bottom of a container. [ ] 
(iii) Weather is stormy at the eye of a cyclone. [ ] 
(iv) During a thunderstorm, it is safer to be in a car. [ ]
Ans: (i) True [T] – Air flows from a region of higher pressure to a region of lower pressure.
(ii) False [F] – Liquids exert pressure on the bottom, sides, and in all directions within a container.
(iii) False [F] – The eye of a cyclone is calm, with no stormy weather.
(iv) True [T] – During a thunderstorm, it is safer to be in a car, as it acts as a protective enclosure (like a Faraday cage) against lightning.}

3. Fig. 6.23 a shows a boy lying horizontally, and Fig. 6.23b shows the boy standing vertically on a loose sand bed. In which case does the boy sink more in sand? Give reasons. 

Ans: 

• The boy will sink more when standing vertically (Fig. 6.23 b). When standing, his weight acts on a smaller area (his feet), resulting in higher pressure that causes more sinking. 
• When lying horizontally (Fig. 6.23 a), his weight is distributed over a larger area, reducing the pressure and minimizing sinking.

4. An elephant stands on four feet. If the area covered by one foot is 0.25 m2 , calculate the pressure exerted by the elephant on the ground if its weight is 20000 N.

Ans: 

• Total area covered by four feet = 4 × 0.25 m² = 1 m².
• Pressure = Force / Area = 20000 N / 1 m² = 20000 N/m² (or 20000 Pa).

5. There are two boats, A and B. Boat A has a base area of 7 m² , and 5 persons are seated in it. Boat B has a base area of 3.5 m² , and 3 persons are seating in it. If each person has a weight of 700 N, find out which boat will experience more pressure on its base and by how much? 
Ans: Weight per person = 700 N.
Boat A: Total weight = 5 × 700 N = 3500 N; Pressure = 3500 N / 7 m² = 500 Pa.
Boat B: Total weight = 3 × 700 N = 2100 N; Pressure = 2100 N / 3.5 m² = 600 Pa.
Boat B experiences more pressure on its base, by 100 Pa.

6. Would lightning occur if air and clouds were good conductors of electricity? Give reasons for your answer. 
Ans: 

• No, lightning would not occur. Lightning results from the buildup and sudden discharge of static charges in clouds, which requires air to act as an insulator. 
• If air and clouds were good conductors, charges would flow continuously without building up enough to cause a discharge.

7. What will happen to the two identical balloons A and B as shown in Fig. 6.24 when water is filled into the bottle up to a certain height. Will both the balloons bulge? If yes, will they bulge equally? Explain your answer. 
Ans: 

• Both balloons will bulge, and they will bulge equally. 
• The pressure exerted by the water depends on the height of the water column in the bottle, not the diameter of the connections or balloons. 
• Since the water is filled to the same height, the pressure is the same, causing equal bulging.

8. Explain how a storm becomes a cyclone. 
Ans: 

• A storm becomes a cyclone over warm ocean waters. Warm, moist air rises, creating a low-pressure area. 
• Surrounding cooler air rushes in, warms, and rises, while water vapor condenses, releasing heat that fuels further rising. 
• Earth’s rotation causes the air to spin, forming a rotating system of clouds, high-speed winds, and rain around a low-pressure center (the eye). 
• This intensifies into a cyclone as the process repeats.

9. Fig. 6.25 shows trees along the sea coast in a summer afternoon. Identify which side is land— A or B. Explain your answer.

Ans: Side B is land. In a summer afternoon, land heats faster than the sea, creating low pressure over land. This causes a sea breeze to blow from the sea (side A) toward the land (side B), bending the trees toward side B.

10. Describe an activity to show that air flows from a region of high pressure to a region of low pressure. 
Ans: 

• Take two similar balloons and a straw. Insert one end of the straw into an uninflated balloon and secure it. 
• Inflate the second balloon, secure its mouth, then insert the other end of the straw into it without leaking air. 
• Air flows from the inflated (high-pressure) balloon to the uninflated (low-pressure) one through the straw, causing the inflated balloon to shrink and the other to expand until pressures equalize.

