Heat - Class 7 Science (CBSE)
Based on the 2025-26 NCERT syllabus for Class 7 Science. This chapter explains heat as a form of energy, how it is measured, and how it transfers between objects.
1. Why this chapter matters
Heat is a fundamental form of energy that affects everything around us -- from cooking food to weather patterns. Understanding heat transfer helps us design better homes, clothing, and cooking utensils. In CBSE exams, this chapter contributes 6-8 marks.
2. Temperature and thermometers
Temperature
Temperature is a measure of how hot or cold an object is. It is measured using a thermometer.
Laboratory thermometer
A laboratory thermometer is used to measure temperature in experiments. It has a bulb containing mercury or alcohol at one end and a scale marked in degrees Celsius.
Clinical thermometer
The clinical thermometer measures body temperature. Its range is 35 C to 42 C. It has a kink that prevents the mercury from falling back after reading.
Digital thermometer
A digital thermometer shows temperature on an electronic display. It is safer because it does not contain mercury.
3. Comparison of thermometers
| Feature | Clinical thermometer | Laboratory thermometer |
|---|---|---|
| Range | 35 C to 42 C | -10 C to 110 C |
| Kink | Present | Absent |
| Uses | Body temperature | General experiments |
| Liquid | Mercury/alcohol | Mercury/alcohol |
| Accuracy | More precise for body | General purpose |
4. Heat transfer: conduction
Conduction is the transfer of heat through a solid material without the movement of the material itself.
Heat flows from the hotter end to the colder end. In metals, heat is transferred through the vibration of atoms and the movement of free electrons.
Conductors and insulators
- Conductors: Materials that allow heat to pass through them easily. Examples: copper, iron, aluminium, silver.
- Insulators: Materials that do NOT allow heat to pass through them easily. Examples: wood, plastic, wool, air, rubber.
Why cooking pans have metal bodies and plastic handles
The metal body conducts heat quickly to cook food. The plastic handle is an insulator, protecting the hand from burns.
5. Heat transfer: convection
Convection is the transfer of heat in liquids and gases by the actual movement of the heated particles.
When a liquid or gas is heated, it expands, becomes less dense, and rises. The cooler, denser fluid sinks to take its place. This creates convection currents.
Sea breeze
During the day, land heats up faster than the sea. The warm air over land rises, and the cooler air from the sea moves in to replace it. This is called a sea breeze.
Land breeze
At night, land cools faster than the sea. The warm air over the sea rises, and the cooler air from the land moves towards the sea. This is called a land breeze.
6. Heat transfer: radiation
Radiation is the transfer of heat without any medium. Heat from the Sun reaches the Earth through radiation, travelling through the vacuum of space.
Hot objects emit infrared radiation. Dark, dull surfaces absorb and emit heat radiation better than light, shiny surfaces.
Applications of radiation
- Solar water heaters have black pipes (black absorbs more radiation).
- Cooking pots are kept shiny to reduce heat loss by radiation.
- White or light-coloured clothes are worn in summer to reflect radiation.
7. Comparison of heat transfer methods
| Feature | Conduction | Convection | Radiation |
|---|---|---|---|
| Medium needed | Solid | Liquid/gas | None (vacuum works) |
| Particle movement | No net movement | Particles move | No particles involved |
| Direction | Hot to cold end | Circular currents | Straight lines from source |
| Example | Metal spoon in hot soup | Boiling water | Sunlight warming Earth |
8. Worked examples
Example 1: Why does a metal chair feel colder than a wooden chair at the same temperature?
Metal is a better conductor of heat than wood. Heat from your hand flows quickly into the metal, making it feel cold. Wood is an insulator, so heat flows slowly, making it feel warmer.
Example 2: In which mode does heat from a campfire reach you?
Heat from a campfire reaches you by radiation. You can feel the heat even if you are not touching the fire or if there is air between.
Example 3: Why are cooking utensils made of metal but their handles made of plastic?
Metal conducts heat well, so the utensil heats up quickly. Plastic is a poor conductor (insulator), so the handle stays cool and safe to hold.
9. Common mistakes and how to fix them
| Mistake | Fix |
|---|---|
| Saying conduction happens in liquids | Conduction is primarily in solids. Convection happens in liquids |
| Confusing temperature with heat | Temperature is a measure; heat is a form of energy |
| Believing all metals have same conductivity | Different metals conduct heat at different rates |
| Thinking wool produces heat | Wool traps air (an insulator) and prevents body heat loss |
| Saying vacuum cannot transfer heat | Radiation works through vacuum (e.g., sunlight) |
10. CBSE exam focus
| Question type | Marks | Frequency |
|---|---|---|
| Define temperature, conduction, etc. | 1-2 marks | 1-2 questions |
| Differentiate conduction/convection/radiation | 3 marks | 1 question |
| Explain sea breeze/land breeze | 3 marks | 1 question |
| Conductors and insulators examples | 2 marks | 1 question |
| Thermometer reading and diagram | 2 marks | Occasional |
11. Self-test
- What is the range of a clinical thermometer?
- Define conduction and give two examples of conductors and insulators.
- Differentiate between sea breeze and land breeze.
- Why are cooking pots black on the outside and shiny on the inside?
- In which mode does heat transfer in a metal rod? Explain.
- Why is it advisable to wear white clothes in summer?
12. Answer key
- The range of a clinical thermometer is 35 C to 42 C.
- Conduction is heat transfer through solids without particle movement. Conductors: copper, aluminium. Insulators: wood, plastic.
- Sea breeze: wind from sea to land during day. Land breeze: wind from land to sea during night.
- Black outside absorbs more heat radiation. Shiny inside reflects heat back, keeping food warm.
- Conduction. Atoms vibrate and transfer energy along the rod from hot to cold end.
- White clothes reflect most of the Sun's radiation, keeping the body cooler.
13. Quick revision
- Temperature is measured with thermometers in degrees Celsius.
- Conduction: heat transfer in solids.
- Convection: heat transfer in fluids (liquids and gases).
- Radiation: heat transfer without a medium.
- Conductors allow heat flow; insulators resist heat flow.
- Sea breeze and land breeze are caused by convection currents.
- Dark surfaces absorb more radiation; shiny surfaces reflect more.
