In-text Questions
Question: Why are cooking utensils generally made of metals?
Answer: Cooking utensils are usually made of metals because metals are very good at letting heat pass through them. Materials that let heat pass easily are called good conductors. This helps the heat from the stove reach the food quickly and cook it properly.
Question: How does heat get transferred in these materials?
Answer: Heat can move from one place to another in three different ways:
- Conduction: This is how heat travels through solids, like a metal pan.
- Convection: This is how heat travels through liquids and gases, like boiling water or warm air.
- Radiation: This is how heat travels through empty space, like the heat we feel from the Sun.
Question: Why does pin I fall before pin II? Why did all the pins not fall together?
Answer: The heat from the candle flame first reaches the part of the metal strip closest to it. This heat then travels along the strip. Since Pin I is closest to the flame, the wax holding it melts first, and it falls. Then the heat reaches Pin II, and its wax melts. The pins don’t fall all at once because it takes time for the heat to travel from the hot end of the strip to the colder end.
Question: From your observations, what can you infer? Do you think that heat is being transferred along the metal strip from the end that is being heated?
Answer: From this activity, we can understand that heat moves from a hotter place to a colder place. Yes, the heat is definitely being transferred along the metal strip from the hot end near the candle to the other end.
Question: If we use a strip made of a material like wood or glass in place of a metal strip to perform Activity 7.1, the pins will not fall. Can you think of the reason for this?
Answer: The pins will not fall if we use a wood or glass strip because these materials are poor conductors of heat (also called insulators). They do not let heat pass through them easily. So, the heat from the candle flame would not be able to travel down the strip to melt the wax.
Question: Does your list include air? If it is there on the list, where have you placed it?
Answer: Yes, air can be on the list. Air is a poor conductor of heat.
Question: Is it possible to construct houses that are not affected much by the outside heat and cold?
Answer: Yes, it is possible! We can build houses using materials that are poor conductors of heat. For example, some houses are built with hollow bricks. The air trapped inside these bricks is a poor conductor and helps to keep the house cool in the summer and warm in the winter.
Question: Why is the smoke going up?
Answer: The air near a fire gets very hot. Hot air is lighter than cool air, so it rises up. Smoke is made of tiny particles mixed with this hot air, so the smoke is carried upwards along with the rising hot air.
Question: How does heat transfer take place in liquids? Do liquids also rise up when heated like air?
Answer: Heat moves through liquids by a process called convection. And yes, just like air, when a part of a liquid is heated, it becomes lighter and rises up.
Question: Why does the streak of coloured water go up in the middle and come down from the sides?
Answer: The water at the bottom of the beaker, right above the candle, gets hot first. This hot water becomes lighter and rises up through the middle. The cooler, heavier water from the sides then sinks down to take its place at the bottom. This water then gets heated and rises too. This creates a circular movement called a convection current.
Question: I wonder how heat from the fire reaches us?
Answer: The heat from a fire reaches us through radiation. Heat travels as invisible waves, similar to how light travels. This is why you can feel the warmth of a fire even if you are not touching it or directly above it.
Question: Why is it more comfortable to wear white or light-coloured clothes during summers and dark-coloured clothes during winters?
Answer:
- In summer, light-coloured clothes are better because they reflect most of the sunlight and heat that falls on them. This helps to keep our bodies cool.
- In winter, dark-coloured clothes are better because they absorb more heat from the sun. This helps to keep our bodies warm.
Question: How does water seep through the surface of the Earth?
Answer: Water seeps through the ground by passing through tiny spaces between soil particles and cracks in rocks. This process is called infiltration.
Question: Why is it so? (Why does water seep fastest through gravel?)
Answer: Water seeps fastest through gravel because the spaces between gravel particles are very large. Sand particles have smaller spaces, and clay particles have the tiniest spaces. Water can flow much more easily through larger spaces.
Activity Questions
Activity 7.1: Conduction of Heat
Question: What will happen to the pins? Will they remain attached to the strip or will they fall?
Answer: The pins will fall from the strip one by one.
Question: Predict the order in which the pins will fall from the strip. Record your observations in Table 7.1.
