Chapter Overview & Weightage
Heat and Temperature is a foundational Class 7 Science chapter. Expect 4–6 marks in school exams — usually a mix of one-mark definitions, two-mark short answers, and one diagram question.
| Question Type | Marks | Common Topics |
|---|---|---|
| Definitions | 1 each | Temperature, Conduction, Convection, Radiation |
| Short answer | 2 | Difference between heat and temperature |
| Diagram | 2–3 | Clinical thermometer, conductors/insulators |
This chapter is the building block for heat-related chapters in Classes 8, 9, and 10. Understanding WHY heat flows — not just that it flows — will help you for years.
Key Concepts You Must Know
1. Heat vs Temperature
Temperature is the measure of how hot or cold an object is. It tells us the average kinetic energy of molecules. Measured in °C, °F, or Kelvin (K).
Heat is a form of energy that flows from a hotter object to a cooler one. It is measured in Joules (J) or calories (cal). A large ice block and a small ice block are at the same temperature but the large block has more heat (energy).
| Property | Heat | Temperature |
|---|---|---|
| What it is | Form of energy | Degree of hotness/coldness |
| SI Unit | Joule (J) | Kelvin (K) or °C |
| Instrument | Calorimeter | Thermometer |
| Depends on | Amount of substance + temperature difference | Average KE of molecules |
2. Thermometers
- Clinical thermometer: Range 35°C to 42°C. Has a kink (constriction) near the bulb that prevents mercury from flowing back — allows reading after removal from body.
- Laboratory thermometer: Range −10°C to 110°C. No kink. Used for general temperature measurements.
3. Modes of Heat Transfer
Conduction: Heat transfers through a substance by particle-to-particle vibration, without bulk movement of particles. Occurs mainly in solids.
Conductors: Materials that allow heat to pass easily — metals (iron, copper, aluminium).
Insulators: Materials that do not allow heat to pass easily — wood, rubber, plastic, air.
Convection: Heat transfers through fluids (liquids and gases) by actual movement of particles. Hot fluid rises, cool fluid sinks — creating convection currents. This is why we boil water from the bottom.
Radiation: Heat transfers without any medium — through electromagnetic waves. The Sun’s heat reaches Earth through empty space via radiation. Dark-coloured surfaces absorb and radiate heat better than light/polished surfaces.
4. Temperature Scales and Conversion
Normal body temperature: 37°C = 98.6°F.
Important Formulas
Water freezes at 0°C (32°F) and boils at 100°C (212°F).
Solved Examples
Example 1 — Definitions (1 mark each)
Q: What is the kink in a clinical thermometer? Why is it important?
Solution: The kink is a narrow constriction in the glass tube just above the bulb. It prevents mercury from flowing back into the bulb after the thermometer is removed from the patient’s body. This allows the temperature to remain stable while reading.
Example 2 — Short Answer (2 marks)
Q: A metal spoon feels colder to touch than a wooden spoon kept in the same room. Why?
Solution: Both spoons are at the same temperature (room temperature). However, metal is a good conductor of heat. When you touch the metal spoon, heat flows rapidly from your hand to the metal — your hand loses heat quickly, so it feels cold. Wood is a poor conductor (insulator), so heat flows slowly from your hand to wood — your hand doesn’t lose heat as fast, so it feels comparatively warmer.
Example 3 — Application (3 marks)
Q: Why are woollen clothes warmer in winter?
Solution: Wool itself is a poor conductor of heat (it is an insulator). But more importantly, woollen fibres trap air between them. Trapped air is also a poor conductor of heat. This layer of trapped air prevents heat from escaping your body to the surroundings. So your body heat is retained, keeping you warm. This is why a thicker woollen sweater is warmer — it traps more air.
Difficulty Distribution
| Difficulty | % | Topics |
|---|---|---|
| Easy | 50% | Definitions, identify conductor/insulator |
| Medium | 35% | Explain why, fill in blanks, differences |
| Hard | 15% | Temperature conversion, diagram labelling |
Expert Strategy
For full marks on “explain why” questions: Always give the mechanism, not just the conclusion. “Metal spoon feels cold because it conducts heat quickly” = 1 mark. Adding “because heat flows from your warm hand to the cooler metal rapidly” = full marks.
Temperature conversion: Know the formula cold. Practice: 25°C = ? °F. Answer: (9/5 × 25) + 32 = 45 + 32 = 77°F. Reverse: 98.6°F to °C: (5/9) × (98.6 − 32) = (5/9) × 66.6 = 37°C.
For diagram questions: label the clinical thermometer with (a) bulb — contains mercury, (b) kink/constriction, (c) scale (35–42°C), (d) glass tube. These four labels cover most marking schemes.
Common Traps
Trap 1: Saying “heat and temperature are the same thing.” They are not. A burning matchstick is at higher temperature than a bucket of warm water, but the bucket contains far more heat (energy). Temperature measures degree of hotness; heat measures total thermal energy.
Trap 2: Saying radiation requires a medium. Radiation travels through vacuum — it’s electromagnetic waves, not molecular motion. Conduction and convection both require a material medium; radiation does not.
Trap 3: Saying the kink “holds” mercury. The kink doesn’t hold mercury in place by clamping it — it prevents backward flow because the constriction has insufficient pressure to push mercury back. Students sometimes draw the kink at the wrong location (it must be near the bulb, not the middle).