FLAME AND COMBUSTION

Combustion and Flame Class 8 Extra Questions Science Chapter 6

Combustion and Flame Class 8 Extra Questions Very Short Answer

Questions

Question 1.
Name the most common fuel used in homes.
Answer:
Liquefied Petroleum Gas. (LPG)

Combustion and flame online course

Question 2.
Name the most common fire extinguisher.
Answer:
Water

Question 3.
What are the states in which a fuel may exist?
Answer:
A fuql may exist in solid, liquid or gaseous state.

Question 4.
Name any two combustible substances.
Answer:
Charcoal, wood

Question 5.
Does magnesium produce heat and light during its combustion?
Answer:
Yes, it does.

Question 6.
What acts as a fuel for our body?
Answer:
Food

Question 7.
Give two examples of non-combustible substances. .
Answer:
Water, sand

Question 8.
How are heat and light produced in the sun?
Answer:
In the sun, heat and light are produced by nuclear reactions.
Question 9.
Where were matchsticks first used?
Answer:
Egypt

Question 10.
What are the three essential requirements for combustion?
Answer:
Fuel, air (to supply oxygen) and heat (to raise the temperature of the fuel
beyond the ignition temperature).

Question 11.
What is the ignition temperature of phosphorus?
Answer:
25°C

Question 12.
What is the colour of an LPG flame?
Answer:
Blue

Question 13.
What type of process is combustion?
Answer:
A chemical process

Question 14.
Name an ideal fuel.
Answer:
Compressed Natural Gas (CNG)

Question 15.
What is the composition of the head of a matchstick?
Answer:
Antimony trisulphide and potassium chlorate.

Question 16.
Which part of a flame does a goldsmith blow for melting gold and silver?
Answer:
The goldsmith blows the outermost zone of a flame for melting gold and
silver.

Question 17.
What is the unit for expressing the calorific value of a fuel?
Answer:
Kilojoules per kilogram (kJ/kg)
Question 18.
Comparing the calorific values of coal and petrol, state which fuel is
better.
Answer:
The calorific value of coal is about 25,000 – 33,000 kJ/kg, whereas that of
petrol is 45,000 kJ/kg. Hence, petrol is a better fuel.

Question 19.
What is deforestation?
Answer:
The cutting down of trees on a large scale is termed as deforestation.

Question 20.
Give any two examples of carbon fuels.
Answer:
Coal, petroleum

Combustion and Flame Class 8 Extra Questions Short Answer Questions

Question 1.
What does magnesium burn to form?
Answer:
Magnesium burns to form magnesium oxide and produces heat and light.

Question 2.
What does coal produce during its combustion?
Answer:
Coal produces carbon dioxide, heat and light during its combustion.

Question 3.
What is combustion?
Answer:
Combustion is a chemical process in which a substance reacts with
oxygen to give off heat. And light

Question 4.
Define ignition temperature of a fuel.
Answer:
The lowest temperature at which a fuel catches fire is called its ignition
temperature.

Question 5.
How does a matchstick catch fire?
Answer:
By rubbing a matchstick against a rough surface (friction), it attains its
ignition temperature and thus catches fire.
Question 6.
Why is sodium kept immersed in kerosene?
Answer:
Sodium has very low ignition temperature, i.e., it catches fire on coming in
contact with air, so it is kept in kerosene.

Question 7.
What are combustible and non-combustible substances?
Answer:
Substances which undergo combustion are said to be combustible,
whereas non-combustible substances are those which don’t burn.

Question 8.
What are inflammable substances? Give examples.
Answer:
The substances which have very low ignition temperature and can easily
catch fire with a flame are called inflammable substances; e.g., LPG,
petrol, alcohol, etc.

Question 9.
What is rapid combustion?
Answer:
When a substance burns instantly and produces a huge amount of heat
and light, the combustion is called rapid combustion; e.g., the instant
burning of LPG in a gas stove.

Question 10.
Define spontaneous combustion.
Answer:
A type of combustion in which the substance suddenly catches fire
without the supply of heat or friction externally is called spontaneous
combustion; e.g., forest fires.

Question 11.
Define explosion.
Answer:
A type of combustion during which a huge amount of heat and light is
evolved with a boom, along with the production of gas, is known as
explosion; e.g., the exploding of fireworks, i.e., crackers, etc.

Question 12.
What is flame?
Answer:
Flame is a region where the burning or combustion of gaseous substances
take place.
Question 13.
Define fuel.
Answer:
Those substances which provide energy on burning are called ‘fuels’; e.g.,
coal, petroleum, LPG, etc.

Question 14.
Give two examples each of solid fuels, liquid fuels and gaseous fuels.
Answer:
Solid fuels – Wood, cow dung, etc.
Liquid fuels – Kerosene, petrol, etc.
Gaseous fuels – Hydrogen, methane, etc.

Question 15.
Give two examples of fuels that are used to generate electricity.
Answer:
Two examples of fuels that are used to generate electricity are
petrochemicals and coal.

Question 16.
Define calorific value.
Answer:
The amount of heat produced on burning or complete combustion of one
kilogram of fuel completely is called its calorific value.

Question 17.
60 kg of fuel was completely burnt for an experiment. The amount of heat
energy was found to be 1,80,000 kJ. Calculate the calorific value of the
fuel.
Answer:
Amount of fuel burnt = 60 kg
Amount of heat produced = 1,80,000 kJ
Calorific value of the fuel = Heat produced Amount of fuel

∴ Calorific value of the fuel is 3,000 kJ/kg.

= 1,80,00060 = 3,000 kJ/kg

Question 18.
Define dark zone of a flame.
Answer:
The innermost zone of a flame around the wick is called its dark zone. It is
the least hottest zone com¬paratively to other.

Question 19.
Name the colours of the flames of following substances:

Barium, arsenic, sodium, magnesium.

 Answer:

Name of the substance Colour of its flame

Barium Pale/Apple green

Arsenic Blue

Sodium Yellow

Magnesium White

Question 20.
Mention any three characteristics of a good fuel.
Answer:
Any three characteristics of a good fuel are following:

 It has high calorific value.

 It is very easy to transport.

 It is cheap, affordable and economic.

 Doesn’t leave any undesirable substance behind

Question 21.
What is global warming?
Answer:
An increase in the average temperature of the earth’s atmosphere,
especially a sustained increase that causes climatic changes, is termed as
‘global warming’.

Global warming is the long-term increase in the Earth's average

temperature. It's caused by the buildup of greenhouse gases in the
atmosphere.
Causes
 Burning fossil fuels like coal, oil, and gas
 Deforestation
 Using air conditioners and refrigerators
 Industrial development
 Farming activities
 Overpopulation
Effects
 Melting glaciers
 Shrinking Arctic
 Rising sea levels
 Floods
 Extreme weather
 Wildfires
 Death of marine species
 Changes in coral reefs
Solutions
 Reducing pollution
 Reducing population growth
 Using less plastic
 Planting more trees
 Using electric and hybrid cars
 Using bicycles and public transit

Combustion and Flame Class 8 Extra Questions Long Answer Questions

Question 1.
Why isn’t hydrogen gas used as a domestic or industrial fuel, although it
has a very high calorific value? State three reasons for the answer.
Answer:
Although hydrogen gas has a very high calorific value, it is not used as a
domestic or industrial fuel due to the following reasons:

 It is expensive.

 It burns with an explosion.

 It is extremely inflammable, so it is risky to store and transport

hydrogen.

Question 2.
Explain how water gets boiled in paper cup without burning it.
Answer:
When we heat the paper cup containing water, the heat given to it is
rapidly transferred to water from the paper cup. The temperature of water
goes on increasing until it attains its boiling point, and starts boiling. As,
during this process, the heat is continuously being transferred to water;
the paper cup does not attain its ignition temperature. Hence, it does not
burn.

Question 3.
Why does a piece of paper burn with yellow flame? Give a reason.
Answer:
The inadequate supply of oxygen during the combustion of the piece of
paper produces solid carbon particles that rise up in the flame. They
become hot and glow to give off yellowish light. This makes the piece of
paper burn with a yellowish flame.

Question 4.
It is observed at petrol pumps and airports, that hydrocarbon fire
extinguishers are used, instead of soda-acid fire extinguisher. Give
reasons why.
Answer:
At petrol pumps and airports, there is more probability of fire break out
due to oil. In such situations, soda-acid fire extinguisher does not work as
it contains water or uses water to take off fire by cooling down the place.
Water being heavier than oil sinks to bottom and hence, fire does not get
controlled. In such a case, hydrocarbon fire extinguisher is very useful, as
it contains turkey red oil, which causes the foaming of carbon dioxide gas
under pressure. The foam covers the surface of the burning substances
and dispels the supply of air to control fire.

Question 5.
Explain complete combustion.
Answer:
This type of combustion involves complete burning of the combustible
substance. No residue is left behind. Ash or smoke is not given off during
or after this type of combustion. Mostly gases, such as hydrocarbons go
through this form of combustion. On combustion, hydrocarbon produces
carbon dioxide, water and heat.

Combustion and Flame Class 8 Extra Questions Higher Order Thinking

Skills

Question 1.
There are three factors which supports burning. Fire extinguisher affects
which factor?
Answer:
Fire extinguishers affect the supply of air.

