NCERT Solutions for Class 7 Science Chapter 3 with Extending and learning, according to the New syllabus of NCERT Books.
Exercise NCERT Solutions
1. State similarities and differences between the laboratory thermometer and the clinical thermometer.
Similarities:
- Both are used to measure temperature.
- Both contain mercury which expands and contracts to show temperature.
Differences:
- Laboratory thermometers can measure a wider range of temperatures, typically from -10°C to 110°C, while clinical thermometers measure a narrow range around human body temperature, from 35°C to 42°C.
- Clinical thermometers have a constriction in the tube that prevents mercury from falling back quickly, allowing the temperature to be read after removing the thermometer from the body.
2. Give two examples each of conductors and insulators of heat.
Conductors of Heat:
- Copper
- Aluminum
Insulators of Heat:
- Wood
- Plastic
3. Fill in the blanks:
- (a) The hotness of an object is determined by its temperature.
- (b) Temperature of boiling water cannot be measured by a clinical thermometer.
- (c) Temperature is measured in degree Celsius.
- (d) No medium is required for transfer of heat by the process of radiation.
- (e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of conduction.
- (f) Clothes of dark colours absorb heat better than clothes of light colours.
4. Match the following:
- (i) Land breeze blows during – (d) Night
- (ii) Sea breeze blows during – (c) Day
- (iii) Dark coloured clothes are preferred during – (b) Winter
- (iv) Light coloured clothes are preferred during – (a) Summer
5. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
Wearing more layers of clothing during winter keeps us warmer because air gets trapped between the layers, which acts as an insulator and prevents the loss of body heat, thus keeping us warm.
6. Look at Fig. 3.13. Mark where the heat is being transferred by conduction, by convection and by radiation.
Conduction: The heat transfer from the burner to the metal pot. Convection: The heat transfer within the boiling water in the pot. Radiation: The heat transfer from the metal pot to the surrounding .
7. In places of hot climate it is advised that the outer walls of houses be painted white. Explain.
In hot climates, white paint is recommended for outer walls because white color reflects most of the sunlight, reducing the amount of heat absorbed by the walls, thus keeping the house cooler.
8. One litre of water at 30°C is mixed with one litre of water at 50°C. The temperature of the mixture will be:
- (a) 80°C
- (b) more than 50°C but less than 80°C
- (c) 20°C
- (d) between 30°C and 50°C ✔️
9. An iron ball at 40°C is dropped in a mug containing water at 40°C. The heat will:
- (a) flow from iron ball to water.
- (b) not flow from iron ball to water or from water to iron ball. ✔️
- (c) flow from water to iron ball.
- (d) increase the temperature of both.
10. A wooden spoon is dipped in a cup of ice cream. Its other end:
- (a) becomes cold by the process of conduction.
- (b) becomes cold by the process of convection.
- (c) becomes cold by the process of radiation.
- (d) does not become cold. ✔️
11. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that:
- (a) copper bottom makes the pan more durable.
- (b) such pans appear colourful.
- (c) copper is a better conductor of heat than the stainless steel. ✔️
- (d) copper is easier to clean than the stainless steel.
NCERT Solutions for Extending Learning – Activities and Projects
1. Visit to a Doctor
(a) Why she dips the thermometer in a liquid before use.
- The thermometer is dipped in a disinfectant solution before use to ensure it is clean and free from any bacteria or viruses. This prevents the transmission of infections from one patient to another.
(b) Why the thermometer is kept under the tongue.
- The thermometer is kept under the tongue because this area is close to the blood vessels, providing an accurate measurement of the body’s core temperature. The mouth is a convenient and consistent site for measuring body temperature.
(c) Whether the body temperature can be measured by keeping the thermometer at some place other than the mouth.
- Yes, body temperature can also be measured at other sites such as the armpit (axillary temperature), rectum (rectal temperature), and ear (tympanic temperature). However, these sites may have different normal temperature ranges and may require specific types of thermometers.
(d) Whether the temperature of different parts of the body is the same or different.
- The temperature of different parts of the body can vary. For example, rectal temperature is usually higher than oral temperature, and axillary temperature is usually lower than oral temperature. These differences are due to the varying degrees of blood flow and exposure to the environment at different sites.
2. Visit to a Veterinary Doctor
Discuss and find out the normal temperature of domestic animals and birds.
- The normal body temperature of domestic animals and birds can vary:
- Dogs: 37.5°C to 39.2°C
- Cats: 38.1°C to 39.2°C
- Cows: 38.5°C to 39.5°C
- Horses: 37.5°C to 38.5°C
- Chickens: 40.6°C to 43°C
- Pigeons: 41°C to 42.6°C
- These temperatures are higher than human body temperature and are adapted to their metabolic rates and living environments.
3. Paper Burning Experiment
Wrap a thin strip tightly around an iron rod. Try to burn the paper with a candle while wrapping it around the rod.
- Observation: When you try to burn the paper tightly wrapped around an iron rod, it does not burn easily.
- Explanation: This happens because the iron rod conducts heat away from the paper quickly. The heat from the candle flame is transferred to the iron rod through conduction, preventing the paper from reaching its ignition temperature. This demonstrates the concept of heat conduction and the effectiveness of metal as a conductor of heat.
