Class 8 Chapter 4 Long Qa Science Cbse Ncert

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Extra 20 important long questions and answers from the chapter:4 "Combustion and Flame" for Class 8 Science (CBSE):-

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:

Easily available

in rural areas.

Cheap and renewable

energy source.

Used in traditional cooking

and heating.

Can be

converted into charcoal

for better efficiency.

Disadvantages:
5. Produces a lot of smoke, causing air pollution.
6. Leads to deforestation if used in large quantities.
7. 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.