Question 2.
Although wood has a very high calorific value, we still discourage its use
as a fuel. Explain.
Answer:
Wood pollutes air very much

Using wood as a fuel would also lead to deforestation on a large scale.

So we still discourage wood as a fuel.

Question 3.
Write in brief about the reasons of forest fires.
Answer:
Reasons of forest fires are:

 Lightning if strikes forest trees or areas may lead to forest fires.

 Human-caused fires.

 During extreme heat of summer, at some places dry grass catches

fire. From this grass, very soon, it spreads to the whole forest.

 Sparks from rock falls in a mountainous region can also be the

reason of forest fires.

 volcanic eruption can also cause forest fires.

Question 4.
Can the process of cellular respiration be called combustion? Why?
Answer:
Yes. Cellular respiration is a set of metabolic reactions and processes that
breaks large molecules into smaller one with the release of heat. So, this
is a slow combustion reaction.

Question 5.
If you hold a piece of iron wire with a pair of tongs inside a candle flame or
a Bunsen burner flame, what will you observe? Will it produce a flame?
Answer:
We will observe that the iron wire will become red hot and start glowing
but it will not produce a flame as it is non-flammable object.

Combustion and Flame Class 8 Extra Questions Value-Based Questions

Question 1.
Sourav was heating oil to fry potato chips. The cooking oil all of a sudden
caught fire. He took water to pour on the fire to extinguish it. But
meanwhile his mother came and switched off the gas and covered the
wok completely with a plate.

 Do you think pouring water to the burning oil would have worked?
Why?

 Do you think what Sourav’s mother had done is right? Why?

  Can you suggest other ways in which we can stop fire due to
burning oil?

 What values of Sourav and Sourav’s mother are shown here?

Answer:

 No. As oil being lighter than water will come up and spill all around
resulting in a major accident.

 Sourav’s mother had done right because she cut off the heat and
oxygen supply from the oil. The fire will ultimately extinguish if it
doesn’t get the required supply of air.

 (c)

 By using excess of baking soda.

 By using fire extinguisher.

 By putting a completely wet towel on the pan to cut off the air
supply.

 Sourav is immature in handling such situation but his mother is

mature, wise with scientific temperament.

Question 2.
During a class discussion on ‘fuels for household’ Sarita suggested petrol.
But her teacher said it is not a safe fuel for household activity and asked
her to sit. Sarita wondered why petrol can’t be used as a fuel for
household activity though its calorific value is high.

 Why petrol is not a safe fuel for household activity?

 What fuels do we use in our houses?

 What value of Sarita is shown here?

Answer:

 Petrol vapourises easily so it can lead to rapid combustion. Hence it

is not safe as household fuel.

 We use LPG, kerosene and wax.

 Sarita has knowledge of fuel but she is little bit confused.

Activities and Projects

Question 1.
Survey the availability of various fuels in your locality. Find out their cost
per kg and prepare a tabular chart showing how many kJ of various fuels
you can get for every rupee.
Answer:
Do it yourself.

Question 2.
Find out the number, type and location of fire extinguishers available in
your school, nearby shops and factories. Write a brief report about the
preparedness of these establishments to fight fire.
Answer:
Do it yourself.

Question 3.
Survey 100 houses in your area. Find the percentage of households using
LPG, kerosene, wood and cattle dung as fuel.
Answer:

Question 4.
Talk to people who use LPG at home. Find out what precautions they take
in using LPG.
Answer:
Some precautions taken by them are following:

 Cleaning of gas burner regularly and properly.

 Changing the delivery pipe regularly.

 Making use of strong delivery pipes.

 Check-up of related appliances at regular intervals.

Question 5.
Make a model of a fire extinguisher. Place a short candle and a slightly
taller candle in a small dish filled with baking soda. Place the dish at the
bottom of a large bowl. Light both the candles. Then pour vinegar into the
dish of baking soda. Take care. Do not pour vinegar on the candles.
Observe the foaming reaction. What happens to the candles? Why? In
what order?
For more information, visit:

 www.newton.dep.anl.gov/askasci/chem03/chem03767.htm

 http://en.wikipedia.org/wiki/combustion

 http://wwwchem.csustan.edu/consumer/fuels/heats%20.htm
Answer:
It is observed that the candles go off. The shorter candle blows off first,
and then the longer one. It hap¬pens because of the production of carbon
dioxide gas, that reaches the shorter candle first, and then the longer
candle.

I. Multiple Choice Questions (MCQs)

Choose the correct option.
Question 1.
Which of the following fuels is used for running automobiles?
(a) CNG
(b) Petrol
(c) Both (a) and (b)
(d) Wood

Question 2.
Magnesium burns to form
(a) calcium carbonate
(b) magnesium oxide
(c) calcium oxide
(d) magnesium sulphate

Question 3.
Coal burns to produce
(a) calcium bicarbonate
(b) magnesium
(c) carbon dioxide
(d) oxygen

Question 4.
Name the chemical process in which a substance reacts with oxygen to
give out heat.
(a) Reaction
(b) Junction
(c) Combustion
(d) All of these
Question 5.
The substance that undergoes combustion is said to be
(a) burning
(b) flame
(c) charcoal
(d) combustible

Question 6.
Combustible substances are also known as
(a) inflammable
(b) flaming
(c) illuminous
(d) non-flammable

Question 7.
Which of the following is a combustible?
(a) Stone piece
(b) Wood
(c) Glass
(d) None of these

Question 8.
In the sun, light and heat are produced by
(a) chemical reactions
(b) nuclear reactions
(c) burning reactions
(d) bunsen burner

Question 9.
Lowest temperature at which a substance catches fire is known as
(a) lowest temperature
(b) burning temperature
(c) ignition temperature
(d) flaming temperature

Question 10.
Long, long ago, which of the following trees was used to produce
matchsticks?
(a) Mango
(b) Deodar
(c) Banyan
(d) Pine

Question 11.
Which chemical is used in the rubbing surface provided for matchsticks?
(a) Sulphur
(b) Gold
(c) Red phosphorus
(d) White phosphorus

Question 12.
Substances which have very low ignition temperature and can catch fire
easily are called
(a) flammable substances
(b) inflammable substances
(c) combustible substances
(d) all of these

Question 13.
Which of the following is an example of inflammable substance?
(a) Iron
(b) Glass
(c) LPG
(d) Stone

Question 14.
Which of the following are required essentially for producing fire?
(a) Glass, coal, water
(b) Fuel, coal, straw
(c) Fire, wood, burner
(d) Fuel, air, heat

Question 15.
The most common element used as fire extinguisher is
(a) CO2
(b) oxygen
(c) phosphorous
(d) oxygen

Question 16.
Baking soda constitutes
(a) hydrogen chloride
(b) sodium oxide
(c) sodium bicarbonate
(d) oxygen

Question 17.
LPG means
(a) Liquefied Petroleum Gas
(b) Liquefied Petrol Gas
(c) Liquid Petrol Godown
(d) Liquid Petroleum Gas
Question 18.
Phosphorus burns at
(a) room temperature
(b) 100°C
(c) cool temperature
(d) any temperature

Question 19.
‘Firework’ is an example of
(a) rapid combustion
(b) explosion
(c) spontaneous combustion
(d) slow combustion

Question 20.
The calorific value of‘hydrogen’ is
(a) 50,000 kJ/kg
(b) 55,000 kJ/kg
(c) 1,50,000 kJ/kg
(d) 6,000 kJ/kg
Answer:
1. (c)
2. (b)
3. (c)
4. (c)
5. (d)
6. (a)
7. (b)
8. (b)
9. (c)
10. (d)
11. (c)
12. (d)
13. (c)
14. (d)
15. (a)
16. (c)
17. (a)
18. (a)
19. (b)
20. (c)

II. Fill in the Blanks

Fill in the blanks with suitable word/s.
1. The substance which vaporises during burning gives _________ .
2. A good fuel should have _________ calorific value.
3. An example of slow combustion is _________.
4. _________ are substances that release energy on combustion.
5. The most common supporter of combustion is _________.
6. Magnesium burns to change into _________.
7. Wood is an example of _________ fuel.
8. _________ is an example of liquid fuel.
9. Carbon dioxide is _________ than oxygen.
10. Oxides of _________ and nitrogen causes acid rain.
11. The increase in amount of _________ gas in atmosphere results in
global warming.
12. Inflammable substances have very low _________ temperature.
13. The _________ zone of a flame is the hottest.
14. During combustion, generally, _________ and _________ energies are
produced.
15. LPG has a calorific value of _________ kJ/kg.
Answer:
1. flame
2. high
3. respiration
4. Fuels
5. oxygen/air
6. magnesium oxide
7. solid
8. Kerosene
9. heavier
10. sulphur
11. carbon dioxide
12. ignition
13. non-luminous
14. heat, light
15. 55,000

III. Match the following

Match the items given in column I suitably with those given in column II.
Answer:
1. (b)
2. (a)
3. (c)
4. (d)
5. (g)
6. (e)
7. (i)
8. (h)
9. (j)
10. (f)

IV. True or False

State whether the given statements are true or false.
1. Automobiles run only by using petrol.
2. CNG means ‘Connecting Neutral Gas’.
3. Coal burns with a flame.
4. Magnesium burns to produce magnesium oxide.
5. Combustion is a physical process.
6. The substance that undergoes combustion is said to be combustible.
7. Fuel may be solid, liquid or gas.
8. Wood is combustible.
9. A matchstick only contains white phosphorus.
10. Kerosene is an example of solid fuel.
11. For combustion, a substance must attain its ignition temperature.
12. The ignition temperature of phosphorus is 25°C.
13. LPG is an example of inflammable substances.
14. Water is a bad conductor of electricity.
15. Fuel, water and heat are three essential requirements for combustion.
16. The most common fire extinguisher is water.
17. Oxygen is heavier than C02.
18. The middle zone of a flame has yellow colour.
19. The calorific value of hydrogen is 1,50,000 kJ/kg.
20. Increase in nitrogen gas in atmosphere has led to ‘global warming’.
Answer:
1. False
2. False
3. False
4. True
5. False
6. True
7. True
8. True
9. False
10. False
11. True
12. True
13. True
14. False
15. False
16. True
17. False
18. True
19. True
20. False

Very Short Answer Questions

1. What are fuels?

Answer: The substances which provide heat and light are called fuels.

