Page 39 - Ripples SCIENCE 7 - TEJPUR Edition 2024 Answer Key

6. How can we change the state of matter?

Ans. To change the state of matter, we can heat (melting or evaporation/boiling) & cool
(condensation or freezing) the matter. Application of pressure also changes the state

of matter.
7. Name the natural source from where we get heat energy.
Ans. The natural source from which we get heat energy is the Sun.

F. Short-answer questions
1. Define the following.

(a) Temperature
Ans. Temperature: It gives us an exact measure of the degree of hotness or coldness of a
body.

(b) Conduction
Ans. Conduction : It is a mode of heat transfer between two solids or part of the same

solid at different temperatures, when the solids or parts are in direct physical contact
with each other.
(c) Convection

Ans. Convection: It is the mode of heat transfer in liquids and gases where the molecules
of liquids and gases themselves carry the heat energy from a hotter area to a cooler

area.
(d) Radiation

Ans. Radiation: It is a mode of heat transfer that does not require a medium.
2. If a pan is removed from the burning gas stove, then why does it cool down after
some time?

Ans. If a pan is removed from the burning gas stove, it cools down after some time because
the gas stove transfers its heat to the surroundings through radiation. In this process,

the pan gradually cools down.
3. Explain the use of kink in a clinical thermometer.

Ans. The kink or constriction in a clinical thermometer prevents the immediate backflow
of the mercury in the bulb of the thermometer.
4. How does heat travel in the air?

Ans. Heat travels in air mainly by convection, where warm air rises and cool air sinks,
creating a cycle that transfers heat. It also travels in air by radiation, like sunlight

warming the Earth.
5. Give an example of the transfer of heat from one body to another.
Ans. An example of heat transfer from one body to another is :

When we touch a cold drink bottle with our warm hands, heat transfers from our
hand to the bottle, making it warmer while our hand feels cooler.

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