Ch 4: Heat — NCERT Curiosity

1 Hot and Cold

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.

The Three Bowls Experiment

This is a classic NCERT experiment that shows why we need a thermometer instead of relying on our hands.

Left Hand (from Hot water)

When placed in lukewarm water, it feels COLD because it was in hot water before.

Right Hand (from Cold water)

When placed in the same lukewarm water, it feels WARM because it was in cold water before.

Conclusion

The same water feels different to each hand! Our sense of touch is not reliable for measuring temperature.

Key Takeaway: This is exactly why we need a thermometer -- our senses can be deceived! A thermometer gives us a precise, objective measurement of temperature that does not depend on our feelings.

2 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.

What is Temperature?

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 degree Celsius (°C).

What is Heat?

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).

Temperature

Degree of hotness or coldness
Measured in °C or °F
Measured with a thermometer
Does NOT depend on amount of substance

Heat

Form of energy
Measured in Joule (J)
Flows from hot to cold
Depends on mass, temperature & material

Key Difference

Temperature = how hot (quality)
Heat = how much energy (quantity)
A cup of tea at 80°C has less heat than a bathtub at 80°C
But both have the same temperature

How Does Heat Flow?

Heat always flows from a hotter body to a colder body until both reach the same temperature. This state is called thermal equilibrium.

1

Hot Body

Has higher temperature, gives away heat energy

→

Heat Flows

Energy transfers from hotter to colder

3

Cold Body

Has lower temperature, receives heat energy

4

Equilibrium

Both reach the same temperature, heat flow stops

Heat always flows from
HOTTER body

→

COLDER body
until both reach same temperature

Example: When you put a cold spoon in a cup of hot soup, heat flows from the hot soup (higher temperature) to the cold spoon (lower temperature). The spoon becomes warm and the soup cools slightly. This continues until both reach the same temperature.

Remember: Heat and temperature are NOT the same! Heat is energy in transit, while temperature is a measure of hotness. A large lake at 30°C contains much more heat energy than a small cup of tea at 80°C, even though the tea has a higher temperature.

3 Thermometer

A thermometer is a device used to measure the temperature of an object or the surroundings. The most common thermometer uses mercury (a liquid metal) or alcohol that expands when heated.

Types of Thermometers

There are two main types of thermometers that NCERT covers for this chapter:

Clinical Thermometer

Laboratory Thermometer

Comparison: Clinical vs Laboratory Thermometer

Feature	Clinical Thermometer	Laboratory Thermometer
Range	35°C to 42°C	-10°C to 110°C
Use	Measuring body temperature	Measuring temperature of liquids, solids in lab
Kink	Yes -- prevents mercury from falling back	No -- no kink present
Jerking required?	Yes -- jerk to bring mercury down before use	No -- mercury falls on its own
Reading method	Can be read after removing from body	Must be read while keeping in the substance
Smallest division	0.1°C	1°C

Temperature Scales

Celsius (°C)

Water freezes at 0°C
Water boils at 100°C
Used in India and most countries
Also called Centigrade scale

Fahrenheit (°F)

Water freezes at 32°F
Water boils at 212°F
Used in USA for daily temperatures
Normal body temp = 98.6°F

Memory Trick: Normal human body temperature = 37°C or 98.6°F. A clinical thermometer only needs to cover a narrow range around body temperature (35°C to 42°C) because that is all we need to check for fever!

How to Read a Thermometer

Reading Tips

Hold the thermometer upright at eye level
Read the scale at the top of the mercury thread
Keep your line of sight perpendicular to the scale
Note the smallest division to read accurately

Precautions

Wash the clinical thermometer before and after use
Jerk clinical thermometer to bring mercury level down before use
Hold by the stem, NOT by the bulb
Never use a clinical thermometer to measure hot liquids
Handle mercury thermometers with care -- mercury is toxic

Why the kink? The clinical thermometer has a small bend (kink) near the bulb. When you remove the thermometer from your mouth, the kink prevents the mercury from flowing back into the bulb. This lets you read the temperature after taking it out. The laboratory thermometer has no kink, so you must read it while it is still in the substance.

4 Transfer of Heat — Conduction

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.

How Conduction Works

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.

Wax drops melt and fall off one by one -- closest to the flame first!

Conductors and Insulators

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).

⚡

Good Conductors

Allow heat to pass quickly

Iron -- used in cooking pans
Copper -- best heat conductor
Aluminium -- cooking utensils
Silver -- best conductor of all
Steel -- industrial use

Heat flows easily

🛡

Poor Conductors (Insulators)

Block or slow heat transfer

Wood -- handles of utensils
Plastic -- handles, covers
Rubber -- gloves
Air -- between window panes
Cloth -- woollen clothes

Heat is blocked

Real-Life Example

Why metal handles get hot?