2. Name two fuels that are used in your homes.

Answer: (i) L.P.G (ii) Kerosene.

3. What fuels are used for running automobiles?

Answer: Petrol, diesel and CNG.

4. What is the full form of CNG?

Answer: CNG stands for Compressed Natural Gas.
5. What is the difference between burning of a candle and
burning of coal?
Answer: A candle burns with a flame whereas coal does not.

6. Classify the fuels.

Answer: The fuels are classified into solid, liquid and gas.

7. What are combustible substances?

Answer: The substances that undergo combustion are called combustible
substances.

8. Do all the fuels burn with a flame?

Answer: No, all the fuels do not burn with a flame.

9. What are the products of combustion?

Answer: Carbon dioxide, water vapour, heat and light.

10. Name some combustible substances.

Answer: Wood, paper, kerosene oil, charcoal etc.

11. Write the names of some non-combustible substances.

Answer: Mud, stone, glass etc.

12. On the basis of the combustion classify the substances.

Answer: These are two types of substances: (i) Combustible (ii)
Non-combustible

13. What is the source of heat and light in the sun?

Answer: In the sun, heat and light are produced by nuclear reactions.

14. What do you mean by an ignition temperature?

Answer: The lowest temperature at which any substance catches fire is
called its ignition temperature.

15. Do all the substances catch fire at the same temperature?

Answer: No, all the substances do not catch fire at the same
temperature. Different substances catch fire at different temperature. The
lowest temp at which any substance catches fire is called its ignition
temp. different substances have different ignition temps for ex wood and
kerosene

16. What are inflammable substances?

Answer: The substances which have very low ignition temperature and
can catch fire easily with a flame are called inflammable substances.

17. Mention some examples of inflammable substances.

Answer: Petrol, alcohol and LPG etc.

18. Name the substances used to extinguish fire.

Answer: Water, sand and fire extinguishers.
19. Are these substances used to extinguish all the types of fire?
Answer: No.

20. What substances are used to extinguish fire in case of electric

short circuit?
Answer: Sand or soil and CO2.

21. Does your city/town have a fire brigade station?

Answer: Yes, there is a fire brigade station in my city.

22. What are the essential requirements for producing fire?

Answer: There are three requirements to produce fire:
(i) Fuel
(ii) Air to supply oxygen
(iii) Ignition temperature.

23. What is the principle to extinguish fire?

Answer: Fire can be controlled by removing one or more of the
requirements—air, fuel or heat.

24. Which is the most common fire extinguisher?

Answer: Water is the most common fire extinguisher.

25. Write the name of different types of fire extinguisher.

Answer: (i) Soda-acid (contains baking soda and acid) fire extinguisher.
(ii) Fire extinguisher cylinders having CO2 stored at high pressure

26. How many types of combustion are there?

Answer: There are three types of combustion:
(i) Rapid combustion
(ii) Spontaneous combustion and
(iii) Explosion.

27. What is flame?

Answer: The burning of vapours forms flame.

28. What is the colour of LPG flame?

Answer: Blue colour.

29. What is the colour of candle flame?

Answer: Yellow flame.

30. What type of substances produce flame?

Answer: The substances which vapourise during burning give flame.

31. Give examples of the substances which give flame.

Answer: Kerosene oil and molten wax.

32. Why does charcoal not produce flame?

Answer: Charcoal does not vapourise, so it does not produce a flame.
33. What are the different zones of flame?
Answer: There are three zones of flame:
(i) Non-luminous zone
(ii) Luminous zone and
(iii) Dark zone.

34. Which zone of flame has highest temperature?

Answer: Non-luminous zone.

35. What do you mean by deforestation?

Answer: The cutting of trees is called deforestation.

Short Answer Type Questions

1: State the difference between burning of a candle and burning

of a fuel like coal

Answer: Candle burns with a flame but coal does not burn with a flame
also coal is a carbon product and its burning is harmful for environment
but candle is made from wax its burning is not as much harmful as
burning of coal.

2: Explain combustion and combustible along with examples.

Answer: The chemical process in which a substance reacts with oxygen

to give off heat is called combustion. In combustion the release of heat
can result in the production of light in the form of either glowing or a
flame. The substance which undergoes combustion is called combustible
or fuel. Fuel may be solid, liquid or gas.
Example: Burning of a coal shows the process of combustion and coal
here is combustible or fuel.

3: Food is a fuel for the body. Justify this statement.

Answer: Food is a fuel for our body because inside our body food is
broken down into simpler form by reaction with oxygen and with the
release of heat and energy.

4: Identify the materials in which combustion can take place

Wood, paper, kerosene oil, iron nails, brick, stone, charcoal.

Answer: Wood, paper, kerosene oil, charcoal

5: On putting glass over a lighted candle, the candle flame

flickers and produce smoke, why so?

Answer: Take two lighted candle A and B, and place them on a table, now
put a transparent glass over candle B and see what happens to the candle
B, we will observe that candle flame flickers and produces smoke and
finally it goes off, this is because on putting glass over it, the air supply
was cut off and candle was unable to burn in the absence of air.

6: What is acid rain? Write its effects.

Answer: The oxides of sulphur and nitrogen dissolve in rain water to form
acids.
Such rain containing acids is called acid rain. It is very harmful for crops,
buildings and soil.

7: When the clothes of a person catch fire, the person is covered

with a blanket to extinguish fire, explain why?

Answer: To cut off the air supply of the fire, so that the fire gets off and
the person could be saved from fire.

8: What is forest fire?

Answer: A forest fire is a natural disaster consisting of a fire which

destroys a forested area, and is dangerous for the people living in forest
area as well as for the wildlife. During extreme heat of summer, at some
places dry grass catches fire, the fire gets spread from grass to tree, and
very soon whole forest catches fire. It is very difficult to control such fires.

9: What do you mean by ignition temperature? Why a matchstick

dose not catch fire on its own at room temperature?

Answer: The lowest temperature at which a substance catches fire or the

lowest temperature at which combustion begins and continues in a
substance is called its ignition temperature.
Match Stick cannot catch fire on its own at room temperature because it
can catch fire only at its ignition temperature not at room temperature.

10: Why does the matchstick start burning on rubbing it on the

side of matchbox?

Answer: The striking surface of the matchbox contains red phosphorus

and the head of the matchstick contains potassium chlorate. So when the
matchstick is rubbed on the matchbox, some of the red phosphorus is
converted to white phosphorus, a chemical i.e. so volatile that it ignites in
air.

11: Why does kerosene oil catch fire faster than wood?

Answer: This is because the specific heat capacity of the wood is more
than the kerosene oil. So, the wood takes time to burn but burns for longer
period than the kerosene oil.

12: Why we should store kerosene oil with proper care?

Answer: Because kerosene oil can catch fire very easily as its ignition
temperature is lower than other combustible material.

13: Explain why inflammable substance can easily set on fire?

Answer: Inflammable substances have very low ignition temperature and

thus they can easily catch fire with a flame, example: diesel, kerosene oil,
alcohol, Liquefied Petroleum Gas (LPG) etc.

14: How do we control fire?

Answer: As we know there are three essential requirements for producing

fire these are fuel, air and heat. Fire can be controlled by removing one or
more of these requirements that is either by cutting of the air supply, or
by bring down the temperature of the fuel or both.

15: How a fireman extinguishes fire?

Answer: Fireman through water with pressure on fire, water helps in

cooling down of the combustible material so that its temperature is
brought below its ignition temperature and fire does not get spread. A part
from this combustible material is surrounded by water vapour which helps
in cutting of the air supply and finally fire is extinguished.

16: Write short note on fire extinguisher.

Answer: The most commonly used fire extinguisher is water, it works

when wood and paper are on fire, but if electrical equipment is on fire
water may conduct electricity and may harm those trying to douse the
fire, also water is not suitable for fires involving oil and petrol, since water
is heavier than oil it sinks below the oil and oil keeps burning on the top.
So if electrical equipment and inflammable materials are on fire Carbon
dioxide is the best extinguisher. Since it is heavier than oxygen it covers
the fire so that the contact between fuel and oxygen is cut off and the fire
is controlled, it also bring s down the temperature of the fuel. For this
purpose carbon dioxide is stored at high pressure as a liquid in cylinders.