Answer:
| Pin falling first | Pin I |
| Prediction | The pins will fall in order, starting with the one closest to the flame: I, then II, then III, and finally IV. |
| Observation | The pins fell in the order I, II, III, and IV. |
| Reasons | Heat travels from the hot end to the cold end by conduction. As the heat reaches each pin, the wax melts, and the pin falls. |
Activity 7.2: Convection in Air
Question: Observe what happens to the cup. Record your observations in Table 7.3 and think of probable reasons.
Answer:
| Observation about the cups | Probable reasons for the observation |
|---|---|
| The paper cup placed above the burning candle rises up. | The candle heats the air below the cup. Hot air is lighter than cool air, so it rises and pushes the cup upwards. |
Activity 7.4: Heating of Land and Water
Question: Did the temperature rise by the same amount for both the soil and the water at the same time?
Answer: No, their temperatures did not rise by the same amount.
Question: If not, which one got heated faster?
Answer: The soil got heated faster than the water.
Question: How much was the rise in temperature of the soil and the water in 20 minutes?
Answer: In 20 minutes, the soil’s temperature rose much more than the water’s temperature. For example, the soil might get hotter by 10°C, while the water only gets hotter by 4°C.
Question: Does the soil also cool faster than water?
Answer: Yes, things that heat up quickly also tend to cool down quickly. The soil cools down faster than the water.
Activity 7.5: Seepage of Water
Question: Predict the amount of water flowing out of each bottle. / Compare the amount of water that comes through each bottle.
Answer:
| Bottles filled with | Prediction | Observation |
|---|---|---|
| Bottle 1 (Clay) | Seepage will be very slow. | Very little water came out. Seepage is very slow. |
| Bottle 2 (Sand) | Seepage will be faster than clay. | A good amount of water came out. Seepage is slow, but much faster than clay. |
| Bottle 3 (Gravel) | Seepage will be very fast. | Almost all the water came out very quickly. Seepage is fast. |
Question: Do your findings match with your predictions?
Answer: Yes, the findings match the predictions. Water flows fastest through gravel and slowest through clay because of the size of the spaces between the particles.
Exercise Questions
1. Choose the correct option in each case.
(i) Your father bought a saucepan made of two different materials, A and B, as shown in Fig. 7.14. The materials A and B have the following properties-
Answer: (c) A is a good conductor and B is a poor conductor of heat.
- Explanation: Part A is the pan itself, which needs to get hot to cook food, so it must be a good conductor. Part B is the handle, which needs to stay cool so you can hold it, so it must be a poor conductor (insulator).
(ii) Pins are stuck to a metal strip with wax and a burning candle is kept below the rod, as shown in Fig. 7.15. Which of the following will happen?
Answer: (b) Pins I and II will fall earlier than pins III and IV.
- Explanation: The candle is closest to pins I and II. The heat will reach them first, melt their wax, and cause them to fall before the heat travels further down the rod to pins III and IV.
(iii) A smoke detector is a device that detects smoke and sounds an alarm. Suppose you are fitting a smoke detector in your room. The most suitable place for this device will be:
Answer: (c) On the ceiling.
- Explanation: Smoke is hot, and hot air always rises. Therefore, smoke will reach the ceiling of a room first, which is the best place to detect it quickly.
2. A shopkeeper serves you cold lassi in a tumbler. By chance, the tumbler had a small leak. You were given another tumbler by the shopkeeper to put the leaky tumbler in it. Will this arrangement help to keep the lassi cold for a longer time? Explain.
Answer: Yes, this arrangement will help keep the lassi cold for a longer time. When you put one tumbler inside another, a layer of air gets trapped between them. Air is a poor conductor of heat (an insulator). This trapped air slows down the transfer of heat from the warm outside air to the cold lassi, helping it stay cold for longer.
3. State with reason(s) whether the following statements are True [T] or False [F].
- (i) Heat transfer takes place in solids through convection. [ F ]
- Reason: Heat transfer in solids takes place mainly through conduction. Convection is the process of heat transfer in liquids and gases.
- (ii) Heat transfer through convection takes place by the actual movement of particles. [ T ]
- Reason: In convection, warmer, lighter particles move upwards, and cooler, denser particles move downwards, carrying heat with them.
