Chapter 3 Heat – Concepts included in the chapter –
- Understanding Heat- Learning about the nature of heat and how it affects different materials.
- Measuring Temperature- Exploring how temperature is measured using different types of thermometers.
- Heat Transfer- Studying how heat moves from one object to another through conduction, convection, and radiation.
Also Check – Class 7 science -Chapter 4 – Heat -Detailed Notes
NCERT Exemplar – Class 7 Science- Chapter 3 – Heat – Solved
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 the wooden tile is not polished.
- (c) reflects more heat than the wooden tile.
- (d) is a poor conductor of heat than the wooden tile.
Answer- (a) is a better conductor of heat than the wooden tile.
Explanation- Marble, being a good conductor of heat, conducts heat away from your feet faster than wood, which is a poor conductor. This rapid heat transfer makes the marble feel colder, even though it’s at the same temperature as the wood.
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 surrounding area.
Answer- (b) air trapped between the layers of newspaper is a bad conductor of heat.
Explanation- The layers of newspaper trap air between them. Air is a poor conductor of heat, which means it doesn’t allow heat to pass through easily. This trapped air helps in keeping the person warm by reducing the loss of body heat to the surroundings.
Also Check – Class 7 science -Chapter 4 – Heat – Definition and Explanation of Important Keywords
Question 3-Paheli and Boojho measured their body temperature. Paheli found hers to be 98.6 °F and Boojho recorded 37°C. Which of the following statements 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) Both have normal body temperature.
Explanation- The normal body temperature is about 98.6 °F (37 °C). Paheli’s temperature in Fahrenheit and Boojho’s temperature in Celsius both indicate normal body temperature. Therefore, neither of them has a higher or lower body temperature than the other, nor are they suffering from a fever.
Question 4-Boojho has three thermometers as shown in Figure 4.1. He wants to measure the temperature of his body and that of boiling water. Which thermometer(s) should he choose?
- (a) Thermometer (i) or (iii) for measuring body temperature and (ii) for measuring the temperature of boiling water.
- (b) Thermometer (i) for measuring the temperature of both.
- (c) Thermometer (ii) for measuring the temperature of both.
- (d) Thermometer (iii) for measuring the temperature of both.
Answer- (a) Thermometer (i) or (iii) for measuring body temperature and (ii) for measuring the temperature of boiling water.
Explanation- Thermometers (i) and (iii) are clinical thermometers, which are suitable for measuring body temperature. Thermometer (ii) is a laboratory thermometer, designed for higher temperatures such as that of boiling water. Clinical thermometers are not suitable for high temperatures like that of boiling water, as they have a limited temperature range, typically up to about 42°C (107.6°F), which is below the boiling point of water (100°C or 212°F).
Also Check -Chapter 3 Heat Activities: Simple Experiments for Class 7
Question 5 -Four arrangements to measure the temperature of ice in a beaker with a laboratory thermometer are shown in Figure 4.2 (a, b, c, and d). Which one of them shows the correct arrangement for accurate measurement of temperature?
- Answer- The answer is (a).
- Explanation- The correct arrangement for measuring temperature with a laboratory thermometer involves ensuring that the bulb of the thermometer is in contact with the ice without touching the bottom or the sides of the beaker. This ensures that the thermometer is measuring the temperature of the ice and not the beaker.
Question 6-Fig 4.3 (a–d) shows a student reading a doctor’s thermometer. Which of the figures indicates the correct method of reading temperature?
- Answer- The answer is (a).
- Explanation- The correct method to read a doctor’s thermometer involves holding the thermometer at eye level and reading the level of the mercury line straight on. This avoids parallax error, which can occur if the thermometer is read from an angle.
Also Check – NCERT Solutions For Class 7 Science Chapter 4 – Heat
Question 7-Figure 4.4 (a–d) shows the readings on four different thermometers. Indicate which of the reading shows the normal human body temperature?
- Answer- The answer is (c).
- Explanation- Normal human body temperature is approximately 37°C (98.6°F). The thermometer showing a temperature close to 37°C indicates normal human body temperature.
Question 8-Shopkeepers selling ice blocks usually cover them with jute sacks. Explain why.
- Answer- Shopkeepers selling ice blocks usually cover them with jute sacks because materials like jute sacks, sawdust, and newspapers act as insulators. They prevent the escape or entry of heat, thus slowing down the melting of 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. Which of the thermometers, A or B, will show a greater rise in temperature?
- Answer- Thermometer B will show a greater rise in temperature.
- Explanation- In this setup, thermometer B is positioned directly above the flame where hot air rises due to convection. Since hot air moves upward, thermometer B, being in the path of this rising hot air, will experience a greater increase in temperature compared to thermometer A, which is positioned to the side of the flame.
Question 10 -To keep her soup warm, Paheli wrapped the container in which it was kept with a woollen cloth. Can she apply the same method to keep a glass of cold drink cool?
