Class 7 Science Chapter 4: Heat — Full NCERT Notes
Introduction: Understanding Heat
Have you ever wondered why a steel spoon left in a cup of hot tea becomes warm after some time, or why we feel cool near a sea breeze in the evening? All these are because of a special form of energy called heat.
In science, heat is the energy that transfers from one object to another because of a difference in temperature. Heat energy makes molecules move faster. When an object absorbs heat, its particles vibrate more rapidly and move apart, making it feel warmer. On the other hand, when an object loses heat, its particles move slowly and come closer, making it feel cooler.
Everything around us — from the air, water, and land to our own body — contains some amount of heat energy. Understanding how heat works helps us cook food, make machines, study weather patterns, and even survive in extreme climates.
Hot and Cold: How Do We Know?
In daily life, we often describe objects as hot or cold. For example, tea feels hot, and ice cream feels cold. But our sense of touch is not always reliable. If you keep one hand in cold water and another in warm water for a minute, and then dip both into a bowl of lukewarm water, the same water feels warm to one hand and cool to the other.
This happens because our sense of touch depends on how heat flows between our body and the object we touch. So, scientists use a measurable scale — temperature — to accurately determine how hot or cold something is.
What is Temperature?
Temperature is the degree of hotness or coldness of a body. It tells us how much heat energy an object possesses. It is measured using a device called a thermometer.
If two objects are at different temperatures, heat always flows from the object at a higher temperature to the one at a lower temperature until both reach the same temperature. This process is known as thermal equilibrium.
Example:
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When you put an ice cube in water, heat flows from water to ice, melting the ice.
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When you touch a metal spoon in a cup of hot tea, heat flows from the tea to the spoon, then to your hand.
Measuring Temperature
The Thermometer
A thermometer is an instrument used to measure temperature. It consists of a narrow glass tube with a small bulb at one end, containing a liquid like mercury or colored alcohol. The liquid expands when heated and contracts when cooled. The scale on the thermometer shows the temperature reading in degrees Celsius (°C) or degrees Fahrenheit (°F).
Clinical Thermometer
A clinical thermometer is used to measure the body temperature of humans. It has a range from 35°C to 42°C (or 94°F to 108°F). Inside it, there’s a thin capillary tube filled with mercury and a kink near the bulb.
The kink prevents the mercury from falling back after the thermometer is taken out of the mouth. It allows doctors to read the temperature easily.
Normal human body temperature:
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37°C or 98.6°F
How to Use a Clinical Thermometer Safely:
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Shake the thermometer gently to bring the mercury level below 35°C.
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Wash it with antiseptic before and after use.
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Place the bulb under the tongue for about one minute.
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Take it out and read the mercury level while keeping it at eye level.
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Do not use a thermometer that is broken or cracked.
Laboratory Thermometer
A laboratory thermometer is used to measure temperatures of liquids and substances in experiments. It generally ranges from –10°C to 110°C. Unlike a clinical thermometer, it does not have a kink because readings need to change continuously.
To get accurate readings, the bulb must be fully dipped in the liquid without touching the sides or bottom of the container.
Digital Thermometers
Nowadays, digital thermometers are becoming more common. They display temperature in numbers on a digital screen and do not use mercury, making them safer and more environmentally friendly.
They are easy to use, accurate, and fast. Many even give an alert sound when the reading is ready.
Transfer of Heat: How Does Heat Move?
Heat can move from one place to another in three main ways:
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Conduction
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Convection
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Radiation
These methods explain how energy flows in solids, liquids, and gases.
1. Conduction
Conduction is the process by which heat is transferred through solids without the particles of the solid moving from one place to another.
When one end of a metal rod is heated, the particles near that end start vibrating faster and pass their energy to the next particles, which in turn pass it along. This continues until the entire rod becomes hot.
Example:
If you place a metal spoon in a hot cup of soup, the handle of the spoon soon becomes warm even though it is not in contact with the soup directly.
Metals such as copper, iron, and aluminum conduct heat well, so they are called good conductors. Substances like wood, plastic, and rubber do not conduct heat well and are known as poor conductors or insulators.
Applications:
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Cooking utensils are made of metals because they conduct heat quickly.
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Handles of utensils are made of wood or plastic to protect hands from heat.
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Electrical wires are coated with plastic to prevent heat and electric shock.
2. Convection
Convection occurs mainly in liquids and gases. It is the process of heat transfer through the movement of the heated particles themselves.