17: Explain the essential requirements for producing fire.

Answer: Following are the essential requirements for producing a fire:

1. Fuel: It is the combustible material

2. Air: Air supplies oxygen which supports combustion and without

which it is impossible to set on a fire.

3. Heat: Heat is important as it raises the temperature of the fuel

beyond the ignition temperature.
18: Why water is not a good fire extinguisher in case of electrical
equipment and inflammable materials?

Answer: The most commonly used fire extinguisher is water, it works

when wood and paper are on fire, but if electrical equipment is on fire
water may conduct electricity and may harm those trying to douse the
fire, also water is not suitable for fires involving oil and petrol, since water
is heavier than oil it sinks below the oil and oil keeps burning on the top.
So if electrical equipment and inflammable materials are on fire Carbon
dioxide is the best extinguisher. Since it is heavier than oxygen it covers
the fire so that the contact between fuel and oxygen is cut off and the fire
is controlled, it also bring s down the temperature of the fuel. For this
purpose carbon dioxide is stored at high pressure as a liquid in cylinders.

19: How many types of combustion are there? Name them.

Answer: There are three types of combustion:

1. Rapid combustion: Bring a burning matchstick or gas lighter near

a gas stove and turn on the knob of gas stove, we will find the gas
burns rapidly and produces heat and light, such combustion is called
as rapid combustion

2. Spontaneous combustion: In this type of combustion a material

suddenly bursts into a flame, without the application of any
apparent. Eg: spontaneous combustion of coal dust.

3. Explosion: When a cracker is ignited a sudden reaction takes place

with the evolution of heat, light and sound, this type of combustion
is called Explosion.

20: What is flame?

Answer: A flame is the visible, gaseous part of a fire. The substance

which vaporise during burning give flames, It is caused by a highly
exothermic reaction taking place in a thin zone.

21: Introduce a glass plate into the luminous zone of the steady
candle flame and hold it for few seconds, then remove it? What
did you observe on the glass plate?

Answer: We will observe a circular blackish ring formed on the glass

plate, this indicates the deposition of unburnt carbon particles present in
the luminous zone of the flame.

22: State some of the characteristics of a good fuel.

Answer: A good fuel is one which is:

 Readily available
  Ignite easily

 Burn well, not with explosion

 Cheap

 Produces a large amount of heat

 Have low smoke and ash content

 Should be easy to store and transport

23: How calorific value of a fuel is related to amount of heat

produced by fuel? Mention calorific value of wood, coal, petrol,
CNG and Biogas.

Answer: Calorific value of the fuel is the amount of heat energy produced
on complete combustion of 1 kg of fuel. It is expressed in kilojoule per kg.
Calorific value of wood is 17000-22000 kJ/kg
Calorific value of coal is 25000-33000 kJ/kg
Calorific value of CNG is 50000 kJ/kg
Calorific value of biogas is 35000 – 40000 kJ/kg

24: Why we say increasing fuel consumption has harmful effects

on environment?

Answer: Increasing fuel consumption has harmful effects on environment

because:

 Carbon fuels like wood, coal and petroleum releases unburnt carbon
particles that are dangerous pollutants causing diseases like asthma
and respiratory disorders

 Incomplete combustion of these fuel releases a very poisonous gas

carbon monoxide

 Release of carbon dioxide gas by combustion of fuels is becoming

one of the cause of global warming

25: Explain global warming and its causes.

Answer: Global warming is the increase in the average temperature of

Earth’s atmosphere and oceans, reason for the same is:
1. Increasing population that leads to increased use of fossil fuels and
agriculture, fossil fuels releases carbon dioxide gas which causes global
warming.
2. Because of increased population more number of people releases
carbon dioxide gas during respiration process and that contributes to
global warming.
3. Increased demand of agriculture for our increasing population is also
one of the reason of global warming as manures used in agriculture
contains methane gas.

26: What are the effects of global warming?

Answer: Global warming is the increase in the average temperature of

Earth’s atmosphere and oceans, it results in melting of the glacier of the
polar region which leads to rise in sea level, causing flood in coastal areas.
Even low lying coastal region may get permanently submerged under
water because of global warming.

27: Explain why it is difficult to burn a heap of green leaves but

we can easily burn dry leaves?

Answer: A heap of green leaves contains lot of water and has very high
ignition temperature, as water is a natural fire extinguisher it do not allow
leaves to catch fire easily where as dry leaves contains no water and have
low ignition temperature thus they can catch fire easily.

28: In an experiment 3.5 kg of a fuel was completely burnt. The

heat produced was measures to be 160,000 kj. Calculate the
calorific value of the fuel.

Answer: Calorific value of the fuel = Amount of heat energy produced /

weight of fuel burnt
= 160000/3.5 kJ/kg
= 45714.28 kJ/kg

29: What is fuel? Give some examples of a fuel.

Answer: The combustible substances which produce heat and sometimes

light on combustion is called fuel. Fuel may be solid, liquid and gas. Wood,
charcoal, petrol and kerosene are some of the examples of fuel.

30: What are combustible and non-combustible substances?

Explain with examples.

Answer: The substances in which combustion takes place are called

combustible substances. For example: wood, paper, coal. The substances
in which no combustion takes place are called non-combustible
substances. For example: Glass, iron nail.

31: Why does a matchstick not burn of its own?

Answer: The ignition temperature for the burning of matchstick is more

than room temperature. So it does not catch fire on its own. When the
stick is rubbed then due to friction it gets its ignition temperature and
starts to burn.
32: Can you burn a piece of wood by bringing a lighted matchstick
near it?

Answer: The piece of wood cannot burn by bringing a lighted matchstick

near it. It is because the heat produced by matchstick is not sufficient to
attain the ignition temperature of wood. So we use paper or kerosene oil
to start fire in wood piece.

33: We can boil water in a paper cup while paper catches fire
easily. Explain the process.

Answer: We take two paper cups. Take some water in one cup and keep
the other empty. Heat both the cups. Empty cup starts to burn but the cup
containing water does not burn. If we continue heating the water in the
cup it starts boiling. The heat supplied to the paper cup is transferred to
water by conduction. So in the presence of water the ignition temperature
of paper is not reached. Hence, it does not burn.

34: How does fire brigade works to extinguish fire?

Answer: When fire brigade arrives, it pours water on the fire. Water cools
the combustible material, so that its temperature is brought below its
ignition temperature. This prevents the fire from spreading. Water vapour
also surrounds the combustible material, helping in the cutting off the
supply of air. So the fire is extinguished.
35: What is the job of a fire extinguisher?

Answer: There are three requirements for producing fire: fuel, air, proper
temperature. Fire can be controlled by removing one or more of these
requirements.
The job of a fire extinguisher is to cut off the supply of air or to bring down
the temperature of the fuel or both. In most of the cases fuel cannot be
eliminated.

36: Explain the structure of a flame.

Answer: There are following three parts of a flame:

(i) Outer zone: It is non-luminous and the hottest zone.
(ii) Middle zone: It is less hot and yellow coloured zone.
(iii) Inner zone: It is dark zone and least hot part.

37: Why is the colour of outer zone is blue while middle zone is
yellow coloured?
Answer: Outermost zone: It is blue coloured part because complete
combustion takes place in this part due to sufficient amount of oxygen. It
is the hottest part of the flame.
Middle Zone: The colour of the middle zone is yellow because
incomplete combustion takes place in this part for the lack of oxygen. It is
less hot part than outer part of the flame.

38: What is calorific value of a fuel?

Answer: The amount of heat energy produced on complete combustion of

1 kg of a fuel is called its calorific value. The calorific value of a fuel is
expressed in a unit called kilojoule per kg (kJ/kg).

Long Answer Type Questions

1: What are the advantages of using CNG and LPG as fuels?

Answer: The advantages of using CNG and LPG as fuels are as follows:

 They both are cleanest burning fuels of all fossil fuels.

 They are less polluting, non-corrosive.

 They can be sent through pipes easily.

 These are easily available and have affordable cost.

 LPG and CNG both are easy to store and transport.

 As there calorific value is very high therefore, they are used for
cooking purpose.

2: Differentiate between LPG and wood as fuel.

Answer:

LPG WOOD

It is costly fuel, but readily available and It is cheap fuel but not readily
easy to transport in cylinders and tankers combustible

It is more energy efficient It is less energy

Its calorific value is 55000 KJ/kg Its calorific value is 17000-25000 KJ/kg

It causes less air pollution and prevent

It causes air pollution and deforestation
deforestation by supplementing the fuel.
 It is not an exhaustible natural resource
It is an exhaustible natural resource.
as trees can be grown in 5-10 years

3: Explain working and principles of a fire extinguisher.

