Revision Notes for Class 7 Science: Chapter 04 Heat
Access comprehensive revision notes for Chapter 04 Heat using the CBSE Class 7 Science Heat and Temperature Notes with Questions Set 05. Designed to align with the 2026-27 academic syllabus for Class 7 Science, these concept summaries help students streamline their exam preparation and review complex topics efficiently.
Review Chapter 04 Heat for Class 7 Science
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Hot and Cold
Heat always flows from a hot object to a cold object. When a hot and a cold object touch, the hot object loses heat and the cold one gains it. The SI unit of heat energy is the joule (J); it is also measured in calories. Our sense of touch tells only whether something is hot or cold, not how hot, so we use temperature.
Temperature
Temperature tells the degree of hotness of an object. Its SI unit is the kelvin (K); common units are degree Celsius (°C) and degree Fahrenheit (°F). Thermometers measure it. There are three scales: Celsius, Fahrenheit and Kelvin (absolute scale). Most thermometers use the Celsius scale. Objects made of different materials gain or lose heat differently: a metal sheet feels colder than a wooden one in ice-cold water, so hotness also depends on the material.
Types of Thermometers
| Thermometer | Details |
|---|---|
| Mercury | First made by Gabriel Fahrenheit in 1714. Mercury stays liquid from −38.87°C to 356.58°C, does not stick to glass, expands uniformly and is easy to see. Example: clinical thermometer |
| Digital | Has a thermistor; the current through it depends on temperature and the value is shown on an LCD. Doctors prefer it as mercury is poisonous and the reading is more accurate. No shaking needed |
| Alcohol | First made by Galileo with spirit of wine. Cheaper and less harmful, and measures down to −115°C, but cannot measure above 78°C. Used as laboratory thermometer (−10°C to 110°C) |
Clinical thermometer: has an arrow at 98.4 or 98.6°F (normal body temperature) and a kink near the bulb that stops the mercury from flowing back before the reading is taken. To use it, shake it so the mercury goes below the mark, place it under the tongue or in the armpit for two minutes, and read at eye level. Wash it in antiseptic before and after use and handle it carefully because it is glass and contains toxic mercury.
Transfer of Heat
Heat is transferred by three modes: conduction, convection and radiation.
| Mode | Meaning | Examples |
|---|---|---|
| Conduction | Transfer of heat through a material from the hot end to the cooler end without movement of the material. Metals are good conductors (free electrons); wood, plastic, glass, liquids and gases are poor conductors, called thermal insulators. A vacuum is a perfect insulator | A steel spoon in hot water gets hot, a plastic one does not; metal pan with plastic handle; ice box; electric iron; oven gloves with trapped air; fat of penguins |
| Convection | Transfer of heat by the movement of the fluid (liquid or gas) itself. Hot fluid becomes less dense and rises; cool fluid sinks, setting up convection currents | Ink in heated water rises at the centre and sinks at the sides; electric kettle; ventilation; air conditioner placed high on the wall; room heater placed low; hot water system; refrigerator |
| Radiation | Transfer of heat as infra-red waves without any medium; works through a vacuum. This is how heat from the Sun reaches the Earth | Heat from the Sun, a fire or a lamp |
Mercury is the only common metal that is liquid at room temperature, and being a metal it conducts heat well, so it is used in thermometers.
Sea Breeze and Land Breeze
By day the land heats faster than the sea. Hot air over the land rises and cooler, denser air from the sea moves in, giving a sea breeze. At night the land cools faster than the sea, so warm air rises over the sea and cool air flows from the land, giving a land breeze.
Absorbing and Emitting Heat by Radiation
Black, dull and rough surfaces are good absorbers and radiators of heat; white, shiny and smooth surfaces are poor ones. People in hot climates wear light-coloured clothes. The cooling fins at the back of a refrigerator are dull and black so they radiate heat quickly. A vacuum flask keeps contents hot or cold by reducing heat transfer by conduction, convection and radiation.
Thermal Expansion and Contraction
Substances expand on heating and contract on cooling. Solids expand very little (a 1 m brass rod grows about 1 mm for a rise of 100°C). Liquids expand more, so we do not fill a container to the brim when boiling. Gases expand the most; air bubbles in dough expand during baking and make holes in bread.
| Example | How it works |
|---|---|
| Railway lines | Small gaps are left because steel expands in hot weather; otherwise the rails would buckle |
| Dental fillings | The filling material must expand like tooth enamel, otherwise hot and cold food causes pain |
| Thermostat | A bimetallic strip of two metals bends when heated because they expand at different rates, switching the heater on and off |
Questions and Answers
1. What do you understand by heat energy? Write its SI unit.
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Answer: Heat energy is the energy that flows from a hot object to a cold one because of a difference in temperature. Its SI unit is the joule.
Teacher's Note: Use the key term in your answer.
2. What does temperature tell? Name the units and scales.
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Answer: Temperature tells how hot an object is. The units are kelvin (SI), degree Celsius and degree Fahrenheit, with the Celsius, Fahrenheit and Kelvin scales.
Teacher's Note: Give an example if the question allows.
3. Convert 50°C into °F and 45°F into °C.
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Answer: °F = (°C × 9/5) + 32, so 50°C = 122°F. °C = (°F − 32) × 5/9, so 45°F is about 7.2°C.
Teacher's Note: Write a short, clear answer.
4. Name the liquids used in the different thermometers.
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Answer: Mercury in the mercury thermometer and alcohol (spirit) in the alcohol thermometer; the digital one has no liquid.
Teacher's Note: Remember this for your exam.
5. On which scale is human body temperature measured?
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Answer: Usually the Celsius scale (about 37°C) or the Fahrenheit scale (98.6°F).
Teacher's Note: Learn this point and write it in your own words.
6. How many modes of heat transfer are there? Name them.
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Answer: Three: conduction, convection and radiation.
Teacher's Note: Use the key term in your answer.
7. Why do metals conduct heat so well?
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Answer: Metals have free electrons that move quickly between the atoms and carry heat energy rapidly.
Teacher's Note: Give an example if the question allows.
8. What is a thermal insulator?
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Answer: A poor conductor of heat, such as wood, plastic or glass.
Teacher's Note: Write a short, clear answer.
9. How does convection take place in liquids and gases?
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Answer: The fluid near the heat source expands, becomes lighter and rises, while cooler denser fluid sinks to take its place, forming a convection current.
Teacher's Note: Remember this for your exam.
10. Why are heaters placed low and air conditioners high?
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Answer: Hot air rises, so a heater at the bottom warms the room by convection; cold air sinks, so an air conditioner at the top cools the room.
Teacher's Note: Learn this point and write it in your own words.
11. Which mode of heat transfer can travel through a vacuum?
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Answer: Radiation.
Teacher's Note: Use the key term in your answer.
12. Name a good and a bad radiator of heat.
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Answer: Good: dull black surface. Bad: shiny white surface.
Teacher's Note: Give an example if the question allows.
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Revision Notes and Key Concepts for Class 7 Science Chapter 04 Heat
Quick Revision Notes: Chapter 04 Heat (CBSE)
Access structured revision notes for Chapter 04 Heat designed in alignment with the latest CBSE curriculum for Class 7 Science. These summaries help clarify core themes and support effective daily study.
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