11. What is a thunderstorm? Explain the process of its formation. 
Ans: 

• A thunderstorm is a storm accompanied by lightning, thunder, and often heavy rain.
• It forms when warm, moist air rises rapidly, cools, and condenses into clouds. 
• Strong upward and downward winds cause ice particles and water droplets to rub, building static charges (positive at the top, negative at the bottom). 
• When charges build sufficiently, air’s insulating property breaks, leading to lightning discharges and thunder.

12. Explain the process that causes lightning. 
Ans:

• Lightning occurs due to charge separation in clouds during a thunderstorm. 
• Strong winds cause ice particles (positively charged) to rise to the cloud’s top and water droplets (negatively charged) to stay at the bottom. This separation builds up opposite charges.
• When the buildup is large enough, air’s insulation breaks, allowing a sudden flow of charges, producing a bright flash (lightning) within a cloud, between clouds, or to the ground.

13. Explain why holes are made in banners and hoardings.

Ans: Holes allow high-speed winds to pass through, reducing the pressure difference between the front and back. Without holes, wind pressure could tear or blow away the banners, especially during storms.

Discover, design, and debate 

1. Hold a strip of paper, 18 cm long and 2 cm wide, between your thumb and forefinger so that it hangs freely. Predict what you will observe if you blow over the paper. Perform the activity now. Note down your observations and interpret your results. 
Ans: 

Prediction:
When you hold a strip of paper by your thumb and forefinger so it hangs freely, and blow air over the top surface of the strip, you might expect the paper to move down due to the force of the air. However, the actual observation defies this intuition.

Observation:
When you blow forcefully over the top of the paper, the strip of paper rises up instead of moving down.

Interpretation:
This occurs due to pressure difference explained by Bernoulli’s principle. Blowing air over the top increases the speed of air above compared to the still air below, resulting in lower pressure on top and higher pressure underneath. The higher pressure below pushes the paper upwards. This is the same principle that helps airplanes fly.

2. List three major cyclones which have occured in India in the last 20 years. List two major destruction caused by each of the cyclones. What measures were taken by the local government and communities to reduce the loss of life and destruction of property? Mention two suggestions you would like to propose to the local government. 
Ans: Three Major Cyclones in India in the Last 20 Years
For all major cyclones, the Indian government has developed:

• The National Cyclone Risk Mitigation Project (NCRMP) to build cyclone shelters, strengthen infrastructure, and promote early warning dissemination.
• Community-based disaster management, where local volunteers are trained to assist in evacuations and first response.

3. Collect data on the strength of thunderstorms for various regions of India. Compare your findings and identify which regions are more prone to thunderstorms. Can you give reasons for your findings?
Ans: 

Regional Data:

• North-East India: Highest thunderstorm frequency, with some locations experiencing over 100 thunderstorm days per year (especially Assam, Meghalaya, Sub-Himalayan West Bengal).
• East India (Odisha, West Bengal, Jharkhand, Chhattisgarh, Bihar): 30–50 thunderstorm days/year; high intensity.
• Southern Peninsula (including Karnataka, Tamil Nadu): 60–80 thunderstorm days/year in some parts, with Bengaluru region averaging ~41 thunderstorm days and ~157 lightning events annually.
• Central/Western India: Lesser frequency but still significant, particularly during pre-monsoon.

Reasons for Regional Variation:

• North-East and eastern India: Thunderstorms, locally called “Nor’westers” (Kalboishakhi), are frequent in the pre-monsoon months due to high moisture, temperature, and terrain effects.
• Southern India: The convergence of Southwest and Northeast monsoon winds, abundant moisture, and heat creates ideal conditions for frequent and intense thunderstorms, especially post-monsoon.
• Coastal and foothill regions: Interaction between land and sea, rapid heating, and orographic uplift strengthen thunderstorm activity.

