Also read: Acids, Bases and Salts Class 7 DAV Chapter 4 Solutions, Notes, Questions & Answers
Chemical Substances and Processes Class 7 DAV Chapter 3 Solutions, Notes, Questions & Answers
A. Fill in the blanks
- Heat is a form of energy.
- The range of clinical thermometer is 35°C to 42°C.
- Water is a poor conductor of heat.
- Land and sea breezes help to maintain the temperature of air.
- Transfer of heat, from the sun to the earth, takes place mainly through the process of radiation.
B. Write True or False for the following statements
- Heat cannot be produced by doing work. — False
- No gap are left between the rails of railway tracks. — False
- Poor conductors of heat are also good insulators of heat. — True
- Ventilators, when provided in rooms, are located near their roofs. — True
- A material medium is required for transfer of heat by the process of radiation. — False
C. Tick (✓) the correct option
1. A copper ball at 30°C is put in a container containing water at 30°C. In this case—
✅ there would be no transfer of heat between the copper ball and water.
2. The reading of the laboratory thermometer, in the figure shown here, is—
✅ 18°C
3. It is not convenient to use the laboratory thermometer to measure our body temperature. This is so because—
✅ as the thermometer is taken out from the mouth, the level of mercury immediately starts falling.
4. When we hold our hands close to the side of a flame, they get warmed up mainly due to—
✅ radiation
5. The freezer compartment in a refrigerator is usually put near its top. This provides good cooling throughout the refrigerator through—
✅ the setting up of convection currents in the air inside the refrigerator.
D. Answer the following questions in brief
1. In which direction does the transfer of heat normally take place?
Heat normally flows from a hotter object to a colder object until both reach the same temperature.
For example, when a hot spoon is placed in cold water, heat flows from the spoon to the water. Similarly, when we hold an ice cube in our hand, heat flows from our hand to the ice cube, causing it to melt.
Thus, heat always moves from the region of higher temperature to the region of lower temperature.
2. What is the cause of heat generation in the following situations?
(a) We apply brakes on our fast moving car.
When brakes are applied, friction is produced between the brake pads and the wheels of the car.
Due to friction, the kinetic energy of the moving car gets converted into heat energy. That is why the brakes become hot after continuous use.
(b) People often jump up and down to feel warmer in cold weather.
When people jump or exercise, their muscles do more work.
This extra muscular activity produces heat inside the body, making the person feel warmer. Increased blood circulation during movement also helps keep the body warm.
3. State two types of physical changes that may take place when a substance is heated.
When a substance is heated, the following physical changes may occur:
(i) Expansion
Most substances expand on heating. Their size or volume increases because the particles move farther apart.
Example:
- Iron rails expand during summer.
- A metal lid becomes loose when heated.
(ii) Change of state
Heating can change the physical state of a substance.
Examples:
- Ice changes into water.
- Water changes into steam on boiling.
In these changes, no new substance is formed; only the state changes.
4. In what way(s) does a clinical thermometer differ from an ordinary thermometer?
| Clinical Thermometer | Ordinary/Laboratory Thermometer |
|---|---|
| Used to measure human body temperature | Used to measure temperature of liquids and other substances |
| Has a range of 35°C to 42°C | Has a wider range, usually −10°C to 110°C |
| Has a kink in the capillary tube | Does not have a kink |
| Mercury level remains fixed for reading | Mercury level falls immediately when removed |
Explanation:
A clinical thermometer is specially designed for medical use, while an ordinary laboratory thermometer is used in science experiments and laboratories.
5. State the mode/different modes of heat transfer, in the following situations.
(a) A paper cup, full of hot soup, lying on a table.
Modes of heat transfer:
- Conduction through the cup
- Convection in the soup and surrounding air
- Radiation from the hot soup
Heat transfer usually occurs through all three modes together in this case.
(b) Cooking vegetables in a pan.
Modes of heat transfer:
- Conduction from flame to pan and from pan to vegetables
- Convection in water or oil inside the pan
(c) Melting of a chocolate bar, in the school bag on a hot day.
Mode of heat transfer:
- Radiation
The heat from sunlight reaches the chocolate mainly through radiation, causing it to melt.
(d) Cooking food in a microwave oven.
Mode of heat transfer:
- Radiation
Microwave ovens use electromagnetic waves (microwaves) to heat and cook food.
6. What is meant by sea breeze? When does it occur?
Sea breeze is the cool wind that blows from the sea towards the land during daytime.
How it occurs:
- During the day, land heats up faster than sea water.
- The air above land becomes hot and rises upward.