Answer: The most commonly used fire extinguisher is water, it works

when wood and paper are on fire, but if electrical equipment is on fire
water may conduct electricity and may harm those trying to douse the
fire, also water is not suitable for fires involving oil and petrol, since water
is heavier than oil it sinks below the oil and oil keeps burning on the top.
So if electrical equipment and inflammable materials are on fire Carbon
dioxide is the best extinguisher. Since it is heavier than oxygen it covers
the fire so that the contact between fuel and oxygen is cut off and the fire
is controlled, it also bring s down the temperature of the fuel. For this
purpose carbon dioxide is stored at high pressure as a liquid in cylinders.

4: Explain why the process of rusting can be called as slow

combustion.

Answer: Rusting of iron is a slow oxidation process in which iron using

oxygen and water is oxidised and is rusted out, it produces heat at very
slow rate. The combustion process is also an oxidation process and is a
chemical reaction by which a fuel and an oxidiser react to produce heat
and light, thus we can say rusting of iron is a slow combustion process,
although combustion is much faster than rusting.

5: What are the essential conditions for combustion? Explain with

the help of an activity.

Answer: Take a candle. Light it and fix it on a table. Put a glass chimney
over the candle and rest it on wooden blocks in such a way that air can
enter the chimney. We see that candle remains lighted. Remove the blocks
and let the chimney rest on the table. We see that the flame flickers and
produces smoke. Now put a glass plate over the chimney. We see that
flame goes off because air is not available. This activity shows that air is
essential to burn a fuel at its ignition temperature.

6: Describe the history of the matchstick.

Answer: History of matchstick is about five thousand years old. The

modern safety match was developed only about 200 years ago. A mixture
of antimony trisulphide, potassium chlorate and white phosphorus with
some glue and starch was applied on the head of a match made of
suitable wood. When it struck against a rough surface, white phosphorus
got ignited and combustion of matchstick started. These days red
phosphorus is used in place of white phosphorus.
7: Write three examples of each type of fuel in tabular form.
Answer:

Solid Fuels Liquid Fuels Gaseous Fuels

Coal Kerosene oil Natural gas

Wood Petrol L.P.G

Dung Cake Diesel Gobar Gas

8: Make a table to show the calorific value of various fuels.

Answer:

Name of fuel Calorific Value kJ/kg

Cowdung cake 6000-8000

Wood 17000-22000

Coal 25000-33000

Petrol 45000

Kerosene 45000

Diesel 45000

Methane 50000

CNG 50000

LPG 55000

Biogas 35000-40000

Hydrogen 150000

9: What is deforestation? Write its effect on environment.

Answer: Cutting of trees is called deforestation.
Effects: Deforestation is very harmful to the environment. The following
are the effects of deforestation:
1. The annual rainfall is disturbed in that area.
2. The frequent floods come.
3. Balance of oxygen and carbon dioxide is disturbed.
4. It causes respiratory problem.
5. Trees provide us with useful substances which are lost due to
deforestation.

10: What are the ill-effects due to the increasing consumption of

fuel?

Answer: Harmful effects of using more fuels:

(i) Carbon fuels like wood, coal and petroleum release unburnt carbon
particles. These fine particles create respiratory disorders and diseases
like asthma.
(ii) Incomplete combustion of these fuels gives carbon monoxide. It is very
harmful gas and causes respiratory disorders. It can kill persons sleeping
in that room.
(iii) Excessive use of fuels causes global warming.
(iv) They cause acid rain which is harmful for crops, buildings and soil.

ccess Important Questions for Class 8 Science Chapter 4 -

Combustion and Flame

Very Short Answer Questions: (1 Mark)

1. ________ do not produce flames but generate heat.

A. Kerosene

B. Oil

C. Petrol

D. Coal

Ans: D. Coal

2. Which of the following is the product of combustion?

A. Carbon dioxide and water

B. Oxygen and Water

C. Only Carbon Dioxide

D. Only Oxygen

Ans: A. Carbon Dioxide and Water

3. Which one of the following is a combustible substance?

A. Wood

B. Nail

C. Steel spoon

D. All of These

Ans: A. Wood

4. __________ is produced when a charcoal is burnt in a closed

room?

A. carbon dioxide

B. Nitrogen Dioxide

C. Carbon Monoxide

D. All of These

Ans: C. Carbon Monoxide

5. What is used as fuel to run automobiles?

A. Petrol

B. Diesel

C. Kerosene

D. Both A and B

Ans: D. Both A and B

6. A good fuel is one which is ______________.

A. Cheap

B. Readily Available
C. Produces Large Amount of Heat

D. All of These

Ans: D. All of these

7. How many zones are there in the flame?

A. One

B. Three

C. Two

D. Four

Ans: B. Three

8. What are the requirements essential for producing fire?

Ans: Requirements for producing fire:

 Fuel

 Air (Oxygen)

 Ignition Temperature

9. Charcoal burns in air to produce ______________, _______________

and ___________.

Ans: Carbon Dioxide, Heat and Light.

10. Name any one chemical substance which helps to put off the
fire other than carbon dioxide.

Ans: Bicarbonates of sodium or potassium will help us to put off the fire.

11. In the Sun, heat and light are produced by ___________.

Ans: Nuclear Reaction.

12. Expand LPG.

Ans: Liquified Petroleum Gas

13. The lowest temperature at which a substance catches fire is
called ___________.

Ans: Ignition Temperature

14. The unit of a calorific value of a fuel is _________________.

Ans: Kilojoules per Kilogram.

Short Answer Questions: (3 Marks)

15. Classify the following into combustible and non-combustible

substances.

iron rod, wood, steel spoon, stone, newspaper, petrol

Ans:

Combustible Non-Combustible

Wood, Newspaper, Petrol Iron rod, Steel rod, Stone

16. Kerosene oil while burying gives flames on the other hand
charcoal does not produce flames? Give a reason.

Ans: Only substances which vaporise during burning can produce flames
such as kerosene whereas charcoal doesn’t vaporise when starting
burning. Thus, charcoal doesn’t produce any flame.

17. If a fire breaks out in the petrol station, how does a fire
extinguisher help to put off the fire?

Ans: A combination of liquid and gaseous carbon is used in fire

extinguishers. Because carbon dioxide is heavier than oxygen, it blankets
the fire. Thus, the fire is controlled because there is no contact between
the fuel and the oxygen.

18. Petrol can be considered as a good fuel. Why?

Ans: Reasons petrol is considered a good fuel:

a. It is affordable.

b. It is easily available.

c. It has a high calorific value.

d. It is easy to transport.

e. It burns easily and produces more heat.

19. What will happen when a paper cup with water is heated?

Ans: When we heat a paper cup containing water, the water will absorb
the heat of the flame. Hence, the ignition temperature of paper will not
reach and it will not start burning easily.

20. Give four examples of inflammable substances.

Ans: Four inflammable substances:

a. Kerosene

b. Petrol

c. LPG

d. Diesel

21. How does combustion start in a matchstick?

Ans: Antimony trisulphide and potassium chlorate are found in the

matchstick's head. The rubbing surface has powdered glass & red
phosphorus. Red phosphorus is transformed to white phosphorus when
the match is hit against the rubbing surface. This combines with the
potassium chlorate in the matchstick head to provide enough heat to
ignite antimony trisulphide and start the combustion process.

22. Define the following.

a. Combustion: Combustion is a chemical reaction in which a material

combines with oxygen present in air to produce heat.

b. Fuel: Fuels are the substances which undergo combustion.

c. Calorific Value: The calorific value of a fuel is the quantity of heat

energy produced when 1 kg of fuel is completely burned.
23. Why is it not advised to use water if an electrical equipment is
on fire?

Ans: Water is a good conductor of electricity which increases the risk to

the helpers. Therefore it is advised not to use water if there is any
electrical equipment on fire.

24. Label the different zones of a candle flame.

Ans:

A – outer zone of complete combustion (Blue coloured)

B – Middle zone of partial combustion (yellow coloured)

C – Innermost zone of unburnt wax vapours (black coloured)

25. In an experiment, 1 kg of fuel was completely burnt. The heat
produced was measured to be 9,000 kJ.

a. What will be the head produced for 7 kg of fuel?

Ans: If the heat produced for 1 kg of fuel is 9000 kJ.

Then heat produced for 7 kg is = 7 X 9000

= 63,000 kJ

b. Calculate the calorific value of the fuel.

Ans: Calorific value of the fuel = Amount of heat energy produced /

weight of fuel burnt

= 63000/8 kJ/kg

= 9,000 kJ/kg

Long Answer Questions: (5 Marks)

26. List the characteristics of an ideal fuel.

Ans: Characteristics of an ideal fuel:

a. It should be affordable.

b. It should be easily available.

c. It should have high calorific value.

d. It should be easy to transport.

e. It should burn easily and produce more heat.

f. It should not leave any unwanted residue behind on burning.

g. It should be more environmentally friendly.

27. With the help of an experiment show that air is necessary for
combustion.

Ans:

i. Take two lighted candles on a table.

ii. Put a glass chimney over one candle s in such a way that air can enter
the chimney.
iii. On the other hand, put a chimney over another candle and place a
glass plate over the chimney.

iv. We observe that the first candle continues to burn whereas flame in the
second candle extinguishes.

This is because the air supply was cut off after placing the glass plate over
the chimney and the candle was not able to burn in the absence of air.
This shows that air (oxygen) is necessary for combustion.