Thunderstorm Strength in Various Regions of India:

Probe and Ponder

1. Why does it feel harder to pedal a bicycle when going uphill than on flat ground?
Ans: 

When cycling uphill, a bicycle has to move against gravity, which pulls both the cycle and the rider downward. More muscular force has to be applied to push the bicycle up the slope as compared to flat ground, where gravity does not resist the forward motion as much. That is why pedaling feels much harder going uphill.

2. Why is it easier to slip on a wet surface?
Ans: 
Wet surfaces have less friction compared to dry surfaces. Friction is the force that helps in gripping the ground when walking or moving. When a surface is wet, the thin layer of water reduces the irregularities between the surfaces of shoes and the floor, causing the friction to decrease. With less friction, it is easy to slip and fall.

3. Why do we feel ‘light’ or like we are ‘floating’ just after our swing reaches its highest point and begins to come down?
Ans: At the highest point of a swing’s motion, the swing (and the person on it) stops for a moment before starting to move back down. For that brief moment, the pull of gravity is almost balanced by the upward force of the swing, making a sensation similar to floating or feeling “light.” This happens because, at that instant, the only force acting is gravity and the person is in a state called “free fall.”

4. Share your questions
Ans:

• Why does sliding work better on ice than on sand?
• What would happen if friction did not exist at all?
• Why do heavier objects sink more in water than lighter ones of the same size?
• Does air also provide friction to moving objects?
• Why can some birds fly easily for long periods without getting tired?

Keep the curiosity alive

1. Match items in Column A with the items in Column B.

Ans: Column A (Type of force)Column B (Example)(i) Muscular force(b) A child lifting a school bag(ii) Magnetic force(e) A compass needle pointing North(iii) Frictional force(a) A cricket ball stopping on its own just before touching the boundary line(iv) Gravitational force(c) A fruit falling from a tree(v) Electrostatic force(d) Balloon rubbed on woollen cloth attracting hair strands

2. State whether the following statements are True or False.
(i) A force is always required to change the speed of motion of an object. 
Ans: True (Force changes speed, direction, or shape).
(ii) Due to friction, the speed of the ball rolling on a flat ground increases. 
Ans: False (Friction slows down or stops moving objects).
(iii) There is no force between two charged objects placed at a small distance apart. 
Ans: False (Electrostatic force acts between charged objects without contact).

3. Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?
Ans: 

• ​The two balloons will repel each other. 
• This happens because rubbing with woollen cloth gives both balloons the same type of static charge, and like charges push each other away due to electrostatic force.

4. When you drop a coin in a glass of water, it sinks, but when you place a bigger wooden block in water, it floats. Explain.
Ans: 

• The coin sinks because its density is higher than water, so the gravitational force pulling it down is stronger than the buoyant force from water pushing it up. 
• The wooden block floats because its density is lower than water, making the buoyant force equal to or greater than gravity, keeping it on the surface.

5. If a ball is thrown upwards, it slows down, stops momentarily, and then falls back to the ground. Name the forces acting on the ball and specify their directions.
Ans: 
(i) During its upward motion: Gravitational force pulls the ball downward, slowing it down, while the initial throwing force acts upward but weakens.
(ii) During its downward motion: Only gravitational force pulls the ball downward, making it speed up.
(iii) At its topmost position: Gravitational force pulls downward; the ball stops momentarily before falling, with zero speed at that instant.

6. A ball is released from the point P and moves along an inclined plane and then along a horizontal surface as shown in the Fig. 5.16. It comes to stop at the point A on the horizontal surface. Think of a way so that when the ball is released from the same point P, it stops 
(i) before the point A 
(ii) after crossing the point A.

Ans: 
(i) To stop before point A, make the surface rougher to increase frictional force, which slows the ball down faster.
(ii) To stop after crossing point A, make the surface smoother to reduce frictional force, allowing the ball to roll farther.

7. Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.
Ans: We slip on smooth surfaces because there is very little friction between our feet and the surface. Without enough friction to grip and oppose sliding, it’s easy to lose balance and fall.