- Cooler air from the sea moves towards the land to take its place.
This movement of cool air from sea to land is called sea breeze.
Time of occurrence:
Sea breeze generally occurs during the daytime, especially in the afternoon.
E. Answer the following questions
1. In the arrangement shown in the figure, pins A, B, C and D are fixed to a circular metal loop with the help of wax.
The circular metal loop is heated at point A with the help of a candle flame.
In which order would the pins fall if AB < AD? Justify your answer.
When the loop is heated at point A, heat travels through the metal loop by the process of conduction. The pins are attached with wax, and when the wax melts due to heat, the pins fall.
Since distance AB is less than AD, heat reaches pin B first. After that, heat travels further and reaches pins C and D. Pin D is farthest from point A, so it falls last.
Order of falling of pins:
B → C → D
Reason:
Heat travels through the metal from the hotter region to the colder region. The pin nearest to the heated point receives heat first, so its wax melts earlier.
2. Give reasons for the following:
(a) Iron rims are heated red hot before ‘fixing’ them on cart wheels.
Iron expands on heating. When the iron rim is heated red hot, it becomes slightly larger and can be fitted easily over the wooden wheel.
After cooling, the iron contracts and fits tightly around the wheel, making it strong and firm.
(b) A clinical thermometer has a slight bend, or kink, in its capillary tube.
The kink prevents the mercury from falling back immediately after removing the thermometer from the mouth.
This helps us read the temperature properly even after the thermometer is taken out.
(c) A new quilt is warmer than an old one.
A new quilt contains more air trapped between its cotton fibres. Air is a poor conductor of heat, so it prevents body heat from escaping.
In an old quilt, the cotton becomes compressed and traps less air, so it provides less warmth.
(d) A brass tumbler feels much colder than a wooden tray on a chilly day.
Brass is a good conductor of heat, so it quickly takes heat away from our hand, making it feel colder.
Wood is a poor conductor of heat, so it does not remove heat quickly from our hand.
Therefore, the brass tumbler feels colder than the wooden tray.
(e) The bottoms of cooking utensils are often kept black.
Black surfaces absorb heat more effectively than shiny surfaces.
Therefore, black bottoms absorb more heat from the flame and help food cook faster and more efficiently.
3. Akshit visited Rishikesh for river rafting during summer holidays. At a campsite there were two tents, one made with a black fabric and the other with a white fabric. Which one should Akshit prefer? Give reason for the choice. Should Akshit prefer the same tent during winters?
During summer, Akshit should prefer the white tent because white colour reflects most of the heat and sunlight. Therefore, the inside of the tent remains cooler and more comfortable.
During winters, Akshit should prefer the black tent because black colour absorbs more heat from sunlight and keeps the tent warmer.
Thus:
- In summer → white tent is better.
- In winter → black tent is better.
4. Explain briefly how winds are caused.
Winds are caused due to uneven heating of the earth’s surface by the sun.
During the day, land gets heated faster than water. The air above land becomes warm, lighter and rises upward. Cooler air from nearby areas moves in to take its place. This movement of air creates wind.
Similarly, differences in temperature between different regions cause air to move from high-pressure areas to low-pressure areas, producing winds.
5. Supriya, while doing an experiment in the Science Laboratory, kept a laboratory thermometer vertically 10 cm away on one side of a candle flame. Her friend Riya kept a similar thermometer ‘Q’ 10 cm above the flame as shown in the figure.
In which of the thermometers, ‘P’ or ‘Q’, the rise in temperature will be faster? Give reason for your answer.
The temperature will rise faster in thermometer Q.
Reason:
Hot air produced by the candle flame rises upward due to convection. Since thermometer Q is placed above the flame, it receives hot air directly and gets heated quickly.
Thermometer P is placed sideways, where much less hot air reaches it. Therefore, its temperature rises more slowly.
6. In the two set-ups X and Y shown in the figure, the wires AB and PQR are made of the same material and have equal thickness. The wire AB in set-up X is equal to the diameter (PR) of the semicircle formed by the wire PQR in set-up Y. Pins P1 and P2 are attached to wires AB and PQR respectively with the help of wax. If both the pins P1 and P2 fall later, give reason for your answer.
Pin P2 will fall later.
Reason:
Heat travels through the wire by conduction. In set-up X, the wire AB is straight, so heat reaches pin P1 through a shorter path.
In set-up Y, heat has to travel along the curved semicircular wire PQR, which is a longer path. Therefore, heat takes more time to reach pin P2.
Hence, the wax holding P2 melts later, so pin P2 falls later.