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!

Why wooden handles stay cool?

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!

Think About It: That is why cooking utensils have wooden or plastic handles! The metal body of the utensil conducts heat for cooking, while the insulating handle protects your hands from getting burned.

Easy way to remember: Metals = Good conductors (let heat pass). Non-metals = Poor conductors / Insulators (block heat). Exception: Graphite (a non-metal) conducts heat, but for NCERT Class 7 this is not required.

5 Transfer of Heat — Convection

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.

How Convection Works in Water

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.

Hot water rises, cool water sinks -- creating a continuous circular convection current

Sea Breeze and Land Breeze

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.

Sea Breeze (Daytime)

Land Breeze (Nighttime)

Sea Breeze (Day)

During the day, land heats up faster than sea
Hot air over land rises up
Cool air from the sea rushes in to take its place
This cool wind from sea to land = Sea Breeze

Land Breeze (Night)

At night, land cools down faster than sea
Sea water is still relatively warm
Warm air over sea rises up
Cool air from land moves towards sea = Land Breeze

Did you know? Convection currents cause winds, ocean currents, and even weather patterns! The monsoons in India are also driven by large-scale convection: the Indian subcontinent heats up in summer, causing moist air from the Indian Ocean to rush in, bringing rain.

Remember: Convection happens only in fluids (liquids and gases), NOT in solids. In solids, heat transfers by conduction. The key difference: in conduction, particles vibrate in place; in convection, particles actually move from one location to another.

☀️ Transfer of Heat — Radiation

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.

How Radiation Works

Dark vs Light Coloured Surfaces

Not all surfaces absorb radiation equally. Dark-coloured surfaces absorb more heat radiation, while light-coloured surfaces reflect more heat radiation.

Examples of Radiation

Sun: Heat from the Sun reaches Earth through radiation across the vacuum of space
Campfire: You feel warmth on your face from a distance
Room heater: Heats the room even without heating the air in between
Hot iron: You can feel heat near a hot press without touching it

Dark vs Light Surfaces

Dark surfaces are good absorbers of radiation
Light surfaces are good reflectors of radiation
The back of a spoon (shiny side) reflects heat; blackened side absorbs heat
Solar cookers have black surfaces inside to absorb maximum heat

That is why we wear light-coloured clothes in summer and dark-coloured clothes in winter! Light colours reflect heat and keep us cool in summer. Dark colours absorb heat and keep us warm in winter. Similarly, the bottom of cooking pots is often painted black to absorb more heat.

Key Difference: Conduction needs a solid medium. Convection needs a fluid medium (liquid/gas). Radiation needs NO medium at all — it can travel through vacuum!

👕 Clothing and Heat

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.

Why Two Thin Layers Are Warmer Than One Thick Layer

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!

☀️ Summer Clothes

Light colours — reflect heat radiation
Cotton fabric — absorbs sweat, allows evaporation
Loose fitting — allows air circulation
Thin material — does not trap too much heat

❄️ Winter Clothes

Dark colours — absorb heat radiation
Woolen fabric — traps air, poor conductor
Multiple layers — trap more air between them
Tight weave — prevents cold air from entering

Fun Fact: Penguins huddle together in groups to keep warm in Antarctica. By pressing close together, they trap warm air between their bodies and reduce exposure to the cold wind. Animals like polar bears have thick fur that traps air, working just like our woolen clothes!

Remember: Woolen clothes do NOT produce heat. They only prevent our body heat from escaping by trapping air. Air is the real insulator here!

🔬 NCERT Activities

All 7 activities from NCERT Chapter 4 — Heat. Try the interactive versions below!

NCERT Activity 4.1

Hot and Cold Sensation — Three Bowls Experiment

Bowl A (Hot)

Bowl C (Lukewarm)

Bowl B (Cold)

Click a button to start the experiment!

NCERT DIAGRAM

Fig 4.1: Three Bowls Experiment

Aim: To show that our sense of touch is not reliable for measuring temperature.

Procedure:

Take three bowls: A (hot water), B (cold water), C (lukewarm water).
Place your left hand in bowl A and right hand in bowl B for one minute.
Now put both hands in bowl C (lukewarm water).

Observation: The left hand (from hot water) feels the lukewarm water as cold. The right hand (from cold water) feels the same water as warm. This proves our sense of touch is unreliable and we need a thermometer for accurate measurements.