8. Is any force being applied to an object in a non-uniform motion?
Ans: Yes, a force is applied to an object in non-uniform motion (when speed or direction changes). Force causes these changes, like speeding up, slowing down, or turning.

9. The weight of an object on the Moon becomes one-sixth of its weight on the Earth. What causes this change? Does the mass of the object also become one-sixth of its mass on the Earth?
Ans: The change happens because the Moon’s gravitational force is weaker than Earth’s (about one-sixth). Weight depends on gravity, so it’s less on the Moon. The mass stays the same everywhere, as it’s the amount of matter in the object.

10. Three objects 1, 2, and 3 of the same size and shape but made of different materials are placed in water. They dip to different depths as shown in Fig. 5.17. If the weights of the three objects 1, 2, and 3 are w1, w2, and w3, respectively, then choose the correct relation.
Fig.5.17
(i) w1 = w2 = w3 
(ii) w1 > w2 > w3 
(iii) w2 > w3 > w1 
(iv) w3 > w1 > w2
Ans: (ii) w1 > w2 > w3 
Heavier objects sink deeper because their weight overcomes buoyant force more, while lighter ones float higher.

Discover, design, and debate 

1. Collect objects made of different materials, such as plastic, wool, silk, rubber, polythene sheet, paper, and metals. Rub one material with another and check if it attracts small pieces of paper or not, that is, whether it gets charged or not. Record your observations in a systematic manner and write a research paper. 
Ans: 

Collecting and Rubbing Materials: Research on Electrostatic Charging

Experiment Steps:

• Gather materials: plastic scale, wool, silk cloth, rubber balloon, polythene sheet, paper, and metals like an iron key or a coin.
• Rub one material (for example, a plastic scale) with another material (like wool or silk).
• Bring the rubbed object close to small pieces of paper and observe if they are attracted.

Observation Table Example:Material RubbedMaterial Used for RubbingAttracts Paper? (Yes/No)Plastic scaleWoolen clothYesBalloonSilk clothYesPlastic scalePolytheneYesMetal coinWoolen clothNoRubberDry paperNo

Conclusion:
Objects made of plastic, wool, or silk become electrically charged when rubbed together and can attract small pieces of paper. Metals usually do not get charged in this way. This is because some materials transfer electrons when rubbed, creating static electric charges.

2. Imagine a scenario where the gravity disappears. Develop a story. Create a cartoon strip to present your story. z Organise a discussion in your class on the topic: Friction—a necessity or a problem? Make a note of the discussion and state where friction is a necessity and when it is a problem. 
Ans: 

Story:
One morning, everyone wakes up to find that gravity has vanished! The moment a ball is thrown, it rises up and never comes down. People start floating off their beds and must grab tightly to the doors and furniture. Water in glasses begins to float around in little blobs, and food hovers above plates. Birds try to fly but end up floating higher and higher. Cars and cycles no longer grip the road, and everyone must wear heavy boots to avoid floating away. Animals and plants on Earth start floating as well. Without gravity, life becomes very difficult, and people must come up with clever ways to stay on the ground or inside their houses.

Cartoon Strip:

• Panel 1: A boy throws a ball and it floats up, surprised expression!
• Panel 2: Breakfast cereal and milk floating in the air in a kitchen.
• Panel 3: The family tying themselves to the sofa to watch TV.
• Panel 4: A cat and dog floating among furniture, while the phone, keys, and homework papers hover around.

3. Make your own spring balance with the help of your teacher and calibrate it using standard weights. Now measure the weights of different objects and calculate the ratio of the weight and mass of different objects. Do you observe a pattern? 
Ans: 

Necessity:

• Friction helps us walk without slipping.
• Allows cars, bicycles, and buses to move and stop safely.
• Needed to write with a pencil or pen.
• Helps in holding and gripping objects.

Problem:

• Friction wastes energy as heat (machines get hot).
• Causes wear and tear of parts in machines, shoes, and tires.
• Makes it harder to move heavy objects.

