NCERT Solutions for Class 7 Science - Heat

Exercises

Q1. State similarities and differences between the laboratory thermometer and the clinical thermometer.
Ans:
Similarities:

• Both thermometers are used to measure temperature.
• Both laboratory and clinical thermometers have long, narrow, uniform glass tubes.
• They both contain mercury that expands or contracts with temperature changes.
• Both have a scale to read the temperature and both are graduated generally on the Celsius scale.

Clinical Thermometer

Laboratory Thermometers

Differences in Thermometers: 

Q2. Give two examples each of conductors and insulators of heat.
Ans: 
Conductors:

1. Copper
2. Aluminium

Insulators:

1. Rubber
2. Glass

Thermal Conductors & Insulators

Q3. Fill in the blanks:

(a) The hotness of an object is determined by its ________.

Ans: Temperature.

View Answer  

• Temperature is a measure of how hot or cold an object is.
• Higher temperatures indicate that the particles are moving faster, which corresponds to greater thermal energy.

(b) Temperature of boiling water cannot be measured by a ________ thermometer.
Ans: Clinical thermometer

View Answer  

• Clinical thermometers are designed to measure body temperature and therefore have a limited range (about 35°C to 42°C).
• The boiling point of water is 100°C at standard atmospheric pressure, which is well above the range of a clinical thermometer.
• Therefore, a laboratory or an alcohol thermometer with a higher range is used to measure the temperature of boiling water.

(c) Temperature is measured in degree ________.
Ans: Celsius

View Answer  

• The Celsius scale (°C) is widely used, where 0°C is the freezing point of water and 100°C is the boiling point at standard atmospheric pressure.
• It is commonly used in science and everyday life for temperature measurement.

(d) No medium is required for transfer of heat by the process of __________.
Ans: Radiation

View Answer  

• Radiation transfers heat in the form of electromagnetic waves and does not need a material medium.
• This is how the Sun heats the Earth through the vacuum of space.

Heat Transfer

(e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of ________.
Ans: Conduction.

View Answer  

• Conduction is heat transfer through direct contact; particles transfer energy to neighbouring particles without bulk movement.
• In this case, heat flows from the hot milk into the spoon, warming the metal and allowing heat to travel along the spoon to its other end.

(f) Clothes of ________colours absorb heat better than clothes of light colours.
Ans: Dark

View Answer  

• Dark colours absorb more wavelengths of light and convert them into heat, while light colours reflect more sunlight.
• This is why wearing dark clothes in the sun makes you feel warmer compared with light-coloured clothes.

Dark colour clothes absorb Heat

Q4. Match the following:

Ans: 

View Answer  

• Land breeze blows during (d) night.
  Explanation: At night the land cools faster than the sea. The cooler, denser air over the land moves towards the warmer sea, producing a breeze from the land to the sea.

• Sea breeze blows during (c) day.
  Explanation: During the day the land heats up faster than the sea. Warm air over the land rises and cooler air from the sea moves in to replace it, causing a sea breeze.

• Dark coloured clothes are preferred during (b) winter.
  Explanation: Dark clothes absorb more heat from sunlight and help retain warmth, so they are useful in cold weather.

• Light coloured clothes are preferred during (a) summer.
  Explanation: Light colours reflect most of the sunlight and thus help keep the wearer cooler in hot weather.

Q5. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing?
Ans: 

• Multiple layers trap several thin pockets of air between them.
• Air is a poor conductor of heat, so these trapped pockets reduce heat loss from the body.
• One thick piece of clothing traps less air overall and therefore is less effective at preventing heat escape than several layers.

Q6. Look at Fig. (in NCERT Text Book). Mark where the heat is being transferred by conduction, by convection and by radiation.

Ans:

Q7. In places of hot climate it is advised that the outer walls of houses be painted white. Explain.
Ans: 

• White colour reflects most of the sunlight and therefore reflects most of the heat falling on it.
• This reflection reduces the amount of heat absorbed by the walls and helps keep the house interior cooler.
• Hence, painting outer walls white is a simple way to lower indoor temperature in hot climates.

Q8. 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
Ans: (d)

View Answer  Explanation:

• When equal masses of water at different temperatures are mixed, heat lost by the hotter water is gained by the colder water.
• Since energy is exchanged until both reach the same temperature, the final temperature must lie between 30°C and 50°C.

Q9. 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.
Ans: (b)

View Answer  Explanation:

• Heat transfer occurs only when there is a temperature difference between two bodies.
• Here both the iron ball and the water are at the same temperature (40°C), so there is no net flow of heat between them.

Q10. 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.
Ans: (d)

View Answer  Explanation:

• Wood is a poor conductor of heat (an insulator).
• Because it does not conduct heat well, the cold from the dipped end does not travel quickly to the other end.
• Therefore, the far end of the wooden spoon remains near the original temperature and does not become cold quickly.

Q11. 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 stainless steel.
(d) copper is easier to clean than stainless steel.
Ans: (c)

View Answer  Explanation:

• Copper is an excellent conductor of heat and spreads heat evenly across the base of the pan.
• Stainless steel alone does not conduct heat as well; combining a copper base with stainless steel gives good heat distribution plus strength and corrosion resistance.
• Thus, a copper bottom improves cooking efficiency while the stainless steel body provides durability.