NCERT Activity 4.2

Reading a Thermometer

Aim: To learn how to read a thermometer correctly.

Procedure:

Click the temperature buttons above to see the mercury level change in the thermometer.
Notice how the mercury rises when the temperature increases and falls when it decreases.
The mercury level indicates the temperature on the scale.

Observation: The mercury level in a thermometer rises with increase in temperature and falls with decrease. At 100°C (boiling point of water), the mercury reaches the top. At 0°C (freezing point), it is at the bottom of the scale.

NCERT Activity 4.3

Conduction Along a Metal Rod

Click "Heat the Rod" to begin!

NCERT DIAGRAM

Fig 4.3: Wax drops on heated rod

Aim: To demonstrate that heat is conducted along a metal rod from the hotter end to the cooler end.

Procedure:

Take an iron rod and fix wax drops at equal intervals along it using melted wax.
Clamp the rod and heat one end using a candle or burner.
Observe the wax drops as the rod heats up.

Observation: The wax drop nearest to the flame melts first, then the next one, and so on. The wax drops melt in order from the heated end to the far end. This shows that heat travels along the rod by conduction.

NCERT Activity 4.4

Conductors and Insulators of Heat

Click on an object to test whether it conducts heat:

Click an object to test it!

Aim: To classify materials as good conductors or poor conductors (insulators) of heat.

Procedure:

Dip each object into hot water and wait for a few seconds.
Feel the other end of the object to check if heat has traveled through it.
If you can feel the heat, it is a good conductor. If not, it is a poor conductor (insulator).

Observation: Metal spoon and aluminium foil become hot quickly — they are good conductors. Wooden spoon, plastic ruler, glass rod, and paper do not become hot quickly — they are poor conductors (insulators).

NCERT Activity 4.5

Convection in Water (KMnO₄ Experiment)

Click "Heat the Water" to see convection!

NCERT DIAGRAM

Fig 4.5: KMnO4 convection experiment

Aim: To demonstrate convection currents in water.

Procedure:

Take a round-bottom flask and fill it with water.
Drop a small crystal of potassium permanganate (KMnO₄) at the bottom.
Heat the flask gently from below using a burner.

Observation: Purple streaks from the KMnO₄ crystal rise upward from the bottom, spread at the top, and then come down along the sides. This creates a circular pattern called convection current. The heated water rises (becomes lighter), and cooler water sinks to take its place.

NCERT Activity 4.6

Sea Breeze and Land Breeze

Aim: To understand the formation of sea breeze and land breeze.

Procedure:

Day: Land heats up faster than sea. Hot air over land rises. Cool air from sea rushes in = Sea Breeze.
Night: Land cools faster than sea. Sea is relatively warmer. Warm air over sea rises. Cool air from land moves to sea = Land Breeze.
Toggle the button above to see how the wind direction reverses between day and night.

Observation: During the day, the breeze blows from sea to land (sea breeze). At night, it reverses and blows from land to sea (land breeze). Both are caused by convection — warm air rising and cool air taking its place.

NCERT Activity 4.7

Dark vs Light Coloured Surfaces and Radiation

Black Can: 25°C White Can: 25°C

Click "Place in Sunlight" to start!

Aim: To show that dark-coloured surfaces absorb more radiation than light-coloured surfaces.

Procedure:

Take two identical metal cans — paint one black and one white.
Fill both with the same amount of water at the same temperature.
Place a thermometer in each and keep both cans in sunlight.
Record the temperature at regular intervals.

Observation: The thermometer in the black can shows a higher temperature than the white can. The black surface absorbs more heat radiation from the sun, while the white surface reflects most of the radiation. This is why we wear light-coloured clothes in summer!

🧠 Practice Questions

Click on any option — the correct one will be highlighted with the answer explanation.

