NCERT Solutions for Class 8 Science Chapter 3 Combustion and Flame

Combustion and Flame Class 8 Questions and Answers

1. List the conditions under which combustion can take place.

Answer:

Three conditions are necessary for combustion:

1. A combustible substance or fuel must be present.

2. Oxygen must be available to support combustion.

3. The fuel must reach its ignition temperature, the minimum temperature at which it catches fire.

If any one of these conditions is absent, combustion cannot continue.

2. Fill in the blanks.

(a) Burning of wood and coal causes ______ of air.

Answer: Pollution

(b) A liquid fuel used in homes is ______.

Answer: Kerosene

(c) Fuel must be heated to its ______ before it starts burning.

Answer: Ignition temperature

(d) Fire produced by oil cannot be controlled by ______.

Answer: Water

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3. Explain how the use of CNG in automobiles has reduced pollution in our cities.

Answer:

CNG is a cleaner fuel than petrol and diesel. Its combustion produces fewer harmful pollutants, so using CNG in automobiles can reduce air pollution compared with more polluting conventional fuels.

4. Compare LPG and wood as fuels.

Therefore, LPG is a better domestic fuel than wood because it burns more cleanly and provides more heat per unit mass.

5. Give reasons.

(a) Water is not used to control fires involving electrical equipment.

Water conducts electricity. Using water on an electrical fire can therefore create a risk of electric shock. Such fires should be handled using an appropriate fire extinguisher.

(b) LPG is a better domestic fuel than wood.

LPG burns more cleanly and produces more heat with less smoke and residue than wood. Therefore, it is more suitable as a domestic fuel.

(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Paper catches fire when its temperature reaches its ignition temperature. When wrapped around an aluminium pipe, much of the heat is transferred from the paper to the metal pipe, preventing the paper from reaching its ignition temperature quickly.

6. Make a labelled diagram of a candle flame.

A candle flame has three main zones:

• Outer zone: The hottest zone; combustion is complete.

• Middle zone: Yellow and luminous; combustion is incomplete.

• Inner zone: Dark zone containing unburnt vapours; it is the least hot.

For the labelled diagram, show the candle at the bottom and label these three zones from outside to inside. The outermost zone should be marked as the hottest zone.

7. Name the unit in which the calorific value of a fuel is expressed.

Answer:

The calorific value of a fuel is expressed in kilojoules per kilogram (kJ/kg).

It represents the heat produced when 1 kg of a fuel burns completely.

8. Explain how CO₂ can control fires.

Answer:

Carbon dioxide helps control fire by cutting off the supply of oxygen to the burning material. Since CO₂ does not support combustion and is denser than air, it can form a layer around the fire and help prevent oxygen from reaching the fuel.

This is why CO₂ is used in certain fire extinguishers, particularly for electrical and some other types of fires.

9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Answer:

Green leaves contain moisture. The moisture absorbs heat and makes it harder for the leaves to reach their ignition temperature.

Dry leaves contain much less moisture, so they can reach their ignition temperature more easily and catch fire quickly.

10. Which zone of a flame does a goldsmith use for melting gold and silver and why?

Answer:

A goldsmith uses the outermost zone of the flame because it is the hottest zone. Complete combustion takes place in this zone, producing a high temperature suitable for melting metals.

11. In an experiment, 4.5 kg of fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.

Given:

Mass of fuel = 4.5 kg

Heat produced = 180,000 kJ

Formula:

Calorific Value=Heat producedMass of fuel\text{Calorific Value} = \frac{\text{Heat produced}}{\text{Mass of fuel}} =1800004.5=\frac{180000}{4.5} =40,000 kJ/kg=40,000\text{ kJ/kg}

Answer: The calorific value of the fuel is 40,000 kJ/kg.

12. Can the process of rusting be called combustion? Discuss.

Answer:

Rusting involves a slow reaction of iron with oxygen and moisture and releases heat very slowly. In the broad sense used in this chapter, it can be described as a slow oxidation process, but it is not ordinary burning because it does not produce the visible heat and light associated with common combustion.

For Class 8 answers, it is useful to distinguish rusting from rapid combustion while recognising that both involve oxidation.

13. Abida and Ramesh were heating water in beakers. Abida kept her beaker in the yellow part of the candle flame, while Ramesh kept his in the outermost part. Whose water will get heated faster?

Answer:

Ramesh's water will get heated faster because the outermost zone of a candle flame is the hottest zone. Complete combustion takes place there, producing a higher temperature than the yellow middle zone.

Read more : CBSE Class 8 Science Electricity - Magnetic and Heating Effects Notes 2026-27

Combustion and Flame Class 8 Important Concepts

What is Combustion?

Combustion is a chemical process in which a substance reacts with oxygen and produces heat and/or light.

Examples include:

• Burning of wood

• Burning of paper

• Burning of LPG

• Burning of coal

Substances that undergo combustion are called combustible substances.

What is Ignition Temperature?

The ignition temperature of a substance is the minimum temperature at which it catches fire.

For example, a lighted matchstick can provide enough heat to raise paper to its ignition temperature, causing it to burn.

What Are the Conditions Necessary for Combustion?

Combustion requires:

• Fuel

• Oxygen

• Heat sufficient to reach ignition temperature

These three requirements can be represented by the fire triangle.

Types of Combustion

Rapid Combustion

Rapid combustion occurs quickly and produces heat and light.

Example: LPG burning in a gas stove.

Spontaneous Combustion

Spontaneous combustion occurs when a substance catches fire without an external flame or spark because sufficient heat builds up within the material.

Explosion

An explosion is a very rapid combustion or reaction that produces a large amount of heat, light, sound and gases in a short time.

Zones of a Candle Flame

A candle flame has three zones.

The outer zone is used for applications that require a higher temperature, such as heating metals.

Combustion and Flame Class 8 Extra Questions and Answers

What are combustible substances?

Substances that can burn in the presence of oxygen and undergo combustion are called combustible substances.

Examples include wood, paper, cotton and kerosene.

What are inflammable substances?

Inflammable substances have very low ignition temperatures and can catch fire easily.

Examples include petrol, alcohol and LPG.

Why should water not be used on an oil fire?

Oil does not mix with water and is generally less dense, so it can remain above the water. Spreading water over burning oil can spread the fire instead of extinguishing it.

Why is LPG considered a good fuel?

LPG has a high calorific value and burns relatively cleanly, making it useful as a domestic fuel.

What is calorific value?

Calorific value is the amount of heat produced by the complete combustion of 1 kg of a fuel. It is expressed in kJ/kg.

Effects of Burning Fuels

Burning fuels can release pollutants such as carbon monoxide, carbon dioxide, smoke and other gases.

Some fuels containing sulphur and nitrogen compounds can also contribute to the formation of pollutants associated with acid rain.

Using cleaner fuels and reducing unnecessary fuel consumption can help reduce pollution.

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Combustion and Flame Class 8 Quick Revision

• Combustion: Reaction with oxygen that produces heat and/or light.

• Fuel: A combustible substance used to produce energy.

• Ignition temperature: Minimum temperature required for a substance to catch fire.

• Calorific value: Heat produced by complete combustion of 1 kg of fuel.

• Unit: kJ/kg.

• Outer flame zone: Hottest zone with complete combustion.

• Middle flame zone: Yellow and luminous.

• Inner flame zone: Dark and least hot.

• CNG: A relatively cleaner fuel used in automobiles.

• LPG: A commonly used domestic fuel.

• CO₂: Does not support combustion and can help extinguish certain fires.

• Green leaves: Do not catch fire easily because of their moisture.

• Dry leaves: Catch fire more easily because they contain less moisture.