YASRAJ STUDIES
DAV Class 7 Science Chapter 5 – Heat Question Answers
DAV Class 7 Science Chapter 5 – Heat Question Answers are given below in a simple, clear, and student-friendly format. This chapter explains heat, temperature, thermometers, transfer of heat, conduction, convection, radiation, thermal expansion, sea breeze, land breeze, and practical applications of heat.
These solutions are useful for DAV Class 7 Science exam preparation, homework, classwork, and revision.
DAV Class 7 Science Chapter 5 – Heat
Introduction
Heat is a form of energy that flows from a hotter object to a colder object. We experience heat in many situations in our daily lives, such as cooking food, warming ourselves in winter, and cooling our homes.
In this chapter, students learn about heat and temperature, different types of thermometers, methods of heat transfer, expansion due to heating, and the formation of winds and breezes.
Below are the complete solved questions and answers for DAV Class 7 Science Chapter 5 – Heat.
A. Fill in the Blanks
1. Heat is a form of __________.
Answer: Energy
2. The range of clinical thermometer is from __________°C to __________°C.
Answer: 35°C to 42°C
3. Water is a __________ conductor of heat.
Answer: Poor
4. Land and sea breezes help to maintain the __________ of air.
Answer: Temperature
5. Transfer of heat from the Sun to Earth usually takes place by __________.
Answer: Radiation
B. Write True or False for the Following Statements
1. Heat cannot be produced by doing work.
Answer: False
2. No gaps are left between the rails of railway tracks.
Answer: False
3. Poor conductors of heat are also good insulators of heat.
Answer: True
4. Ventilators, when provided in rooms, are located near their roofs.
Answer: True
5. A material medium is required for transfer of heat by the process of radiation.
Answer: 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—
Answer: (d) 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—
Answer: (d) 18°C
3. It is not convenient to use the laboratory thermometer to measure our body temperature. This is so because—
Answer: (b) As the thermometer is taken out from the mouth, the level of mercury in it immediately starts falling.
4. When we hold our hands near one side of a flame, they get warmed due to—
Answer: (c) Radiation
5. The freezer compartment in a refrigerator is usually put near its top. This provides good cooling throughout the refrigerator by making use of—
Answer: (c) 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?
2. What is the cause of heat generation in the following situations?
(a) We apply brakes on our fast-moving car.
(b) People often jump up and down to feel warmer in cold weather.
3. State two types of physical changes that may take place when a substance is heated.
Answer:
Two physical changes are:
- Change of state
- Thermal expansion
4. In what way does a clinical thermometer differ from an ordinary/laboratory thermometer?
Answer:
| Clinical Thermometer | Ordinary/Laboratory Thermometer |
|---|---|
| Its range is generally 35°C to 42°C. | Its range is generally wider, such as 0°C to 100°C. |
| It has a kink in its capillary tube. | It does not have a kink. |
| The kink prevents the liquid column from falling immediately. | The reading should be taken while the thermometer is in contact with the object. |
| It is mainly used to measure body temperature. | It is used to measure the temperature of different substances. |
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.
Answer:
- From soup to cup – Conduction
- From soup to surrounding air – Convection
(b) Cooking vegetables in a pan.
Answer:
- From pan to vegetables – Conduction
- Within the vegetables – Conduction and convection
- From the flame to the utensil – Conduction, convection and radiation
(c) Melting of a chocolate bar in the school bag on a hot day.
Answer:
- From surrounding air to the school bag – Convection and radiation
- From the school bag to the chocolate – Conduction and convection
(d) Cooking food in a microwave oven.
Answer: Radiation
6. What is meant by sea breeze? When does it occur?
It generally occurs during the daytime.
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 a candle flame. In which order would the pins fall if AB < AD? Justify your answer.
Answer:
The pins would fall in the following order:
A → B → D → C
Since AB < AD, heat reaches B before it reaches D. Therefore, the wax at B melts before the wax at D. Pin B falls next, followed by D and then C.
2. Give reasons for the following.
(a) Iron rims are heated red hot before ‘fixing’ them on cart wheels.
(b) A clinical thermometer has a slight bend, or kink, in its capillary tube.
(c) A new quilt is warmer than an old one.
(d) A brass tumbler feels much cooler than a wooden tray on a chilly day.
(e) The bottoms of cooking utensils are often kept black.
3. Akshit visited Rishikesh for river rafting during summer holidays. At a campsite, there were two tents, one made with black fabric and the other with white fabric. Which one should Akshit prefer? Give reasons for the choice. Should Akshit prefer the same tent during winters?
During winter, he should prefer the black tent.
Black surfaces absorb more heat radiation. Therefore, a black tent can absorb more heat and may be more suitable during winter.
4. Explain briefly how winds are caused.
5. Supriya, while doing an experiment in the Science Laboratory, kept a laboratory thermometer ‘P’ 10 cm away on one side of the flame of a candle. Her friend Riya kept a similar thermometer ‘Q’ 10 cm above the flame of the candle as shown in the figure.
In which of the thermometers, ‘P’ or ‘Q’, will the rise in temperature be faster? Give reasons for your answer.
6. In the two set-ups X and Y, the wires AB and PQR are made of the same material and have equal thickness. The length of wire AB in set-up X is equal to the diameter of the semi-circle formed by wire PQR in set-up Y. Pins P₁ and P₂ are attached to the wires with the help of wax. Which of the two pins, P₁ or P₂, will fall off later? Give reason for your answer.
Hence, P₂ receives heat later and falls off later than P₁.
