Heat and it's effects,Class 8 ICSE Physics

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HEAT AND ITS EFFECTS

Class 8 ICSE Physics

Explore the science of heat, changes of state, thermal expansion and everyday applications.

Learning Objectives Rainbow Mindmap Thinking MCQs Interactive Quiz Real-Life Science

1. Learning Objectives

After studying this lesson, students will be able to:

• Define heat and state its SI unit.
• Explain the difference between heat and temperature.
• Describe the effects of heat on temperature, size and state.
• Explain thermal expansion in solids, liquids and gases.
• Differentiate between linear, superficial and cubical expansion.
• Explain melting, freezing, boiling, condensation, sublimation and solidification.
• Identify practical applications of thermal expansion.
• Use scientific reasoning to solve application-based questions.
• Follow safe and responsible practices while handling heat.

2. A Story: Riya and the Stubborn Jar

One Sunday morning, Riya wanted to open a glass jar of pickle. However, its metal lid was tightly stuck. She tried turning it several times, but it would not open.

Her grandfather suggested holding the lid under warm running water for a short time. Riya followed his advice and tried again. To her surprise, the lid opened easily!

"Grandfather, why did the warm water help?" she asked.

"Heat caused the metal lid to expand slightly," he explained. "The lid became a little larger, loosening its grip on the glass jar."

Later, Riya noticed small gaps between sections of railway tracks. Grandfather explained that these gaps allow the rails to expand during hot weather.

Riya realised that heat is not just something that makes objects hot. It can change temperature, size and even the state of matter. Understanding these effects helps us solve everyday problems.

Think and Answer:
Which effect of heat helped Riya open the jar?
Answer: Thermal expansion of the metal lid.

3. Rainbow Mindmap: Heat

Revise the entire chapter visually using the Rainbow Mindmap. Observe the seven sections and connect each concept with examples from everyday life.

How to use the mindmap:

1. Read the central definition of heat.
2. Identify its unit.
3. Understand the effects of heat.
4. Connect the concepts with real-life examples.
5. Map the three types of thermal expansion.
6. Apply the concepts to numerical and thinking questions.
7. Explain the concepts in your own words.

4. What Is Heat?

Heat is energy transferred from a body at a higher temperature to a body at a lower temperature because of a temperature difference.

Heat flows naturally from a hotter object to a colder object until they reach thermal equilibrium.

Heat

Heat is energy transferred because of a temperature difference.

SI unit: joule (J)

Temperature

Temperature indicates how hot or cold an object is.

It is commonly measured in degrees Celsius (°C).

Units of Heat

• SI unit: joule (J)
• 1 kilojoule (kJ) = 1000 joules (J)
• 1 calorie (cal) ≈ 4.2 joules (J)
• 1 joule ≈ 0.24 calorie

Remember: Heat and temperature are related but are not the same quantity.

5. Effects of Heat

Effect 1: Change in Temperature

When a substance absorbs heat, its temperature usually rises. When it loses heat, its temperature usually falls.

Example: Water becomes warmer when heated.

Effect 2: Change in State

Heat can change a substance from solid to liquid or from liquid to gas. Removing heat can reverse these changes.

Example: Ice melts into water.

Effect 3: Change in Size

Most solids, liquids and gases expand on heating and contract on cooling. This is called thermal expansion.

Example: Metal rails expand in hot weather.

Effect 4: Chemical Change

Heat can initiate or speed up chemical changes. Some processes produce new substances.

Example: Cooking food and burning paper.

6. Change in State: Six Important Processes

A substance can change from one state of matter to another when heat is absorbed or released.

1. Melting

Solid → Liquid

A solid absorbs heat and changes into a liquid.

Example: Ice changing into water.

2. Freezing

Liquid → Solid

A liquid loses heat and changes into a solid.

Example: Water forming ice.

3. Boiling or Vaporisation

Liquid → Gas

A liquid changes into vapour. During boiling at a fixed pressure, the temperature remains constant.

Example: Water boiling in a pan.

4. Condensation

Gas → Liquid

A gas loses heat and changes into a liquid.

Example: Water droplets forming on a cold glass.

5. Sublimation

Solid → Gas

Some solids change directly into gas without first becoming liquid.

Example: Dry ice changing directly into carbon dioxide gas.

6. Deposition or Solidification of Vapour

Gas → Solid

A gas changes directly into a solid without passing through the liquid state. This is called deposition.

Example: Frost forming from water vapour.

Scientific note: Freezing normally describes the change from liquid to solid. The direct change from gas to solid is specifically called deposition.