- (iii) Areas with clay materials allow more seepage of water than those with sandy materials. [ F ]
- Reason: Sandy materials have larger spaces between particles, which allows water to seep through much faster than through clay, which has very small spaces.
- (iv) The movement of cooler air from land to sea is called land breeze. [ T ]
- Reason: At night, land cools faster than the sea. The cooler, heavier air from the land moves towards the warmer sea. This is called a land breeze.
4. Some ice cubes placed in a dish melt into water after sometime. Where do the ice cubes get heat for this transformation?
Answer: The ice cubes get heat from their surroundings, such as the air in the room and the dish they are in. Heat naturally moves from warmer places (the air) to colder places (the ice), causing the ice to melt.
5. A burning incense stick is fixed, pointing downwards. In which direction would the smoke from the incense stick move?
Answer: The smoke from the incense stick would still move upwards. Even though the stick is pointing down, the smoke is hot. Since hot air and smoke are lighter than the surrounding cool air, they will always rise.
6. Two test tubes with water are heated by a candle flame as shown in Fig. 7.16. Which thermometers (Fig. 7.16a or Fig. 7.16b) will record a higher temperature? Explain.
Answer: The thermometer in Figure 7.16a will show a higher temperature rise for the whole test tube of water.
- Explanation: When you heat water from the bottom (Fig. 7.16a), the hot water rises and the cool water sinks, creating a convection current that heats all the water in the tube efficiently. When you heat from the top (Fig. 7.16b), the hot water stays at the top because it is lighter. No convection current is formed, and the rest of the water stays cool. Therefore, the setup in Fig. 7.16a is much better at heating the entire volume of water.
7. Why are hollow bricks used to construct the outer walls of houses in hot regions?
Answer: Hollow bricks are used because they have air trapped in their hollow spaces. Air is a poor conductor of heat (an insulator). This trapped air acts as a barrier, preventing the heat from the hot sun outside from entering the house and helping to keep the inside cool.
8. Explain how large water bodies prevent extreme temperature in areas around them.
Answer: Water heats up and cools down much more slowly than land.
- During the day, the land gets hot quickly, but the sea stays cool. A cool breeze blows from the sea to the land (sea breeze), which keeps the coastal area from getting too hot.
- At night, the land cools down quickly, but the sea stays warm. The air over the sea is warmer, which prevents the coastal area from getting too cold. This is why places near the sea have a more moderate climate (not too hot, not too cold).
9. Explain how water seeps through the surface of the Earth and gets stored as groundwater.
Answer: When it rains, some of the water soaks into the ground. It travels downwards through the spaces in the soil and cracks in the rocks. This process is called infiltration. The water continues to seep down until it reaches a layer of solid rock that it cannot pass through. The water then fills up all the empty spaces in the ground above this rock layer, like a giant underground sponge. This water stored underground is called groundwater.
10. The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.
Answer: This statement is true. The water cycle moves water all around the planet.
- Redistribution: The sun’s heat evaporates water from oceans. Winds carry the clouds formed from this water vapour over land, sometimes hundreds of miles away. The rain that falls from these clouds distributes water to places far from the sea.
- Replenishment: The rain and melting snow fill up our rivers, lakes, and seep into the ground to refill the groundwater. This process replaces the water that we use, that flows to the sea, or evaporates.
Exploratory Projects
Activity: Tightly wrap a thin paper strip around a metallic rod. Try to burn the paper with a candle while rotating the rod continuously. Does the paper burn? Explain your observations.
- Observation: The paper will not burn easily.
- Explanation: Metal is a very good conductor of heat. As you heat the paper with the candle, the metal rod quickly pulls the heat away from the paper and spreads it along the rod. Because the heat is carried away so fast, the paper never gets hot enough to catch fire.
Activity: Take a sheet of paper. Draw a spiral on it…Cut the paper along the spiral. Suspend the paper…above a burning candle. Observe what happens. Provide an explanation for your observation.
- Observation: The paper spiral will start to spin or rotate.
- Explanation: The flame of the candle heats the air directly above it. This hot air becomes lighter and rises. As the stream of hot air rises, it pushes against the surfaces of the paper spiral, causing it to turn. This is a perfect example of a convection current in action.