NCERT Solutions for Class 7 science Chapter 7: Heat Transfer in Nature Class 7

How does heat get transferred in these materials? (cooking utensils made of metals) (Page 90)
Answer:
The heat is transferred in these materials through a process called conduction. During conduction, the particle that is heated passes the heat to its adjacent particles one after another. The particles remain fixed in their positions.

Identify whether the given materials are good or poor conductors of heat. (Page 91)

Why is the smoke going up? (Page 92)
Answer:
The smoke is going up because hot air rises. When smoke is produced, it is usually warm or hot. Due to convection, the hot air (and smoke with it) becomes lighter than the surrounding cooler air and rises upwards.

How does heat transfer take place in liquids? Do liquids also rise up when heated, like air? (Page 93)
Answer:
When water is heated in a container, the water at the bottom becomes hot, expands and becomes lighter, causing it to rise. As the warmer water rises, cooler and heavier water moves down to take its place. This process is called convection, and it continues till the entire liquid is heated. Thus, liquids rise when heated, just like air.

I wonder how heat from the fire reaches us? (Page 96)
Answer:
The heat from the fire reaches us through the process of radiation. In this process, heat is transferred by radiating energy to the surroundings. Heat transfer by radiation does not require any medium.

How does water seep through the surface of the Earth? (Page 99)
Answer:
Water seeps through the surface of the Earth by a process called infiltration. From the Earth’s surface the water seeps through the soil containing sand, gravel or clay. Once it seeps, it gets stored in the pore spaces of sediments and the opening in rocks known as aquifers.

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

Fig. Saucepan

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, the arrangement will keep the lassi cold for a longer time. The air trapped between the two tumblers acts as an insulator since air is a poor conductor of heat. It reduces the transfer of heat from the surroundings to the lassi, helping it stay cool for a longer period.

State with reason(s) whether the following statements are True [T] or False [F],
(i) Heat transfer takes place in solids through convection.
Answer:
False, heat transfer takes place in solids through conduction.

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 absorb heat from their surroundings, including the air and the surface they are placed on. This heat causes the ice to melt, changing it from a solid state to a liquid state, turning it into water.

A burning incense stick is fixed, pointing downwards. In which direction would the smoke from the incense stick move? Show the movement of smoke with a diagram.
Answer:
The smoke from the incense stick will move in the upward direction because smoke is a mixture of hot gases and tiny solid particles. Since hot air rises due to convection, the smoke moves upwards.

Two test tubes with water are heated by a candle flame as shown in Fig. 7.16. Which thermometers (Fig. 7.16 a or Fig. 7.16 b) will record a higher temperature? Explain.

Fig. Two thermometers dipped in two test tubes
Answer:
In Fig. (a), the thermometer is placed at the top of the test tube and candle flame heats the bottom of the test tube directly. The water at the bottom gets heated first. As the water heats up, it rises due to convection, carrying heat to the top, where the thermometer is placed. So, the thermometer quickly records a temperature rise.

Why are hollow bricks used to construct the outer walls of houses in hot regions?
Answer:
The air gets trapped in the hollow bricks and air is a poor conductor of heat. This trapped air acts as an insulator and prevent the heat from entering the house, keeping the interior cooler.

Explain how large water bodies prevent extreme temperature in areas around them.
Answer:
Large water bodies like seas and oceans prevent extreme conditions in areas around them. In coastal areas during day, the land heats up faster than water, so cooler air from the sea flows towards the land (sea breeze), keeping the temperature moderate. At night, land cools down faster than water. The air above the sea remains warmer, while the cooler air from the land moves towards the sea (land breeze). This movement of air helps prevent extreme temperatures in coastal regions.

Fig. (a) Sea breeze (b) Land Breeza

Explain how water seeps through the surface of the Earth and gets stored as groundwater.
Answer:
Water seeps through the surface of the Earth by gradually passing through the soil and porous rocks. This process is known as infiltration. This water fills the pores and cracks in rocks and soil and gets stored underground as groundwater. The layers of sediments and rocks that store water in their pore spaces are known as aquifers.

The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.
Answer:
The water cycle helps in the continuous redistribution and replenishment of water on Earth through the movement of water in different stages, such as evaporation, condensation and precipitation. Water from oceans, rivers and lakes evaporates due to the Sun’s heat, forms clouds through condensation, and falls back to Earth as rain or snow through a process called precipitation. The rain refills lakes, rivers and groundwater, ensuring the continuous availability of water on Earth.