7. Thermal Expansion

Thermal expansion is the increase in the dimensions of a substance when its temperature rises.

When heated, particles gain energy. In solids, they vibrate more strongly about their positions, usually increasing the average separation between them. In liquids and gases, heating generally increases the volume occupied by the substance.

On cooling, most substances contract.

Types of Thermal Expansion in Solids

1. Linear Expansion

Increase in the length of a solid on heating.

ΔL = αL₀ΔT

L₀ = original length
α = coefficient of linear expansion
ΔT = rise in temperature

2. Superficial Expansion

Increase in the surface area of a solid on heating.

ΔA = βA₀ΔT

A₀ = original area
β = coefficient of superficial expansion

3. Cubical Expansion

Increase in the volume of a solid on heating.

ΔV = γV₀ΔT

V₀ = original volume
γ = coefficient of cubical expansion

Important Relationships

For an isotropic solid undergoing small temperature changes:

β ≈ 2α

γ ≈ 3α

Factors affecting linear expansion:

1. Original length of the rod.
2. Rise in temperature.
3. Material of the rod.

8. Applications of Thermal Expansion

• Railway tracks: Expansion arrangements allow rails to accommodate temperature changes.
• Bridges: Expansion joints allow bridge materials to expand and contract.
• Metal jar lids: Warming a metal lid may loosen it because the metal expands.
• Electric wires: Wires generally sag more in hot weather because they expand.
• Thermometers: Many liquid-in-glass thermometers use the expansion of a liquid when its temperature rises.
• Hot-air balloons: Heating the air inside a balloon decreases its density at approximately the same pressure, helping the balloon rise.

Safety reminder: Never pour boiling water into a cold glass container. Sudden, uneven heating can cause thermal stress and break the glass. Do not handle hot lids, pans or liquids without suitable protection and adult supervision.

9. Thinking-Based MCQs

Read each situation carefully. Choose the best answer and check the scientific explanation after submitting.

1. A metal lid is stuck on a glass jar. Warm water is poured over the lid. Why may it become easier to open? The lid contracts when heated. The metal lid expands slightly when heated. The glass jar loses its mass. The metal changes into a liquid.

2. A metal rod is heated from 20°C to 40°C. Which change is most likely? Its length increases slightly. Its mass doubles. Its particles disappear. It must melt.

3. Railway tracks can bend in very hot weather if expansion is not adequately accommodated. Why? Rails contract on heating. Rails expand, and restricted expansion can cause buckling. Rails lose all their heat. Rails become gases.

4. Equal lengths of two different metals are heated through the same temperature rise. Will they necessarily expand by equal amounts? Yes, all metals expand equally. No, expansion depends partly on the material. Yes, because their masses must be equal. No, because metals never expand.

5. A cold glass develops cracks when hot water is poured into it suddenly. What is the main reason? The glass becomes weightless. Uneven expansion produces thermal stress. The glass contracts everywhere equally. The water changes into ice.

6. Ice melts into water when it absorbs heat. Which statement is correct during melting at the melting point? The temperature must rise continuously. The absorbed energy helps change the state while the temperature remains constant. No energy is absorbed. Water immediately becomes a gas.

7. Which example represents cubical expansion? Increase in the length of a rod. Increase in the area of a metal sheet. Increase in the volume of a metal block. Decrease in the length of a rod on heating.

8. A hot-air balloon rises because the heated air inside the balloon generally becomes: Denser than the surrounding air. Less dense than the surrounding air. A solid. Completely weightless.

9. A student heats a metal washer. What happens to its inner diameter? It generally increases. It becomes zero. It always decreases. It cannot change.

10. Which statement about heat is correct? Heat always flows naturally from colder to hotter objects. Heat flows naturally from hotter to colder objects. Heat and temperature are identical. Heat has no relation to temperature differences.

10. Quick Revision Quiz

Test your understanding of the chapter.

1. What is the SI unit of heat? Metre Joule Kilogram Degree Celsius

2. The increase in length of a solid on heating is called: Linear expansion Superficial expansion Condensation Freezing

3. The increase in the surface area of a heated solid is called: Linear expansion Superficial expansion Sublimation Freezing

4. The increase in volume of a heated solid is called: Linear expansion Superficial expansion Cubical expansion Condensation

5. Which process changes a gas directly into a solid? Melting Boiling Deposition Evaporation

6. Which of the following is a common application of thermal expansion? Expansion joints in bridges Measuring mass with a balance Measuring distance with a ruler Counting objects

7. Approximately how many joules are there in one calorie? 0.24 J 4.2 J 100 J 1000 J

8. Which is an example of a chemical change caused by heat? Ice melting A metal rod expanding Cooking an egg Water cooling

11. Apply Your Knowledge: A Numerical

A metal rod has an original length of 2 m. Its coefficient of linear expansion is 1.2 × 10⁻⁵ °C⁻¹. Calculate the increase in length when its temperature rises by 50°C.