28. Explain the different types of combustion with examples.

Ans: There are three different types of combustions:

a. Rapid Combustion: It's a form of combustion in which a substance

burns quickly, releasing heat and light. Example: LPG.

b. Spontaneous Combustion: The sort of combustion in which a

substance spontaneously erupts into flames without any apparent cause.

Example: Phosphorus. It burns in air at ambient temperature without

igniting.

c. Explosion: When a substance is ignited, it undergoes a rapid reaction

that produces heat, light, and sound. The reaction produces a significant
amount of gas and heat, which is then released. This type of combustion
is called an explosion.

Example: Bursting of firecrackers.

29. Explain in detail the harmful effect caused by burning of fuel.

Ans: Harmful effects of burning of fuels:

a. The carbon based wood, coal etc leaves unburnt substances which
causes air pollution. They can be harmful for humans too, causing
respiratory diseases.

b. Most of the fuel releases carbon dioxide on burning. This process slowly
increases carbon content in the atmosphere leading to bigger problems
like global warming.

c. Incomplete combustion of fuel can produce carbon monoxide instead of

dioxide which when inhaled can rapidly combine with our blood and work
as poison.
d. In a few cases sulphur dioxide is produced on burning of fuel which is
harmful to both humans and the environment.

ultiple Choice Questions

Question. 1 A substance which reacts with oxygen giving heat is

called a combustible substance. Which one of the following is a
combustible substance?
(a) Iron nail (b) Glass (c) Stone piece (d) Wood
Answer. (d) Wood is a combustible substance. It is a solid fuel and
produces smoke during combustion.

Question. 2 Which one of the following has the highest calorific

value?
(a) Kerosene (b) Biqgas (c) LPG (d) Petrol
Answer. (c) LPG has the highest calorific value, i.e. 55000 kJ/kg.
It means that if 1 kg of LPG is burnt completely, then it will produce 55000
kilojoules of heat energy.
Note
Calorific value
45000 kJ/kg 35000 to 40000 kJ/kg 45000 kJ/kg

Question. 3 Magnesium ribbon on burning in air produces

(a) magnesium oxide, water and light (b) magnesium oxide and
heat
(c) magnesium oxide, heat and light (d) magnesium oxide, water
and heat
Answer. (c) Magnesium ribbon on burning in air produces magnesium
oxide, heat and light, Magnesium ribbon is a very reactive metal.

Question. 4 Which of the following is not a combustible

substance?
(a) Camphor (b) Glass (c) Straw (d) Alcohol
Answer. (b) Those substances which do not burn are called non-
combustible substance. Some of the non-combustible substances are
stone, glass, cement, soil, sand, iron nails, etc.

Question. 5 The substance that does not burn with flame is

(a) LPG (b) camphor (c) dry grass (d) charcoal
Answer. (d) Charcoal is a solid fuel which does not vaporise on heating.
So, charcoal does not burn by producing a flame. It only glows on
combustion.
Question. 6 On placing an inverted tumbler over a burning candle,
the flame extinguishes after some time. This is because of non-
availability of (a) oxygen (b) water vapours (c) carbon dioxide (d)
wax
Answer. (a) The flame extinguishes because of non-availability of oxygen.
Air or oxygen is necessary for combustion.

Question. 7 If a person’s clothes catch fire, the best way to

extinguish the fire is to
(a) throw water on the clothes
(b) use fire extinguisher
(c) cover the person with a woollen blanket
(d) cover the person with a polythene sheet
Answer. (c) Cover the person with a woollen blanket so that the supply of
air to the burning clothes is cut off and hence, the burning (or fire) stops.

Question. 8 The substance expected to have the highest ignition

temperature out of the following is
(a) kerosene (b) petrol (c) coal (d) alcohol
Answer. (c) Coal has the highest ignition temperature.
Note The lowest temperature at which a substance catches fire and starts
burning, is called its ignition temperature. Kerosene, petrol and alcohol
have low ignition temperature.

Question. 9 Choose the correct statement about inflammable

substances from the following.
They have
(a) low ignition temperature and cannot catch fire easily
(b) high ignition temperature and can catch fire easily
(c) low ignition temperature and can catch fire easily

(d) high ignition temperature and cannot catch fire easily

Answer. (c) Inflammable substances have low ignition temperature and
can catch fire easily.

Question. 10 Choose the incorrect statement from the following.

Forest fires are usually due to
(a) carelessness of humans (b) heat of Sun
(c) cutting of trees (d) lightning strike
Answer. (c) Forest fires are not due to cutting of trees,

Question. 11 The calorific value of a fuel is expressed in a unit

called
(a) kilojoule per litre (b) kilogram per millilitre
(c) kilojoule per gram (d) kilojoule per kilogram
Answer. (d) The calorific value of a fuel is expressed in the unit of
kilojoule per kilogram (kJ/kg),

Question. 12 In villages, people use wood as fuel because

(a) it is considered to be an ideal fuel (b) of its easy availability
and low cost (c) it is environment friendly (d) it catches fire easily
Answer. (b) In villages, people use wood as fuel because of its easy
availability and low cost.

Question. 13 Which one among the following is considered as the

cleanest fuel?
(a) Cowdung cake (b) Petrol
(c) Kerosene (d) Hydrogen gas
Answer. (d) Hydrogen gas is considered as cleanest fuel. Unlike carbon
based fuels, hydrogen produces no harmful by-products on combustion.
Only energy and clean water are produced.

Question. 14 Choose the incorrect statement from the following.

A good fuel is one which
(a) is readily available
(b) produces a large amount of heat
(c) leaves behind many undesirable substances
(d) burns easily in air at a moderate rate
Answer. (c) A good fuel does not produce any harmful gases or leaves
any residue after burning. So, incorrect statement is option (c).

Q. 15 Shyam was cooking potato curry on a chulha. To his

surprise, he observed that the copper vessel was getting
blackened from outside. It may be due to –
(a) proper combustion of fuel (b) improper cooking of potato
curry
(c) improper combustion of fuel (d) burning of copper vessel
Answer. (c) Due to incomplete combustion of fuel, copper vessel was
getting blackened from outside.

Very Short Answer Type Questions

Question. 16 Fill in the blanks.

(a)A ……….. process in which a substance reacts with ……….. to
give off heat
is called combustion.
(b) When the clothes of a person catch …………the person is
covered with a…………… to extinguish fire.
(c) The………….. temperature at which a substance catches fire is
called its…………..temperature.
(d) The substances which gave very …………..ignition temperature
and can easily catch fire with a flame are
called………….substances.
(e) The substances which vaporise during……………..give flame.
Answer.(a) chemical, oxygen
(b) fire, blanket
(c) lowest, ignition
(d) low, inflammable
(e) burning

Question. 17 Some words (underlined) in the following sentences

are jumbled up. Write them in theirjrorrect form.
(a) Seldie is a combustible substance.
(b) Slaas is a non-combustible material.
(c) Chittsmack does not burn by itself.
(d) Some substances on combustion produce thea and mafel.
(e) The amount of heat energy produced on complete combustion
of 1 kg of a fuel is called its ficalroic value.
Answer. (a) Diesel (b) Glass
(c) Matchstick (d) heat, flame
(e) calorific

Question. 18 Two glass jars A and B ajre filled with carbon dioxide
and oxygen gases respectively. In each jar, a lighted candle is
placed simultaneously. In which jar, will the candle remain lighted
for a longer time and why?
Answer. In jar B, the candle remains lighted for a longer time because
oxygen is a supporter of combustion. So, we can say that oxygen (air) is
necessary for combustion to take place.

Question. 19 Anu wants to boil water quickly in a test tube. On

observing the j different zones of the flame, she is not able to
decide which zone of the flame will be best for boiling water
quickly. Help her in this activity.
Answer. Anu should keep her test tube in the outermost zone or non-
luminous zone of the flame because it is the hottest zone of a flame and
has more temperature.
In the outer zone of a flame, complete combustion of the fuel takes place
because there is plenty of air around it so, water in a test tube will boil
quickly in this zone.

Question. 20 Why is the use of diesel and petrol as fuels in

automobiles being replaced by Compressed Natural Gas (CNG) in
big cities?
Answer. It is because when CNG burns, it produces harmful products in
very small amount. It is a clean fuel because it burns without producing
smoke. The use of CNG as fuel in automobiles has reduced air pollution In
big cities.

Short Answer Type Questions

Question. 21 Boojho wants to separate the following materials as

combustible and non-combustible. Can you help him? Charcoal,
chalk, stone, iron rod, copper coin, straw, cardboard, glass, paper,
candle, wood.
Answer. 1.Combustible — charcoal, straw, cardboard, paper, candle,
wood.
2.Non-combustible — chalk, stone, iron rod, copper coin, glass.

Question. 22 State whether the following statements are

True/False.
(i) Air is necessary for combustion.
(ii) Magnesium is a non-combustible metal.
(iii) Carbon dioxide is an excellent fire extinguisher.
(iv) Calorific value of wood is higher than that of coal.
Answer. 1. True, air contains oxygen and oxygen is a supporter of
combustion.
2. False, magnesium is a combustible metal.
3.True
4.False, calorific value of coal is higher than that of wood.

Question. 23 Match the items of Column I with the items of

Column II.