Multiple Choice Questions

1. Heat travels from the Sun to the Earth by which process?
- (a) Conduction
- (b) Convection
- (c) Radiation
- (d) All of these
Answer: (c) Radiation. Heat from the Sun travels through the vacuum of space as radiation. Conduction and convection need a medium, but there is no medium in space.
2. A clinical thermometer has a range of:
- (a) 0°C to 100°C
- (b) -10°C to 110°C
- (c) 35°C to 42°C
- (d) 30°C to 50°C
Answer: (c) 35°C to 42°C. A clinical thermometer is designed to measure body temperature, which normally ranges between 35°C and 42°C.
3. Which of the following is a good conductor of heat?
- (a) Wood
- (b) Plastic
- (c) Copper
- (d) Air
Answer: (c) Copper. Metals like copper, iron, and aluminium are good conductors of heat. Wood, plastic, and air are poor conductors (insulators).
4. Sea breeze blows during:
- (a) Day time, from sea to land
- (b) Night time, from sea to land
- (c) Day time, from land to sea
- (d) Night time, from land to sea
Answer: (a) During the day, land heats up faster than sea. Hot air over land rises and cool air from the sea rushes in. This is called sea breeze.
5. Woolen clothes keep us warm because:
- (a) Wool produces heat
- (b) Wool traps air which is a poor conductor of heat
- (c) Wool conducts heat from the surroundings to our body
- (d) Wool absorbs heat from the sun
Answer: (b) Wool does not produce heat. It traps air in its fibres, and air is a poor conductor of heat. This trapped air prevents our body heat from escaping.
6. In which mode of heat transfer do particles actually move from one place to another?
- (a) Conduction
- (b) Convection
- (c) Radiation
- (d) None of these
Answer: (b) In convection, the heated fluid (liquid or gas) actually moves from one place to another, carrying heat with it. In conduction, particles only vibrate in place. In radiation, no medium is needed at all.
7. Dark-coloured clothes are preferred in winter because:
- (a) They reflect more heat
- (b) They absorb more heat from the sun
- (c) They are thicker
- (d) They trap more air
Answer: (b) Dark-coloured surfaces are good absorbers of heat radiation. They absorb more heat from the sun and keep us warmer in winter.
8. The normal temperature of the human body is:
- (a) 98.6°C
- (b) 36°C
- (c) 37°C
- (d) 40°C
Answer: (c) 37°C (or 98.6°F). A temperature significantly above 37°C usually indicates a fever.

Short Answer Questions

1. Why do we wear light-coloured clothes in summer?

Ans: Light-coloured clothes reflect most of the heat radiation from the sun. This keeps our body cooler. Dark-coloured clothes absorb more heat, making us feel hotter.
2. What is the difference between conduction and convection?

Ans: In conduction, heat transfers through a solid medium without the particles moving from their position — they only vibrate. In convection, the heated particles of a fluid (liquid/gas) actually move from one place to another, carrying heat with them. Conduction occurs mainly in solids; convection occurs in liquids and gases.
3. Why does the mercury in a clinical thermometer not fall on its own?

Ans: A clinical thermometer has a small kink (bend) near the bulb. This kink prevents the mercury from flowing back into the bulb on its own. That is why we need to jerk the thermometer sharply to bring the mercury level down before each use.
4. Explain how sea breeze is formed.

Ans: During the day, the land heats up faster than the sea. The air over the land becomes hot and rises up. The cooler air from above the sea rushes in to take its place. This cool wind blowing from the sea towards the land is called sea breeze. It is caused by convection.
5. Why is wearing two thin layers of clothing warmer than one thick layer?

Ans: Two thin layers trap a layer of air between them. Air is a very poor conductor of heat (a good insulator). This extra trapped air layer acts as additional insulation, preventing body heat from escaping. One thick layer has no such air gap between layers.
6. Give an example to show that metals are good conductors of heat.

Ans: When we stir hot tea with a metal spoon, the handle of the spoon also becomes hot after some time. This happens because the heat from the tea travels through the metal spoon by conduction. This shows that metals are good conductors of heat. In contrast, a wooden spoon handle does not become hot because wood is a poor conductor.

📚 Chapter Summary

Key Concepts at a Glance

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.

Quick Reference: Conduction vs Convection vs Radiation

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

All 7 NCERT Activities at a Glance

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

Final Revision Checklist — Before Your Exam:
1. Can you explain the three bowls experiment?
2. Can you tell the difference between clinical and laboratory thermometer?
3. Can you explain conduction, convection, and radiation with examples?
4. Can you name 5 conductors and 5 insulators of heat?
5. Can you explain why we wear light clothes in summer and dark in winter?
6. Can you explain sea breeze and land breeze?
7. Can you explain why woolen clothes keep us warm?
If you answered YES to all, you are well prepared!

Heat

Left Hand (from Hot water)

Right Hand (from Cold water)

Conclusion

Temperature

Heat

Key Difference

Clinical Thermometer

Laboratory Thermometer

Celsius (°C)

Fahrenheit (°F)

Reading Tips

Precautions

Good Conductors

Poor Conductors (Insulators)

Why metal handles get hot?

Why wooden handles stay cool?

Sea Breeze (Daytime)

Land Breeze (Nighttime)

Sea Breeze (Day)

Land Breeze (Night)

Examples of Radiation

Dark vs Light Surfaces

☀️ Summer Clothes

❄️ Winter Clothes

Share These Notes