Conclusion:
Friction is necessary in daily life for grip and safety, but too much friction can make tasks difficult and can damage objects or waste energy.

4. An electroscope is a device which can determine whether an object is electrically charged. You can make your own electroscope (Fig. 5.18) in your class with the help of your teacher, test the device. Explore in what other ways you may use this electroscope.
Ans: 

Steps:

• Create a spring balance with help from a teacher, calibrate it using known weights (like 100g, 200g).
• Hang objects (eraser, pencil box, water bottle, stone) and note their weights.
• Measure the mass of the same objects using a digital or beam balance.

Table:ObjectMeasured Mass (g)Measured Weight (N)Weight/Mass (N/g)Eraser500.50.01Pencil box1501.50.01Water bottle30030.01

Pattern:
The ratio of weight to mass is nearly the same for all objects (about 0.01N/g), showing that weight depends on mass and gravity.

5. An electroscope is a device which can determine whether an object is electrically charged. You can make your own electroscope (Fig. 5.18) in your class with the help of your teacher, test the device. Explore in what other ways you may use this electroscope.
Ans: 
​Steps to Make an Electroscope:

• Use a clean, dry jar with a plastic or cardboard lid.
• Insert a metal wire through the lid; attach thin strips of aluminium foil to the inside end of the wire.
• Charge a plastic scale by rubbing with wool, then bring it close to the exposed end of the wire.

Observation:
When a charged object is brought near the wire, the aluminium strips repel each other and move apart. This indicates the presence of an electric charge.

Other Uses:

• To check if an object is electrically charged or not.
• Can be used to show that charges can transfer from one object to another.
• Can help detect static electricity in different classroom experiments.

Probe and Ponder

1. If we don’t have an electric lamp while making an electric circuit with an electric cell, is there any other way to find out if current is flowing in the circuit?

• ​Yes, there are other ways to check if current is flowing. One way is to use a magnetic compass. 
• If you place a compass near the wire and close the circuit, the needle of the compass will deflect when the current flows through the wire. This shows that electricity is passing through the circuit. 
• Another way is to use a device like an electric bell or buzzer, which will make a sound if current is present.

2. Is it possible to make temporary magnets? How can these be made?

• ​Yes, temporary magnets can be made easily. Wrapping a long insulated wire around an iron nail and connecting both ends of the wire to a battery turns the nail into an electromagnet (a temporary magnet). 
• When electric current flows through the wire, the nail acts like a magnet and can attract magnetic materials. When the current is switched off, the nail loses its magnetism.

3. We can generate heat by burning fossil fuels and wood; but how is heat generated in various electrical appliances?

• ​In electrical appliances, heat is generated due to the heating effect of electric current. 
• When electric current passes through a wire or coil (often made of materials like nichrome), the wire offers resistance to the current. This resistance causes some electrical energy to change into heat energy, making the wire hot. 
• That’s why devices like heaters, irons, and toasters become warm when switched on.

4. How do we know if a cell or a battery is dead? Can all cells and batteries be recharged?

• ​A cell or battery is considered dead if it can no longer provide enough current to light a bulb, move a motor, or run any electrical device. Sometimes the device works weakly or not at all, which signals that the battery is dead. 
• Not all cells and batteries can be recharged. Dry cells, like those used in TV remotes, are single-use and cannot be recharged. 
• Rechargeable batteries, such as those in mobile phones and laptops, can be used again and again by recharging them with a charger.

5. Share your questions

• Why does reversing the battery terminals in a coil change the direction of the compass needle?
• What happens if we use longer wires to make an electromagnet?
• Which fruit or vegetable makes the strongest electric cell?
• How does a rechargeable battery work differently from a single-use battery?
• What materials are best for making heating elements in electric appliances?

Keep the curiosity alive

1. Fill in the blanks: 
(i) The solution used in a Voltaic cell is called ________. 
(ii) A current carrying coil behaves like a _______ . 
Ans: 
​(i) The solution used in a Voltaic cell is called electrolyte.
(ii) A current carrying coil behaves like a magnet.