When water in a pan is heated, the water near the bottom becomes hot, expands, and rises. The cooler, denser water at the top moves down to replace it. This circular motion is called a convection current. Eventually, all the water gets heated.
Convection in Air:
Air also shows convection. The air near a heat source becomes warm and rises, and cooler air moves in to take its place. This process repeats continuously, creating air circulation.
Examples in Daily Life:
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Heating water or milk in a pot.
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Hot air balloons rise because the air inside them is heated and becomes lighter.
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Winds and breezes on Earth are caused by convection of air.
Sea Breeze and Land Breeze
Sea Breeze: During the day, the land gets heated faster than the sea. The air above the land becomes hot and rises. Cooler air from the sea moves in to replace it, forming a gentle sea breeze that blows from the sea toward the land.
Land Breeze: At night, the land cools faster than the sea. The warmer air over the sea rises, and the cooler air from the land moves toward the sea, forming a land breeze.
These natural breezes help regulate coastal temperatures and are caused entirely by convection currents in air.
3. Radiation
Radiation is the transfer of heat without any medium. It can occur even through a vacuum. The Sun’s heat reaches the Earth through radiation.
All objects radiate (emit) and absorb heat. However, dark and dull surfaces absorb and emit radiation better than shiny and light-colored surfaces.
That’s why:
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We wear light-colored clothes in summer — they reflect sunlight.
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We wear dark-colored clothes in winter — they absorb more heat.
Experiments on Radiation
Take two tin cans, one painted black and another shiny silver. Fill both with equal amounts of hot water and leave them in a cool room. After a while, the black can cools faster because it emits heat more effectively.
This shows that black surfaces are better at emitting heat, while shiny surfaces reflect heat.
The Concept of Heat Capacity
Different substances require different amounts of heat to change their temperature. This is called heat capacity or specific heat.
For example, it takes more time to heat water than to heat the same amount of oil, even with the same flame. This means water has a higher specific heat capacity.
That’s why coastal areas have moderate climates — the sea absorbs and releases heat slowly, keeping the air temperature steady.
Thermos Flask — Preventing Heat Loss
A thermos flask keeps liquids hot or cold for a long time by preventing heat transfer through conduction, convection, and radiation.
It consists of:
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A double-walled glass container with a vacuum between the walls (prevents conduction and convection).
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Inner walls coated with silver (reduces radiation).
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Cork or plastic stopper (insulator).
Thus, heat cannot easily enter or leave, keeping the temperature constant.
Role of Heat in Daily Life
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Cooking: Heat is used to cook food and change its chemical properties.
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Ironing: Heat straightens fabric fibers.
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Refrigeration: Heat is absorbed by cooling systems to preserve food.
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Weather: Sun’s heat drives the water cycle, wind, and seasons.
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Health: Body temperature maintenance is vital for life processes.
Regulation of Body Temperature
Our body maintains a steady temperature of about 37°C. When we feel hot, sweat glands release moisture that evaporates and cools the body. When it’s cold, we shiver — our muscles contract rapidly, generating heat.
This natural system keeps our internal temperature balanced regardless of external conditions.
Heat and Climate
Heat plays a huge role in Earth’s climate. It influences rainfall, monsoons, winds, and ocean currents. Uneven heating between land and sea creates pressure differences that lead to wind movement.
The Sun’s radiant heat also drives photosynthesis, oceanic evaporation, and the greenhouse effect — all crucial for maintaining life.
Heat Energy and Its Effects
Heat can cause:
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Expansion: Metals and liquids expand when heated.
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Change in State: Ice melts into water; water turns into vapor when heated.
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Chemical Change: Heat can cause new substances to form — for example, cooking food.
Interesting Facts about Heat
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Space is a vacuum, yet we feel the Sun’s heat — that’s radiation!
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Black cars heat faster in the sun because black absorbs more radiation.
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Air conditioners cool rooms by removing heat, not by adding cold air.
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The desert becomes very hot in the day but cold at night because sand loses heat quickly.
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Penguins have feathers that trap air to reduce heat loss.
Conclusion
Heat is one of the most essential forms of energy in our lives. From the warmth of the Sun to the comfort of our home, from cooking food to running machines — heat is everywhere. Understanding its flow through conduction, convection, and radiation helps us use it efficiently and safely.
The study of heat not only strengthens our scientific knowledge but also connects us deeply with nature, weather, and daily human comfort.