Answer. The correct matching is as given:

(a)—(iv), (b)-(vi), (c)-(v), (d)—(iii), (e)-(i), (f)-(ii)

Question. 24 Match the following for the flame of a candle.

Answer. The correct matching is as given:
(a)—(iii) (x), (b)—(ii) (z), (c)—(i) (y)

Question. 25 If you hold a piece of iron wire with a pair of tongs

inside a candle flame or a Bunsen burner flame, what will you
observe? Will it produce a flame?
Answer. Iron wire will become red hot and glow. It will not produce a
flame.

Question. 26 Fill in the blanks using the words given in the box.
ignition, petrol, combustion, calorific value, combustible,
inflammable
(a) A chemical process in which a substance reacts with oxygen to
give off heat is called ………………
(b) Wood, paper, CNG are ……………substances.
(c) The lowest temperature at which a substance catches fire is
called its …………… temperature.
(d) Ignition temperature of……………… is lower than that of wood.
(e) The substances which have very low……………… temperature
and can easily catch fire with a flame are called……………..
substances.
(f) The amount of heat energy produced on complete combustion
of 1 kg of a fuel is called its …………….
Answer. (a) combustion
(b) combustible (c) ignition
(d) petrol
(e) ignition, inflammable
(f) calorific value

Question. 27 People usually keep angithi/burning coal in their

closed rooms during winter season. Why is it advised to keep the
door open?
Answer. When coal burns in air, it produces carboh dioxide which is non-
poisonous. But, when angithi or coal burns in a closed room, the air or
oxygen cannot enter the room.
So, in the incomplete supply of oxygen, coal burns to produce carbon
monoxide which is highly poisonous gas. It causes suffocation in a closed
room. So, door and windows should be open.

Question. 28 Write True/False against the following statements

and also correct the false statement.
(a) A physical process in which a substance reacts with oxygen to
give off heat is called combustion.
(b) Water is the best extinguisher for fires involving electrical
equipment.
(c) Alcohol, CNG and LPG are inflammable substances.
(d) Increased concentration of nitrogen in air is believed to cause
global warming.
(e) Greater the calorific value, better is the fuel.
(f) Middle zone is the hottest zone of a flame.
(g) The substances which vaporise during burning, give flame.
Answer. (a) False, a chemical process in which a substance reacts with
oxygen to give off heat is called combustion.
(b) False, carbon dioxide is the best extinguisher for fires involving
electrical equipment.
(c) True
(d) False, increased concentration of carbon dioxide in air is believed to
cause global warming.
(e) True
(f) False, outer zone is the hottest zone of a flame.
(g) True

Question. 29 Cracker on ignition produces sound. Why?

Answer. When a cracker is ignited, a sudden reaction (very rapid) takes
place with the evolution of heat, light and sound and a large amount of
gas is liberated. Such a reaction is called explosion. The gases produced
are heated by the heat evolved in the reaction.
The hot gases expand rapidly and cause an explosion (producing a loud
sound).

Question. 30 What do you understand by fuel efficiency?

Answer. Fuel efficiency is determined by its calorific value which is the
amount of heat energy produced on complete combustion of 1 kg of a
fuel. The calorific value of a fuel is expressed in kJ/kg.
A good fuel should have high calorific value. That means it should produce
large amount , of heat per unit mass.

Long Answer Type Questions

Question. 31 You are provided with three watch glasses

containing milk, petrol and mustard oil respectively. Suppose you
bring a burning candle near these materials one by one, which
material(s) will catch fire instantly and why?
Answer. The watch glass containing petrol will catch fire instantly
because its ignition temperature is very low. Also, petrol is an inflammable
substance, i.e. it can easily catch fire with a flame.

Question. 32 Manu was heating oil to fry potato chips. The

cooking oil all of a sudden caught fire, he poured water to
extinguish the fire. Do you think, this action was suitable? If yes,
why? If not, why not? In such a condition what should Manu have
done?
Answer. Pouring water to extinguish the fire due to oil was not suitable
action. It is because oil is lighter than water. So, water will settle down
below the oil. The oil floats on water and continues to burn.
In such condition, sand or soil should be used. They cut off the supply of
air to the fire. Thus, fire can be controlled. But sand or soil will not be
available at that place.
So, Manu should have switched off the flame of the burner and put a lid on
the frying pan. By doing this, the contact between fuel and oxygen is cut
off and the flame will go off,

Question. 33 What are the three essential requirements to

produce fire? How fire extinguisher is useful for controlling the
fire?
Answer. Three essential requirements to produce fire are as follows:
(i) Fuel (combustible substance) (ii) Air (or oxygen) and
(iii) Heat to acquire the ignition temperature.
The job of fire extinguisher is to cut off the supply of air or to bring down
the temperature of fuel or both.
The most common fire extinguisher is water. But water works only when
things like wood and paper are on fire. Water extinguishes fire by cooling
the burning substance.
For fires involving electrical equipment and inflammable materials like
petrol, then carbon dioxide (C02)is the best extinguisher.
C02 being heavier than oxygen, covers the fire like a blanket. Since, the
contact between the fuel and oxygen is cut off, so the fire can be
controlled. The added advantage of C02 is that in most cases, it does not
harm the electrical equipment.

Question. 34 Give two examples each for a solid, liquid and

gaseous fuel along with some important uses.
Answer. Solid fuels Examples are wood and coal. These are used to cook
food in homes. Coal is also used in industries.
Liquid fuels Examples are kerosene and petrol. Kerosene is used in stoves
to cook food and in lamps and petrol is used as a fuel in automobiles.
Gaseous fuels Examples are natural gas and petroleum gas. These are
used in . industries, CNG is used to run automobiles.

Question. 35 The calorific values of petrol and CNG are 45000

kJ/kg and 50000 kJ/kg respectively. If you have vehicle which can
run on petrol as well as CNG, which fuel will you prefer and why?
Answer. We will prefer CNG (Compressed Natural Gas) because the
calorific value of CNG is higher than that of petrol. CNG will produce large
amount of heat energy than petrol.
At the same lime, it produces the least air pollutants. CNG will be more
economical.

Question. 36 Although wood has a very high calorific value, we

still discourage its use as a fuel. Explain.
Answer. Burning of wood has several disadvantages. These are as
follows:
(i) Burning of wood produces a lot of smoke which causes respiratory
diseases.
(ii) The cutting down of trees to obtain as a wood fuel leads to
deforestation which is very harmful to the environment.
(iii) Trees provide us many-useful substances. To obtain fuel wood, when
trees are cut down, then all useful substances which can be obtained from
trees are lost.

Question. 37 Forest fire produces a lot of air pollution. Write in

brief about the reasons of forest fires.
Answer. Reasons of forest fires are :
(i) At high temperature, sometimes dry grass catches fire which spreads
throughout the forest.
(ii) Camp fire may also be a reason.
(iii) Due to the spark of lightning from the sky.
(iv) The use of fires by villagers to ward off wild animals.
(v) Fire lit intentionally by people living around forests for recreation.
(vi) Fires started accidentally by careless visitors to forests. When they
throw away lighted cigarettes in the forest.
(vii) The friction of bamboos due to high wind velocity and rolling stones.

Question. 38 Complete the crossword with the help of the clues.

Across
1. Non-metal which catches fire if exposed to air. (10)
3. The lowest temperature at which a substance catches fire is
called its …………. temperature. (8)
5. The most common fire extinguisher. (5)
Down
2. A chemical process in which a substance reacts with oxygen to
give off heat. (10)
4. Petrol is used as a ………….. in automobiles. (4)
6. It is as hard as stone and black in colour. (4)
Answer.Across
1. Phosphorus 3. Ignition 5. Water
Down
2. Combustion 4, Fuel 6. Coal

Question. 1 A substance that reacts with oxygen giving heat, is

called a combustible substance. Which one of the following is a
combustible substance?

(a) Iron nail (b) Glass (c) Stone piece (d) Wood

Answer. (d) Wood is a combustible substance. It is a solid fuel and

produces smoke during combustion.

Question. 2. Which one of the following has the highest calorific

value?

(a) Kerosene (b) Biqgas (c) LPG (d) Petrol

Answer. (c) LPG has the highest calorific value, i.e., 55000 kJ/kg.

It means that if 1 kg of LPG is burnt completely, it will produce 55000

kilojoules of heat energy.

Note
Calorific value

45000 kJ/kg 35000 to 40000 kJ/kg 45000 kJ/kg

Question. 3 Magnesium ribbon on burning in air produces

(a) magnesium oxide, water, and light (b) magnesium oxide, and
heat (c) magnesium oxide, heat, and light (d) magnesium oxide,
water, and heat

Answer. (c) Magnesium ribbon, burning in air, produces magnesium

oxide, heat, and light; magnesium ribbon is a very reactive metal.

Question. 4 Which of the following is not a combustible

substance?

(a) Camphor (b) Glass (c) Straw (d) Alcohol

Answer. (b) Those substances which do not burn are called a non-
combustible substance. Some non-combustible substances are stone,
glass, cement, soil, sand, iron nails, etc.

Question. 5 The substance that does not burn with flame is

(a) LPG (b) camphor (c) dry grass (d) charcoal

Answer. (d) Charcoal is a solid fuel that does not vaporise on heating. So,
charcoal does not burn by producing a flame. It only glows on combustion.

