Class 7 Science Chapter 3 HOTS Questions - Heat

Q1: Two objects at the same temperature, one smaller than the other, are placed together. In which direction will the heat be transferred? Give reason.

Ans. The direction of heat transfer would be from larger to smaller objects because the larger object has more surface area, due to which it loses its heat much faster

Q2: In a mercury thermometer, the level of mercury rises when its bulb comes in contact with a hot object. What is the reason for this rise in the level of mercury?
Ans: As the temperature increases, an expansion in mercury takes place, which leads to a rise in the level of mercury in a thermometer.

Q3: The radiators in cars are painted black. Explain why.
Ans: Since black bodies are better radiators, radiators in cars are painted black.

Q4:  Why are the pipes of the solar heater and the containers of the solar cooker painted black?

Ans. The pipes of the solar heater and the containers of the solar cooker are painted black because black colour absorbs heat more than any other colour, and this will help to keep the pipes and containers warmer

Q5: At a campsite, there are tents of two shades. One is made with black fabric and the other with white fabric. Which one will you prefer for resting on a hot summer afternoon? Give a reason for your choice. Would you like to prefer the same tent during winter?
Ans: We would prefer a white fabric tent in case of summer because it reflects all the radiation from the sun and keeps us cool inside the tent.
But in the case of winter, we should not use a white fabric tent. We should use a black fabric tent as it absorbs all colours of light from the sun and keeps us warm inside the tent.

Q6: Explain the reason for the following statement, “When heat is applied at the bottom of the water vessel, then it gets heated more quickly than when it is heated at the top.”
Ans: The particles of water become lighter and rise in the middle. Due to this, the cold particles of water, which are heavier, come down along the sides of the vessel. So, the convection currents of heat are set and water becomes hot. The moment when the heat is applied at the top, then the water present in the upper portion becomes hot, and these hot water molecules get fixed over the surface. Since water is a bad conductor of heat, due to water present at the lower end (or bottom) of the vessel remains.

Q7: The freezer is located at the top of the refrigerator. Explain why.

Ans: The freezer is located at the top of the refrigerator because cold air is denser and sinks, allowing the cooler air from the freezer to circulate down and help cool the refrigerator section below, making the cooling process more efficient.

Q8: To keep her soup warm, Paheli wrapped the container in which it was kept with woollen clothes. Can she apply the same method to keep a glass of cold drink cool? Give a reason for your Answer.
Ans: Yes, she can apply the same method to keep a glass of cold drink cool because wool is a thermal insulator, and it cannot allow heat to pass through it.

Q9: A laboratory thermometer A is kept 7 cm away on the side of the flame while a similar thermometer B is kept 7 cm above the flame of a candle as shown in the figure. Which of the thermometers A or Swill show a greater rise in temperature? Give a reason for your answer.

Ans: Thermometer B will show a greater rise in temperature because heated air above the candle rises immediately and increases the temperature of the bulb of thermometer B quite more than A.

Q10: A circular metal loop is heated at point O as shown in the figure.

(a) In which direction, would heat flow in the loop?
(b) In which order, are the pins at points P, Q and R fixed with the help of wax fall if points O, P, Q and R are equidistant from each other?
Ans:
(a) Heat will flow in both directions from O to P and from O to R.
(b) First of all, the pin at P and R will fall simultaneously. After that, the pin at Q will fall.

Q11: You may have noticed that a few sharp jerks are given to a clinical thermometer before using it. Why is it done so?
Ans: Jerks are given a clinical thermometer before using it to settle down the mercury level below normal temperature so that the measurement taken of the body is accurate.

Q12: For setting curd, a small amount of curd is added to warm milk. The microbes present in the curd help in setting if the temperature of the mixture remains approximately between 35°C to 40°C. At places where room temperature remains much below the range, setting of curd becomes difficult. Suggest a way to set curd in such a situation.
Ans: For the setting of curd where temperature is below room temperature, the container in which curd is to be made must be kept in a thermally insulated cover or it can be wrapped with wool or jute sacks so that temperature is maintained for the setting of curd.

Q13: A beggar wrapped himself with a few layers of newspaper on a cold winter night. This helped him to keep himself warm because
(a) friction between the layers of newspaper produces heat
(b) air trapped between the layers of newspaper is a bad conductor of heat
(c) newspaper is a conductor of heat
(d) newspaper is at a higher temperature than the temperature of the surroundings
Ans: (b) air trapped between the layers of newspaper is a bad conductor of heat

Q14: Boojho has three thermometers as shown in the figure. He wants to measure the temperature of his body and that of boiling water. Which thermometer(s) should he choose?

(a) Thermometers (i) and (iii) for measuring body temperature and (ii) for measuring the temperature of boiling water
(b) Thermometer (i) for measuring temperature of both
(c) Thermometer (ii) for measuring temperature of both
(d) Thermometer (iii) for measuring temperature of both
Ans: (a) Thermometers (i) and (iii) are clinical thermometers and (ii) are laboratory thermometers.

Q15: Figure (a, b, c, d) shows a student reading a doctor’s thermometer. Which of the figure indicates the correct method of reading temperature?
Ans: (a) In the first case, the eyes of the student are at the level of the thermometer, so the correct way to measure temperature is by using a clinical thermometer.

Q16: Why does the level of mercury rise when its bulb comes in contact with a hot object?
Ans: Mercury expands when it is heated. Hence, it rises in the capillary tube.

Q17: While constructing a house in coastal area, in which direction should the windows preferably face and why?
Ans: The windows should preferably face towards the sea as the sea breeze will keep the house cool during the day.

Q18: Ventilators are situated close to the ceilings and not near the floor. Why?
Ans: Warm air, being lighter, rises, and cool air takes its place. This warm air finds its exit from the ventilators located close to ceilings and thus helps in the circulation of fresh air.

Q19: Shopkeepers selling ice blocks usually cover them with jute sacks or sawdust. Explain why.
Ans: Jute sacks or saw dust act as insulating material and reduce the melting of ice. Thus, blocks of ice are covered with jute sacks or sawdust.

Q20: Why are milk vans carrying milk from the factory to the depots painted silver or white?
Ans: Milk vans are painted silver or white because white absorbs minimum light or reflects maximum light and thus keeps the milk fresh.