- Answer- Yes, she can apply the same method.
- Explanation- A woollen cloth acts as an insulator, meaning it slows down the transfer of heat. When used to wrap a hot object like a soup container, it slows down the loss of heat, keeping the soup warm. Similarly, when used to wrap a cold object like a glass of cold drink, it slows down the gain of heat from the surroundings, keeping the drink cool.
Also Check – Class 7- Chapter 3- Heat- 5 Worksheets Solved and Unsolved
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- The level of mercury rises due to the expansion of mercury when it is heated.
- Explanation- Mercury is a liquid metal that expands when heated. When the bulb of a mercury thermometer comes in contact with a hot object, the heat causes the mercury inside the bulb to expand. Since the liquid has no other place to go, it rises up in the thermometer tube, indicating an increase in temperature.
Question 12 -A circular metal loop is heated at point O as shown in Figure 4.6.
- (i) In which direction would heat flow in the loop?
- Answer- Heat will flow in both directions from O, towards P and towards R.
- (ii) In which order 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?
- Answer- The pins at R and P will fall simultaneously, followed by the pin at Q.
- Explanation- Heat conducts symmetrically in both directions along the metal loop. Since P and R are equidistant from the heat source at O, the pins at P and R will fall at the same time. Q, being farther away from the heat source, will see its pin fall later.
Question 13 -In the arrangements A and B shown in Figure 4.7, 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?
- Answer- In case B, the pins are likely to fall at different times.
- Explanation- In case B, heat is applied at one end of the iron rod. This means the heat will reach pin P first, causing it to fall before pin Q. In arrangement A, heat is applied in the middle of the loop, causing heat to travel symmetrically to both pins P and Q, making them fall simultaneously.
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- To set curd in colder environments, the container with the milk and curd mixture can be wrapped in an insulating material like a woollen cloth. Another method is to place the container near a warm place, such as near a stove or in a warm room.
- Explanation- The insulating material helps maintain the temperature of the mixture within the optimal range for the microbes to act. Keeping the container in a warmer area of the house also helps maintain the necessary temperature for setting the curd.
Question 15-Why are a few sharp jerks given to a clinical thermometer before using it?
- Answer- Sharp jerks are given to a clinical thermometer before using it to ensure that the mercury level falls below the 35°C mark. This is necessary because the mercury thread might have expanded and stayed at a position higher than the minimum temperature mark (35°C) due to previous use or ambient temperature.
Question16 -Why is it advised not to hold the thermometer by its bulb while reading it?
- Answer- It is advised not to hold the thermometer by its bulb while reading it because the heat from the hand can transfer to the mercury in the bulb, causing it to expand. This expansion would give a higher temperature reading, leading to an inaccurate measurement.
Question 17 -At a campsite, there are tents of two shades – one made with black fabric and the other with white fabric. Which one would you prefer for resting on a hot summer afternoon? Would you like to prefer the same tent during winter?
- Answer- On a hot summer afternoon, the white fabric tent would be preferable as white reflects most of the sunlight, keeping the inside of the tent cooler. In contrast, the black fabric tent would absorb more heat, making it hotter inside.
- For winter- The preference would shift to the black fabric tent. Black absorbs more heat, which would be beneficial in keeping the tent warmer in cold conditions.
Question 18 -While constructing a house in a coastal area, in which direction should the windows preferably face and why?
- Answer- In coastal areas, it is preferable for windows to face towards the sea.
- Explanation- This is because the sea breeze, which generally moves from sea to land during the day, can enter the house and help in keeping it cool. Additionally, this orientation can also provide a scenic view of the sea, which is often desirable in coastal areas.
Also Check – Sea Breezes and Land Breezes – Class 7 Science explained in details
Question 19 -Observe the picture given in Figure 4.8. Water is being boiled in a pan of wide base.
- (i) Which position, P or T, will feel warmer?
- Answer- Position P will feel warmer.
- (ii) Fill up the boxes P and T to indicate the mode of flow of heat to the hand.
- Answer- P – Convection; T – Radiation.
- Explanation- In position P, the hand is above the pan where the hot air rises due to convection, making it feel warmer. In position T, the hand is to the side and feels heat mainly through radiation.
Also Check – Difference Between Sea Breeze And Land Breeze
Question 20 -Look at Figure 4.9. The length of wire PQ in case 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. Which pin will fall first?
- Answer- The pin on wire PQ in case A will fall first.
- Explanation- The heat will reach the pin on the straight wire PQ more quickly than it will reach the pin on the semicircular wire CDE. This is because heat travels more efficiently through a straight path compared to a curved path, resulting in the pin on the straight wire falling first.
Also Check – Conduction, Convection, and Radiation- Class 7 Science Explained
Also Check – Rapid Revision – Class 7 Science -Chapter 3 – Heat– Complete Notes