Value-Based Question
During the summer holidays, Hridyika visited her grandparents in their village. During her morning walks, she observed a girl playing music on a homemade musical instrument. Hridyika wanted to learn the skill but felt hesitant to ask the girl.
Her mother advised her to politely request the girl to teach her. She explained that just as heat flows from a hot object to a cold object, knowledge and skills can be passed from a mentor to a learner.
1. State the values displayed by Hridyika’s mother.
- Humility
- Persuasion skills
- Respect for knowledge and learning
2. Name the three different modes of transfer of heat and give one example of each.
Answer:
The three modes of heat transfer are:
- Conduction – Heating of an iron rod.
- Convection – Heating of water.
- Radiation – Heat received from the Sun.
3. Have a group talk in which students talk about how they learnt some skill from a mentor.
Answer: Do it yourself.
Something To Do
1. Boil some water in a vessel. Cover it with a plate. Remove the plate after sometime. Allow it to cool. We will see droplets of water on the surface of the plate. Try to find out why and how these water droplets are formed.
2. Describe the various ways in which heat energy can enter/escape from our houses. Learn about the measures that are now being taken to reduce such heat transfers.
Explanation:
- Conduction: Heat can pass through walls, floors, roofs, doors and windows.
- Convection: Moving air can carry heat into or out of a house.
- Radiation: Sunlight can transfer heat into a house.
Heat transfer can be reduced by using insulating materials, curtains, suitable roofing materials, double-glazed windows and proper ventilation.
3. Take a cup of hot water/tea. Measure its temperature accurately with a thermometer. What will happen to the hot water/tea if it is left in a room for, say, one hour? Measure the temperature at fixed intervals of five minutes and record your observations in the given table. Using these readings, draw a temperature-time graph.
Answer: Do it yourself.
Students should:
- Measure the initial temperature.
- Record the temperature every five minutes.
- Continue the observations for one hour.
- Plot time on the X-axis and temperature on the Y-axis.
- Join the points to obtain the temperature-time graph.
4. Take some moderately hot water in a tumbler. Dip one end of a steel spoon, a plastic spoon, a glass rod, a wooden stick and a copper rod in the hot water. After waiting for a few minutes, carefully touch the other end of each item. Observe which ones become hot and which ones do not. Hence, categorise them as ‘Heat conductors’ or ‘Heat insulators’.
Answer:
| Heat Conductors | Heat Insulators |
|---|---|
| Steel spoon | Plastic spoon |
| Copper rod | Glass rod |
| Wooden stick |
Plastic, glass and wood are poor conductors of heat and therefore act as heat insulators.
Important Points to Remember
- Heat is a form of energy.
- Heat normally flows from a hotter body to a colder body.
- Heat can be produced by doing work.
- The three modes of heat transfer are conduction, convection and radiation.
- Metals are generally good conductors of heat.
- Wood, plastic and glass are generally poor conductors of heat.
- Radiation does not require a material medium.
- A clinical thermometer generally has a range of 35°C to 42°C.
- A clinical thermometer has a kink in its capillary tube.
- Most substances expand when heated.
- Gaps are provided between railway tracks to allow for thermal expansion.
- Sea breeze blows from sea to land during the day.
- Land breeze blows from land to sea at night.
- Ventilators are placed near the roof because warm air rises.
- Black surfaces are good absorbers of heat radiation.
- White surfaces reflect most heat radiation.
- Winds are caused by the uneven heating of the Earth's surface.
Quick Revision – Heat
Heat
Heat → Form of Energy
Heat flows from a hotter body to a colder body.
Three Modes of Heat Transfer
1. Conduction
Heat transfer through direct contact
Example: Heating an iron rod.
2. Convection
Heat transfer through movement of liquids or gases
Example: Heating water.
3. Radiation
Heat transfer without a material medium
Example: Heat from the Sun reaching Earth.
Thermometers
Clinical Thermometer
- Range: 35°C to 42°C
- Used for measuring body temperature
- Has a kink
Laboratory Thermometer
- Has a wider temperature range
- Does not have a kink
- Used for measuring the temperature of substances in experiments
Thermal Expansion
Heating → Expansion
Most substances expand when heated and contract when cooled.
Example: Gaps are left between railway tracks to provide space for expansion.
Sea Breeze
Daytime → Land heats faster → Warm air rises → Cool air from sea moves towards land
Therefore:
Sea → Land = Sea Breeze
Land Breeze
Night → Land cools faster → Air over sea remains comparatively warmer → Air moves from land towards sea
Therefore:
Land → Sea = Land Breeze
Heat Transfer – Easy Chart
| Process | Meaning | Example |
|---|---|---|
| Conduction | Heat through direct contact | Iron rod |
| Convection | Heat through moving fluids | Boiling water |
| Radiation | Heat without medium | Sun's heat |
Conclusion
DAV Class 7 Science Chapter 5 – Heat explains how heat is produced, measured and transferred from one place to another.
The chapter helps students understand important concepts such as temperature, thermometers, conduction, convection, radiation, thermal expansion, sea breeze, land breeze and insulation.
Students can also relate these concepts to everyday situations such as cooking food, using refrigerators, railway tracks, quilts, cooking utensils and ventilation in houses.
Understanding the difference between conduction, convection and radiation is especially important for solving conceptual and application-based questions in examinations.
These DAV Class 7 Science Chapter 5 – Heat Question Answers are useful for homework, classwork, exam preparation and quick revision.
YRS YASRAJ STUDIES
DAV Class 7 Science | Chapter 5 | Heat
Complete Question Answers + Explanations + Important Points + Quick Revision
