Chapter 4 · Class 7 Science · NCERT
We come across hot and cold objects every day -- a cup of tea is hot, ice cream is cold, and water from the tap may feel warm. But can we always trust our senses to tell us how hot or cold something is?
Surprisingly, our sense of touch is not reliable for measuring temperature! The same object can feel hot to one hand and cold to the other. Let us see how.
This is a classic NCERT experiment that shows why we need a thermometer instead of relying on our hands.
When placed in lukewarm water, it feels COLD because it was in hot water before.
When placed in the same lukewarm water, it feels WARM because it was in cold water before.
The same water feels different to each hand! Our sense of touch is not reliable for measuring temperature.
We often use the words "heat" and "temperature" as if they mean the same thing. But in science, they are quite different! Let us understand the difference.
Temperature is the degree of hotness or coldness of a body. It tells us how hot or cold an object is. Temperature is measured using a thermometer and its SI unit is Kelvin (K). In everyday use, degree Celsius (°C) is commonly used.
Heat is a form of energy that flows from a hotter body to a colder body. It is the energy that is transferred between objects due to a difference in temperature. The SI unit of heat is Joule (J).
Heat always flows from a hotter body to a colder body until both reach the same temperature. This state is called thermal equilibrium.
Heat can travel from one place to another. The process of heat transfer through solids is called conduction. In conduction, heat is transferred from one particle to the next without the particles actually moving from their position.
Imagine you heat one end of a metal rod. The particles at the heated end start vibrating faster and pass their energy to neighbouring particles. This chain continues along the rod. The particles themselves stay in place -- only the energy is transferred.
Materials that allow heat to pass through them easily are called conductors. Materials that do NOT allow heat to pass easily are called insulators (or poor conductors).
Allow heat to pass quickly
Block or slow heat transfer
Metals are good conductors. When you heat a metal pan, the heat quickly travels through the metal to the handle, making it too hot to touch!
Wood is a poor conductor (insulator). It does not allow heat to pass through easily. That is why cooking utensils have wooden or plastic handles!
Convection is the transfer of heat in liquids and gases by the actual movement of the heated particles from one place to another. Unlike conduction, the particles themselves move, carrying heat energy with them.
When water is heated at the bottom of a container, the water near the heat source gets hot, expands, becomes lighter, and rises up. The cooler, heavier water from above sinks down to take its place. This creates a circular movement called a convection current.
Convection currents in the atmosphere cause sea breeze and land breeze. These happen because land and water heat up and cool down at different rates.
We know heat can travel through solids (conduction) and through liquids/gases (convection). But how does the heat from the Sun reach us? There is no medium (no air, no liquid) in the vacuum of space!
The answer is Radiation — a mode of heat transfer that does NOT need any medium. Heat can travel through vacuum in the form of invisible rays called infrared radiation.
Not all surfaces absorb radiation equally. Dark-coloured surfaces absorb more heat radiation, while light-coloured surfaces reflect more heat radiation.
Why do we wear woolen clothes in winter? It is not because wool produces heat — it does not! Wool is a poor conductor of heat (an insulator). It traps air between its fibres, and air itself is a very poor conductor. This trapped air prevents our body heat from escaping, keeping us warm.
Wearing two thin sweaters is actually warmer than one thick sweater of the same total thickness. This is because the air trapped between the two layers acts as an extra insulating layer. More trapped air = better insulation!
All 7 activities from NCERT Chapter 4 — Heat. Try the interactive versions below!
Click a button to start the experiment!
Aim: To show that our sense of touch is not reliable for measuring temperature.
Procedure:
Aim: To learn how to read a thermometer correctly.
Procedure:
Click "Heat the Rod" to begin!
Aim: To demonstrate that heat is conducted along a metal rod from the hotter end to the cooler end.
Procedure:
Click on an object to test whether it conducts heat:
Aim: To classify materials as good conductors or poor conductors (insulators) of heat.
Procedure:
Click "Heat the Water" to see convection!
Aim: To demonstrate convection currents in water.
Procedure:
Aim: To understand the formation of sea breeze and land breeze.
Procedure:
Click "Place in Sunlight" to start!
Aim: To show that dark-coloured surfaces absorb more radiation than light-coloured surfaces.
Procedure:
Click an option — correct answer highlights automatically.
Write answers in 1–2 lines. Direct recall from NCERT.
Write in 30–40 words with 2 clear NCERT points.
Write in 60–80 words with 3 clear NCERT points.
Long answers (5 marks). Use sub-headings and specific NCERT facts.
