"Combustion and Flame" for Class 8 Science (CBSE):-

Extra 20 important long questions and answers from the chapter:4

1. What is combustion? What are the necessary conditions for combustion?

Ans: Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light.

Necessary conditions for combustion:

Presence of fuel – A substance that burns (e.g., wood, petrol).

Supply of oxygen – Oxygen is needed for burning.

Attainment of ignition temperature – The fuel must reach its minimum temperature to catch fire.

Absence of non-combustible substances – Water, sand, or CO₂ can stop combustion.

Proper air circulation – Combustion is incomplete in low oxygen supply.

 

2. What are the different types of combustion? Explain with examples.

Ans: There are three types of combustion:

Rapid combustion – A substance burns quickly and produces heat & light.

Example: Burning of LPG or kerosene.

Spontaneous combustion – A substance catches fire on its own without heating.

Example: White phosphorus catching fire in air.

Explosion – A sudden release of heat, light, and gases with a loud sound.

Example: Firecrackers bursting.

 

3. What are the different zones of a flame?

Ans: A flame has three zones:

Dark inner zone – Contains unburnt fuel vapours.

Luminous middle zone – Incomplete combustion occurs, yellow in colour.

Non-luminous outer zone – Complete combustion occurs, hottest part, blue in colour.

 

4. What is ignition temperature? Why does a matchstick not catch fire on its own?

Ans: Ignition temperature is the minimum temperature required to catch fire.

A matchstick does not catch fire on its own because:

Its ignition temperature is higher than room temperature.

Friction is needed to produce heat.

The matchstick head contains red phosphorus, which ignites when rubbed against a rough surface.

 

5. Why is LPG preferred over coal for domestic use?

Ans:

Higher calorific value – Produces more heat per unit.

Smokeless burning – Does not produce soot.

Easy storage and transport – Stored in cylinders.

Complete combustion – Less pollution.

More convenient – No need to carry and cut wood or coal.

 

6. Why does a candle flame have different colours?

Ans:

Inner dark zone – Unburnt wax vapours, least hot.

Middle yellow zone – Partial combustion, luminous.

Outer blue zone – Complete combustion, hottest.

 

7. What are fuels? What are the characteristics of an ideal fuel?

Ans: Fuels are substances that burn to produce heat energy.

Characteristics of an ideal fuel:

High calorific value – Produces more heat.

Smokeless burning – Does not cause pollution.

Easily available – Readily found.

Economical – Affordable.

Non-toxic – Does not produce harmful gases.

Easy storage and transport – Should be safe to handle.

 

8. What are the disadvantages of using fossil fuels?

Ans:

Causes air pollution – Releases CO₂, SO₂, and NO₂ gases.

Leads to global warming – CO₂ increases Earth’s temperature.

Causes acid rain – Sulphur dioxide mixes with rainwater.

Non-renewable – Takes millions of years to form.

Health hazards – Causes breathing problems.

Oil spills harm marine life.

 

9. What are fire extinguishers? How do they work?

Ans: Fire extinguishers help put out fires by cutting off oxygen supply or reducing temperature.

Types of fire extinguishers:

Water extinguishers – Cools down flames.

Carbon dioxide extinguishers – Smothers fire by displacing oxygen.

Foam extinguishers – Used for oil fires.

Dry powder extinguishers – Used for electrical fires.

 

10. Why is CNG considered a better fuel than petrol and diesel?

Ans:

Lower pollution – Produces fewer harmful gases.

Higher calorific value – Gives more energy per unit.

Cheaper and easily available.

Does not produce smoke.

Less carbon monoxide emission.

 

11. What are the different types of fuels? Give examples of each.

Ans: Fuels are classified into three types based on their physical state:

Solid fuels – Example: Coal, Wood, Charcoal.

Liquid fuels – Example: Petrol, Diesel, Kerosene.

Gaseous fuels – Example: LPG, CNG, Biogas.

Solid fuels are easily available but cause pollution.

Liquid fuels burn more efficiently and are used in vehicles.

Gaseous fuels are cleaner, more efficient, and eco-friendly.

The choice of fuel depends on its calorific value, availability, and pollution level.

 

12. What is the difference between complete and incomplete combustion? Give examples.

Ans:

Complete Combustion	Incomplete Combustion
Takes place in sufficient oxygen.	Takes place in limited oxygen.
Produces carbon dioxide (CO₂).	Produces carbon monoxide (CO).
Releases more heat energy.	Releases less heat energy.
Smokeless burning, no soot.	Produces soot and smoke.
Example: LPG burning in a gas stove.	Example: Burning of wood with low oxygen.
Environmentally friendly.	Causes pollution and breathing problems.
More efficient.	Less efficient.