2. Choose the correct option: 
(i) Dry cells are less portable compared to Voltaic cells. (True/False) 
(ii) A coil becomes an electromagnet only when electric current flows through it. (True/False) 
(iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells. (True/False)
Ans: (i) Dry cells are less portable compared to Voltaic cells. False 
Dry cells are more portable as they use a paste electrolyte and are compact.
(ii) A coil becomes an electromagnet only when electric current flows through it. True.
(iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells. False 
More cells provide stronger current, leading to a stronger magnetic field and more attraction.

3. An electric current flows through a nichrome wire for a short time. 
(i) The wire becomes warm. 
(ii) A magnetic compass placed below the wire is deflected. 
Choose the correct option:
 (a) Only (i) is correct 
(b) Only (ii) is correct 
(c) Both (i) and (ii) are correct 
(d) Both (i) and (ii) are not correct
Ans: Correct option: (c) Both (i) and (ii) are correct 
This demonstrates the heating and magnetic effects of electric current.

4. Match the items in Column A with those in Column B

Ans: Column AColumn B(i) Voltaic cell(d) Generates electricity by chemical reactions(ii) Electric iron(c) Works on heating effect of electric current(iii) Nichrome wire(a) Best suited for electric heater(iv) Electromagnet(b) Works on magnetic effect of electric current

5. Nichrome wire is commonly used in electrical heating devices because it 
(i) is a good conductor of electricity. 
(ii) generates more heat for a given current. 
(iii) is cheaper than copper. 
(iv) is an insulator of electricity.
Ans: (ii) generates more heat for a given current (It offers high resistance, converting more electrical energy to heat).

6. Electric heating devices (like an electric heater or a stove) are often considered more convenient than traditional heating methods (like burning firewood or charcoal). Give reason(s) to support this statement considering societal impact. 
Ans. Electric heating devices provide instant, controllable heat without smoke or ash, reducing indoor air pollution and health risks like respiratory issues. They eliminate the need for fuel collection, saving time and reducing deforestation, which benefits the environment and promotes sustainable living in communities.

7. Look at the Fig. 4.4a. If the compass placed near the coil deflects: (i) Draw an arrow on the diagram to show the path of the electric current. (ii) Explain why the compass needle moves when current flows. (iii) Predict what would happen to the deflection if you reverse the battery terminals.
Ans. (i) The path of the electric current would flow from the positive terminal of the cell, through the coil from end A to end B, and back to the negative terminal.
(ii) The compass needle moves because the current flowing through the coil creates a magnetic field, which interacts with the needle’s own magnetism, causing deflection.
(iii) Reversing the battery terminals would reverse the current direction, flipping the electromagnet’s poles and causing the compass needle to deflect in the opposite direction.

8. Suppose Sumana forgets to move the switch of her lifting electromagnet model to OFF position (in introduction story). After some time, the iron nail no longer picks up the iron paper clips, but the wire wrapped around the iron nail is still warm. Why did the lifting electromagnet stop lifting the clips? Give possible reasons.
The electromagnet stops lifting clips because the battery likely depletes over time, stopping the current flow and eliminating the magnetic field. The wire remains warm due to residual heat from earlier current or possible ongoing low-level resistance heating if there’s a partial connection.

9. In Fig. 4.11, in which case the LED will glow when the switch is closed?

Ans: The LED will glow in (a) with lemon juice, as it acts as an electrolyte enabling chemical reactions to generate current between the iron nail and copper strip. It will not glow in (b) with pure water, which is a poor conductor and does not facilitate the necessary reactions.

10. Neha keeps the coil exactly the same as in Activity 4.4 but slides the iron nail out, leaving only the coiled wire. Will the coil still deflect the compass? If yes, will the deflection be more or less than before?
Ans: Yes, the coil will still deflect the compass as current through it produces a magnetic field. However, the deflection will be less than before, since the iron nail acts as a core that strengthens the magnetic effect.