Step 1: Write the formula

ΔL = αL₀ΔT

Step 2: Substitute the values

ΔL = 1.2 × 10⁻⁵ × 2 × 50

Step 3: Calculate

ΔL = 1.2 × 10⁻³ m

Converting metres into millimetres:

ΔL = 1.2 mm

Final Answer: The rod increases in length by 1.2 mm.

12. Moral Values and Life Skills

1. Safety First

Heat is useful, but careless handling can cause burns and fires. Follow safety rules when using hot objects and appliances.

2. Responsibility

Switch off appliances when they are no longer needed and use energy thoughtfully.

3. Care for Others

Warn others when an object is hot. Never leave hot liquids where young children can accidentally touch them.

4. Scientific Honesty

Observe experiments carefully, record actual results and explain what the evidence shows.

5. Environmental Awareness

Avoid wasting electricity and fuel. Responsible energy use helps conserve resources.

6. Problem-Solving

Use scientific knowledge to understand everyday problems and design safer structures.

13. Interesting Activity: Can Warm Air Inflate a Balloon?

Objective: To observe the expansion of air when heated.

Materials

• An empty plastic bottle
• A flexible balloon
• A bowl of warm water
• A bowl of cool water

Procedure

1. Fit the balloon securely over the mouth of the empty plastic bottle.
2. Place the bottle in warm water, with adult supervision.
3. Observe the balloon and record any changes.
4. Move the bottle to cool water and observe again.

Observation

The balloon may inflate in warm water and become smaller as the air cools. The extent of the change depends on the temperature and experimental setup.

Scientific Explanation

When air is heated, it expands if it can move into an available space. When it cools, it contracts.

Safety: Use only warm water, never boiling water. Ask an adult to supervise. Never heat a sealed bottle.

14. Curiosity Corner: Did You Know?

Why Does Ice Melt?

Ice absorbs energy from its surroundings. At normal atmospheric pressure, pure ice melts at approximately 0°C.

Why Do Bridges Have Expansion Joints?

Bridge materials expand in warmer conditions and contract in cooler conditions. Joints accommodate these changes.

Why Do Metal Lids Loosen?

A metal lid expands slightly when warmed, which may make it easier to remove from a jar.

Why Does a Hot-Air Balloon Rise?

Heating the air inside makes it less dense than the surrounding air under typical operating conditions. Buoyancy helps the balloon rise.

15. Revision Questions

1. Define heat and state its SI unit.
2. Differentiate between heat and temperature.
3. What is thermal expansion?
4. Name the three types of thermal expansion in solids.
5. State three factors affecting linear expansion.
6. Differentiate between melting and freezing.
7. What is the difference between sublimation and deposition?
8. Why are expansion joints provided in bridges?
9. Explain why a metal washer's inner diameter generally increases on heating.
10. Give two examples of chemical changes caused by heating.

1. Heat is energy transferred due to a temperature difference. Its SI unit is the joule (J).
2. Heat is energy transferred between bodies because of a temperature difference. Temperature indicates how hot or cold a body is.
3. Thermal expansion is the increase in the dimensions of a substance when heated.
4. Linear expansion, superficial expansion and cubical expansion.
5. Original length, rise in temperature and material of the rod.
6. Melting is the change from solid to liquid. Freezing is the change from liquid to solid.
7. Sublimation is the direct change from solid to gas. Deposition is the direct change from gas to solid.
8. Expansion joints allow structures to expand and contract with temperature changes and help prevent damage.
9. When a metal washer is heated uniformly, its material expands in all directions, so the hole generally becomes larger.
10. Cooking food and burning wood are examples.

16. Key Takeaways

• Heat is energy transferred from a hotter body to a colder body.
• The SI unit of heat is the joule.
• Heat can change temperature, state and size.
• Thermal expansion occurs when the dimensions of a substance increase on heating.
• Solids undergo linear, superficial and cubical expansion.
• Heating and cooling cause changes of state under suitable conditions.
• Thermal expansion is important in railway tracks, bridges and everyday objects.

Think like a scientist. Observe carefully, ask questions, test ideas and explain your answers with evidence.

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