 

13. What are the advantages and disadvantages of using wood as a fuel?

Ans:

Advantages:

1-Easily available in rural areas.

2-Cheap and renewable energy source.

3-Used in traditional cooking and heating.

4-Can be converted into charcoal for better efficiency.

Disadvantages:
1. Produces a lot of smoke, causing air pollution.
2. Leads to deforestation if used in large quantities.
3. Has a low calorific value compared to modern fuels like LPG.

 

14. Why do firefighters use carbon dioxide (CO₂) to put out fires?

Ans:

CO₂ is heavier than air, so it displaces oxygen.

Fire needs oxygen to burn, so CO₂ cuts off oxygen supply.

CO₂ does not support combustion and cools the fire.

It is safe for electrical and oil fires where water cannot be used.

It forms a blanket over the fire, preventing re-ignition.

CO₂ fire extinguishers are widely used in offices, kitchens, and factories.

Unlike water, CO₂ does not damage electrical equipment.

 

15. How can fire be controlled in different situations?

Ans: Fire can be controlled by removing one of the three essential components: Fuel, Oxygen, or Heat.

Water – Used for controlling wood and paper fires by cooling them.

Carbon dioxide (CO₂) – Used for electrical and oil fires.

Foam extinguisher – Covers fire and cuts off oxygen supply.

Sand or soil – Used for putting out petrol and kerosene fires.

Fire blankets – Used for small kitchen fires.

Automatic sprinkler systems – Used in buildings for fire safety.

Fire breaks – Creating gaps between trees to prevent forest fires.

 

16. Why does kerosene burn with a flame but coal does not?

Ans:

Kerosene is a liquid fuel, while coal is a solid fuel.

Liquid fuels like kerosene vaporize easily, forming a flame.

Coal undergoes slow combustion and does not vaporize.

The presence of volatile substances in kerosene helps in burning.

Coal burns by glowing red-hot, instead of forming a flame.

Charcoal, which is a processed form of coal, also burns without a flame.

Fuels that produce vapours during combustion burn with a flame.

 

17. How is biogas a better fuel than cow dung cakes?

Ans:

Biogas:

Made from cow dung, plant waste, and sewage.

Produces methane, which burns cleanly.

Has a high calorific value and releases more heat.

Produces no smoke, reducing air pollution.

Provides fertilizer as a byproduct.

Helps in waste management and is environmentally friendly.

Used in cooking and electricity production.

Cow Dung Cakes:

Produce a lot of smoke, causing respiratory problems.

Have a low calorific value, giving less heat.

Lead to nutrient loss as dung is not used as fertilizer.

Take a long time to burn.

Contribute to indoor air pollution.

 

18. Why is burning fossil fuels harmful to the environment?

Ans:

Releases carbon dioxide (CO₂), leading to global warming.

Produces sulphur dioxide (SO₂) and nitrogen oxides (NO₂), causing acid rain.

Causes air pollution, leading to respiratory diseases.

Produces carbon monoxide (CO), a poisonous gas.

Leads to climate change, affecting weather patterns.

Non-renewable – Once used, they cannot be replaced quickly.

Oil spills from petroleum transportation harm marine life.

 

19. What are the characteristics of a good fuel?

Ans:

High calorific value – Produces more heat energy per unit.

Non-polluting – Should not release harmful gases.

Easily available and affordable – Should be cost-effective.

Safe to store and transport – Should not explode easily.

Ignition temperature should be moderate – Not too high or too low.

Should burn completely without residue – No ash or smoke should be left.

Should not produce toxic gases – Carbon monoxide and sulphur oxides should be minimal.

 

20. How does the candle flame help in understanding combustion?

Ans:

The dark inner zone contains unburnt wax vapours.

The middle luminous zone undergoes partial combustion, producing a yellow glow.

The outermost blue zone is where complete combustion occurs, making it the hottest part.

The flame shows that gaseous fuels burn with a flame, while solid fuels like charcoal do not.

The tilt of the candle affects the burning rate, as more oxygen supply increases combustion.

The flame flickers when exposed to wind, proving that air (oxygen) is needed for burning.

Placing a glass over the candle extinguishes it, showing that combustion stops without oxygen.