11. We have four coils, of similar shape and size, made up from iron, copper, aluminium, and nichrome as shown in Fig. 4.12. When current is passed through the coils, compass needles placed near the coils will show deflection.

When current is passed through the coils, compass needles placed near the coils will show deflection. 
(i) Only in circuit (a) 
(ii) Only in circuits (a) and (b) 
(iii) Only in circuits (a), (b), and (c) 
(iv) In all four circuits
Ans: 
Option (iv) In all four circuits (All materials conduct current, producing a magnetic field that deflects the compass, though nichrome’s higher resistance may result in weaker current and deflection).

Discover, design, and debate

Q1. Make coils of turns 25, 50, 75, and 100. Connect them to the same cell one by one. Note the deflection in a magnetic compass placed in the same position in all the cases. Report your observations. Draw conclusion of the effect of number of turns of the coil on the strength of the electromagnet. 
Ans: 
Activity and Observations:
Coils are made with 25, 50, 75, and 100 turns and each is connected to the same electric cell. A magnetic compass is placed near the end of each coil, and the deflection of the compass needle is observed.Number of TurnsDeflection of Compass Needle25Small deflection observed50More deflection than 25 turns75Even greater deflection100Maximum deflection

Conclusion:
As the number of turns in the coil increases, the strength of the electromagnet also increases. This is seen because the compass needle deflects more each time the number of turns increases. This happens because more turns mean a stronger magnetic field is produced in the coil when current flows.

Q2. Take two thin nichrome wires of equal length and different thickness (approximately one of these wire thickness to be double of the other, say 0.3 mm and 0.6 mm). Connect them one by one in a circuit which has a switch and a cell, and allow the current to flow for 30 s in each case. Momentarily touch these wires. Which wire heats up more? Now repeat the same activity with two nichrome wires of same diameter but of different lengths. Prepare a brief report of your activity. 
Ans: 
​Part 1: Same Length, Different Thickness
Two nichrome wires of the same length (example: 10cm) but different thickness (0.3mm and 0.6mm) are used one by one in a circuit, and current flows for 30 seconds.Wire ThicknessObservation after 30s0.3 mm (thin)Feels hotter to touch0.6 mm (thick)Warms up less, not as hot as 0.3mm

Result:
The thinner nichrome wire (0.3mm) becomes hotter than the thicker wire (0.6mm), because thinner wires have higher resistance and produce more heat when current flows.

Part 2: Same Thickness, Different Lengths
Two wires of same thickness but different lengths (10cm and 20cm) are tested the same way.Wire LengthObservation after 30s10 cm (short)Warms up20 cm (long)Warms up, sometimes more than short

Result:
The longer wire can become warmer because it has more resistance than the shorter wire.

Brief Report:
Nichrome wires with more resistance (thinner or longer) heat up more when electric current flows through them. That is why thin and long wires are used as heating elements in appliances.

Q3. Try to make an electric cell using various fruits and vegetables. Also try with electrodes of different metals. Prepare a brief report.
Ans: Activity:
Fruits like lemon, potato, and tomato are tried as electric cells by inserting two different metal strips (such as copper and zinc, or copper and iron) into them. Several fruits or vegetables may be connected in series to increase the power, and an LED or small bulb is connected to check if electricity is produced.

Observations:

• When copper and zinc metals are used in a lemon, the LED glows dimly.
• More lemons connected together make the LED glow brighter.
• With other fruits and vegetables such as potatoes and tomatoes, electricity is still produced, but lemons usually work the best because of their sour and acidic juice.
• If two metals that are far apart in reactivity (like zinc and copper) are used, the electricity produced is more.
• If plain water is tried instead of fruit, almost no electricity is produced.

Brief Report:
Fruits like lemon or tomato, when combined with different metals, can act as simple electric cells and produce a small amount of electricity. The acidic juice in fruits helps in carrying the current. The choice of metals is important—combinations like zinc and copper are the best for making a fruit cell. This type of cell can glow a small bulb or LED if several are connected together, but they do not provide as much electricity as common batteries.