(1) Conduction: Transfer of heat through solid material. Particles do NOT move — they only vibrate and pass energy to adjacent particles. Requires solid medium. Example: Metal spoon in hot tea gets hot at the handle. Metals are good conductors; wood, plastic, rubber are poor conductors (insulators).
(2) Convection: Transfer of heat in fluids (liquids and gases). Heated particles actually move, setting up convection currents. Requires liquid or gas medium. Examples: (a) Boiling water — hot water rises, cool water sinks; (b) Sea breeze and land breeze; (c) Atmospheric winds.
(3) Radiation: Transfer of heat without any medium — travels through vacuum at speed of light. Examples: (a) Sun's heat reaching Earth; (b) Warmth felt near a campfire without touching it; (c) Solar cookers.
Key differences: Conduction = particles vibrate (solid); Convection = particles move (fluid); Radiation = no particles needed (vacuum possible). All three can occur simultaneously — near a fire, radiation warms you, convection carries hot air upward, and touching a heated surface causes conduction.
Introduction: Our choice of colours and materials is closely linked to how they absorb, reflect, or conduct heat.
(1) Light-coloured clothes in summer: Light/white surfaces are good reflectors of radiation. They reflect most sunlight, preventing heat from reaching our body — we feel cooler.
(2) Dark-coloured clothes in winter: Dark/black surfaces are good absorbers of radiation. They absorb more sunlight and warm our body.
(3) Metal vessels with insulating handles: Metal conducts heat efficiently from stove to food. Wood/plastic handles are insulators — they don't conduct heat to our hands.
(4) Woolen clothes in winter: Wool fibres trap air. Air is a very poor conductor. The trapped air prevents body heat from escaping. Wool does NOT produce heat — it conserves it.
(5) Solar cookers painted black: Black interior absorbs maximum solar radiation. Glass cover traps heat (greenhouse effect). This enables cooking using sunlight without fuel.
| Concept | Key Points |
|---|---|
| Hot and Cold | Our sense of touch is unreliable for measuring temperature. Same water can feel different to each hand. |
| Temperature | Degree of hotness/coldness of a body. Measured in °C or °F using a thermometer. |
| Clinical Thermometer | Range: 35°C to 42°C. Has a kink. Used to measure body temperature. |
| Laboratory Thermometer | Range: -10°C to 110°C. No kink. Read while in the substance. |
| Transfer of Heat | Heat flows from a hotter body to a cooler body until both reach the same temperature. |
| Conduction | Heat transfer through a material without particles moving. Occurs mainly in solids. |
| Conductor | Material that allows heat to pass easily. Example: metals (copper, iron, aluminium). |
| Insulator | Material that does NOT allow heat to pass easily. Example: wood, plastic, air, wool. |
| Convection | Heat transfer by actual movement of heated particles. Occurs in liquids and gases. |
| Radiation | Heat transfer without any medium. Can travel through vacuum. Example: Sun to Earth. |
| Dark Surfaces | Good absorbers of radiation. Heat up faster. Used in solar cookers. |
| Light Surfaces | Good reflectors of radiation. Stay cooler. Light-coloured clothes in summer. |
| Woolen Clothes | Trap air (poor conductor). Prevent body heat from escaping. Do NOT produce heat. |
| Feature | Conduction | Convection | Radiation |
|---|---|---|---|
| Medium needed? | Yes (solid) | Yes (liquid/gas) | No (can travel through vacuum) |
| Particle movement | Vibrate in place | Actually move | No particles needed |
| Occurs in | Mainly solids | Liquids and gases | All media + vacuum |
| Speed | Slow | Medium | Fast (speed of light) |
| Example | Hot spoon handle | Boiling water, sea breeze | Sunlight, campfire warmth |
| Direction | Along the material | Circular currents | All directions from source |
| Activity | What It Demonstrates | Key Observation |
|---|---|---|
| 4.1 Three Bowls | Unreliable sense of touch | Same lukewarm water feels different to each hand |
| 4.2 Thermometer | Reading temperature | Mercury level rises with temperature |
| 4.3 Wax on Rod | Conduction in metals | Wax drops melt in order from heated end |
| 4.4 Conductor Test | Conductors vs Insulators | Metals conduct; wood/plastic/air do not |
| 4.5 KMnO4 in Water | Convection currents | Purple streaks rise, spread, and come down |
| 4.6 Sea/Land Breeze | Convection in air | Wind direction reverses between day and night |
| 4.7 Dark vs Light Cans | Radiation absorption | Black can heats up faster than white can |