NCERT Exemplar Problems Class 7 Science – Heat

Question 1:
A marble tile would feel cold as compared to a wooden tile on a winter morning because the marble tile
(a) is a better conductor of heat than the wooden tile
(b) is polished while wooden tile is not polished
(c) reflects more heat than wooden tile
(d) is a poor conductor of heat than the wooden tile
Answer:
(a) Since, a marble tile is a good conductor of heat as compared to the wood, therefore a marble tile will allow heat to pass through itself easily, keeping it cool on a winter morning.

Question 2:
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
Answer:
(b) The air trapped between the layers of newspaper does not allow heat to pass through, so it helps the beggar to keep himself warm.

Question 3:
Paheli and Boojho measured their body temperatures. Paheli found tier’s to be 98.6°F and Boojho recorded 37°C. Which of the following statement is true?
(a) Paheli has a higher body temperature than Boojho
(b) Paheli has a lower body temperature than Boojho
(c) Both have normal body temperature
(d) Both are suffering from fever
Answer:
(c) The temperature 37°C is described in Celsius scale while the temperature 98.6°F is described in Fahrenheit scale but they are actually equal. Both these values represent the normal average temperature of a human body.

Question 4:
Boojho has three thermometers as shown in 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
Answer:
(a) Thermometers (i) and (iii) are clinical thermometers used to measure body temperature and (ii) is laboratory thermometer used to measure the temperature of boiling water or any object other than human body.

Question 5:
Four arrangements to measure temperature of ice in beaker with laboratory thermometer are shown in figure (a, b, c, d). Which one of them shows the correct arrangement for accurate measurement of temperature?

Answer:
(a) The correct arrangement is shown in figure (a). For accurate measurement of temperature, a laboratory thermometer must be kept upright not tilted and the bulb should be completely surrounded by the substance of which the temperature is to be measured. Also, the bulb should not touch the surface of the container.

Question 6:
Figure (a, b, c, d) shows a student reading a doctor’s thermometer. Which of the figure indicates the correct method of reading temperature?

Answer:
(a) In first case, eyes of student are in level of mercury in thermometer, so it is the correct way to read the temperature in a clinical thermometer.

Question 7:
Figure (a, b, c, d) shows the readings of four different thermometers. Indicate which of the reading shows the normal human body temperature?

Answer:
(c) As we know that normal body temperature is 37°C. So, the correct option is (c),

Very Short Answer Type Questions

Question 8:
Shopkeepers selling ice blocks usually cover them with jute sacks. Explain why.
Answer:
Since, a jute sack is a thermal insulator, therefore, it helps ice not to meit immediately. So, shopkeepers must use insulating materials like sack, saw dust, newspaper etc, to cover the ice.

Question 9:
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 figure. Which of the thermometers A or 8, will show a greater rise in temperature? Give reason for your answer.

Answer:
Thermometer S whose bulb is just above the candle flame, will show a greater rise in temperature because hot air above the candle rises up immediately and increases the temperature of bulb of thermometers higher than that of A.

Question 10:
To keep her soup warm, Paheli wrapped the container, in which it was kept a woollen cloth. Can she apply the same method to keep a glass of cold drink cool? Give reason for your answer.
Answer:
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.

Question 11:
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?
Answer:
As the temperature increases, expansion of mercury takes place in the capillary tube which leads to the rise in the level of mercury in thermometer.

Short Answer Type Questions

Question 12:
A circular metal loop is heated at point 0 as shown in figure.
(a) In which direction, would heat flow in the loop?
(b) In which order, the pins at points P, Q and R fixed with the help of wax fall if points 0, P, Q and R are equidistant from each other?

Answer:
(a) Heat will flow in both the directions i.e. from 0 to P and O to ft.
(b) First of all pins at P and ft will fall simultaneously and after that pin at 0 will fall because equal amount of heat will reach the pins at P and ft first.

Question 13:
In the arrangements A and B as shown in figure, pins P and Q are fixed to a metal loop and an iron rod with the help of wax. In which case are both the pins likely to fall at different times? Explain.

Answer:
As from the given figures A and 8, in case of A, pins at P and 0 get equal amount of heat, so pins fixed at P and Q in case of A will fall simultaneously.
But in case of 8 pin at point P falls before the pin at Q as because heat will reach the pin at P first than the pin at Q.

Question 14:
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.
Answer:
For the setting of curd at places 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 either by a woollen material or a jute sack so that temperature is maintained for the setting of curd.
The container can also be kept in the sun or near the gas stove while cooking food for the setting of curd.

Question 15:
You may have noticed that a few sharp jerks are given to clinical thermometer before using it. Why is it done so?
Answer:
Jerks are given to clinical thermometer before using it to settle down the mercury level below normal temperature so that the measurement of temperature of a body can be taken accurately.

Question 16:
It is advised not to hold the thermometer by its bulb while reading it. Explain, why.
Answer:
We are advised not to hold the thermometer bulb while reading ii, as the level of mercury increases from the actual reading due to our body temperature.

Question 17:
At a camp site there are tents of two shades. One made with black fabric and the other with white fabric. Which one will you prefer for resting on a hot summer afternoon? Give reason for your choice. Would you like to prefer the same tent during winter?
Answer:
We will prefer a white fabric tent in case of summer because it reflects all the radiations from the sun and keeps us cool inside the tent.
No, in case of winter, we should use a black fabric tent as it absorbs all colours of light from the sun and keeps us warm inside the tent.

Question 18:
While constructing a house in a coastal area, in which direction should the windows preferably face and why?
Answer:
Windows should preferably face towards the sea beach because the sea broeze coming from sea keeps the house cool during the day time.

Question 19:
Observe the picture given in figure. Water is being boiled in a pan of wide base.
(a) Which position P or T will feel warmer?
(b) Fill up the boxes P and T to indicate the mode of flow of heat to the hand.

Answer:
(a) From the given diagram, position Pwill feel warmer because of hot air rising up.
(b) The flow of heat to P is a convection process because the air .near the boiling water gets not and rises. However, on the sides such as at T, there is no convection and air does not feel as hot as at the top. Therefore, flow of heat to T is a radiation process.

Question 20:

The length of wire PQ in case of A is equal to the diameter of the semicircle formed by the wire CDE, in case B. One pin is attached to each wire with the help of wax as shown in figure. Which pin will fall first? Explain.
Answer:
The pin on the wire PQ in case A will fall first because the heat will reach to the pin on straight wire PQ before the heat reaches the pin on the semicircular wire CDE in case B.