Question. 6 On placing an inverted tumbler over a burning candle,

the flame extinguishes after some time. This is because of the
non-availability of (a) oxygen, (b) water vapours, (c) carbon
dioxide, (d) wax.

Answer. (a) The flame extinguishes because of the non-availability of

oxygen. Air or oxygen is necessary for combustion.

Question. 7 If a person’s clothes catch fire, the best way to

extinguish the fire is to

(a) throw water on the clothes

(b) use fire extinguisher

(c) cover the person with a woolen blanket

(d) cover the person with a polythene sheet

Answer. (c) Cover the person with a woolen blanket so that the air supply
to the burning clothes is cut off; hence, the burning (or fire) stops.

Question. 8 The substance expected to have the highest ignition

temperature out of the following is
(a) kerosene (b) petrol (c) coal (d) alcohol

Answer. (c) Coal has the highest ignition temperature.

Note The lowest temperature at which a substance catches fire and starts
burning is called its ignition temperature. Kerosene, petrol, and alcohol
have low ignition temperatures.

Question. 9 Choose the correct statement about inflammable

substances from the following.

They have

(a) low ignition temperature and cannot catch fire easily

(b) high ignition temperature and can catch fire easily

(c) low ignition temperature and can catch fire easily

(d) high ignition temperature and cannot catch fire easily

Answer. (c) Inflammable substances have low ignition temperatures and

can catch fire easily.

Question. 10 Choose the incorrect statement from the following.

Forest fires are usually due to

(a) the carelessness of humans, (b) the heat of Sun (c) the cutting
of trees, (d) lightning strike

Answer. (c) Forest fires are not due to the cutting of trees,

Question. 11 The calorific value of a fuel is expressed in a unit

called

(a) kilojoule per litre (b) kilogram per millilitre (c) kilojoule per
gram (d) kilojoule per kilogram

Answer. (d) The calorific value of a fuel is expressed in the unit of

kilojoule per kilogram (kJ/kg),

Question. 12 In villages, people use wood as fuel because

(a) it is considered to be an ideal fuel (b) of its easy availability

and low cost (c) it is environment friendly (d) it catches fire easily

Answer. (b) In villages, people use wood as fuel because of its easy
availability and low cost.

Question. 13 Which one among the following is considered the

cleanest fuel?

(a) Cow dung cake (b) Petrol (c) Kerosene (d) Hydrogen gas
Answer. (d) Hydrogen gas is considered as cleanest fuel. Unlike carbon-
based fuels, hydrogen produces no harmful by-products on combustion.
Only energy and clean water are produced.

Question. 14 Choose the incorrect statement from the following.

A good fuel is one which

(a) is readily available

(b) produces a large amount of heat

(c) leaves behind many undesirable substances

(d) burns easily in the air at a moderate rate

Answer. (c) Good fuel does not produce harmful gases or leave any
residue after burning. So, the incorrect statement is option (c).

Question. 15 Shyam was cooking potato curry on a chulha. To his

surprise, he observed that the copper vessel was getting
blackened from the outside. It may be due to –
(a) proper combustion of fuel, (b) improper cooking of potato
curry (c) improper combustion of fuel, (d) burning of copper
vessel

Answer. (c) Due to incomplete fuel combustion, the copper vessel was
getting blackened from the outside.

CBSE Syllabus for Class 8

CBSE Class 8 Science Syllabus CBSE Class 8 Information Techno

Syllabus

CBSE Class 8 Social Science Syllabus CBSE Class 8 Maths Syllabus

CBSE Class 8 Hindi Syllabus CBSE Class 8 English Syllabus

Very Short Answer Type Questions

Question. 16 Fill in the blanks.

(a)A ……….. process in which a substance reacts with ……….. to

give off heat
is called combustion.
(b) When the clothes of a person catch …………the person is
covered with a…………… to extinguish fire.

(c) The………….. temperature at which a substance catches fire is

called its…………..temperature.

(d) The substances which give very …………..ignition temperature

and can easily catch fire with a flame are
called………….substances.

(e) The substances which vaporize during……………..give flame.

Answer.(a) chemical, oxygen

(b) fire, blanket

(c) lowest, ignition

(d) low, inflammable

(e) burning

Question. 17 Some words (underlined) in the following sentences

are jumbled up. Write them in their correct form.

(a) Seldie is a combustible substance.

(b) Slaas is a non-combustible material.

(c) Chittsmack does not burn by itself.

(d) Some substances on combustion produce thea and mafel.

(e) The amount of heat energy produced on complete combustion

of 1 kg of a fuel is called its ficalroic value.

Answer. (a) Diesel (b) Glass (c) Matchstick (d) heat, flame (e) calorific

Question. 18 Two glass jars, A and B, are filled with carbon

dioxide and oxygen gases, respectively. In each jar, a lighted
candle is placed simultaneously. In which jar will the candle
remain lighted for a longer time, and why?

Answer. In jar B, the candle remains lit for a longer time because oxygen
supports combustion. So, we can say that oxygen (air) is necessary for
combustion to take place.

Question. 19 Anu wants to boil water quickly in a test tube. On

observing the j different zones of the flame, she cannot decide
which zone of the flame will be best for boiling water quickly.
Help her in this activity.
Answer. Anu should keep her test tube in the outermost or non-luminous
zone of the flame because it is the hottest flame zone and has more
temperature.
In the outer zone of a flame, the fuel’s complete combustion occurs
because there is plenty of air around it, so water in a test tube will boil
quickly in this zone.

Question. 20 Why is using diesel and petrol as fuels in

automobiles being replaced by Compressed Natural Gas (CNG) in
big cities?

Answer. When CNG burns, it produces harmful products in very small

amounts. It is a clean fuel because it burns without producing smoke.
Using CNG as fuel in automobiles has reduced air pollution In big cities.

Short Answer Type Questions

Question. 21 Boojho wants to separate the materials as

combustible and non-combustible. Can you help him? Charcoal,
chalk, stone, iron rod, copper coin, straw, cardboard, glass, paper,
candle, wood.

Answer. 1. Combustible — charcoal, straw, cardboard, paper, candle,

wood.

2. Non-combustible — chalk, stone, iron rod, copper coin, glass.

Question. 22 State whether the following statements are

True/False.

(i) Air is necessary for combustion.

(ii) Magnesium is a non-combustible metal.

(iii) Carbon dioxide is an excellent fire extinguisher.

(iv) Calorific value of wood is higher than that of coal.

Answer. 1. True, air contains oxygen, and oxygen is a supporter of

combustion.

2. False; magnesium is a combustible metal.

3. True

4. False; the calorific value of coal is higher than wood.

Question. 23 Match the items of Column I with the items of
Column II.

Answer. The correct matching is as given: (a)—(iv), (b)-(vi), (c)-(v), (d)—

(iii), (e)-(i), (f)-(ii)

Question. 24 Match the following for the flame of a candle.

Answer. The correct matching is as given: (a)—(iii) (x), (b)—(ii) (z), (c)—(i)
(y)

Question. 25 What will you observe if you hold a piece of iron wire
with a pair of tongs inside a candle flame or a Bunsen burner
flame? Will it produce a flame?

Answer. The iron wire will become red hot and glow. It will not produce a
flame.

Question. 26 Fill in the blanks using the words in the box.

Ignition, petrol, combustion, calorific value, combustible,
inflammable

(a) A chemical process in which a substance reacts with oxygen to

give off heat is called ………………

(b) Wood, paper, and CNG are ……………substances.

(c) The lowest temperature at which a substance catches fire is

called its …………… temperature.

(d) Ignition temperature of……………… is lower than that of wood.

(e) The substances with very low……………… temperature and can

easily catch fire with a flame are called…………….. substances.

(f) The amount of heat energy produced on complete combustion

of 1 kg of a fuel is called its …………….

Answer. (a) combustion

(b) combustible

(c) ignition

(d) petrol

(e) ignition, inflammable

(f) calorific value

Question. 27 People usually keep angithi/burning coal in their

closed rooms during winter. Why is it advised to keep the door
open?
Answer. When coal burns in the air, it produces carbon dioxide, which is
non-poisonous. But, when angithi or coal burns in a closed room, the air or
oxygen cannot enter the room.
So, in the incomplete supply of oxygen, coal burns to produce carbon
monoxide, a highly poisonous gas. It causes suffocation in a closed room.
So, the door and windows should be open.

Question. 28 Write True/False against the following statements

and also correct the false information.

(a) combustion is a physical process in which a substance reacts

with oxygen to give off heat.

(b) Water is the best extinguisher for fires involving electrical

equipment.

(c) Alcohol, CNG, and LPG are inflammable substances.

(d) Increased nitrogen concentration in the air is believed to

cause global warming.

(e) Greater the calorific value, the better the fuel.

(f) Middle zone is the hottest zone of a flame.

(g) The substances which vaporise during burning give flame.

Answer. (a) False; a chemical process in which a substance reacts with

oxygen to give off heat is called combustion.

(b) False; carbon dioxide is the best extinguisher for fires involving
electrical equipment.

(c) True

(d) False; increased concentration of carbon dioxide in the air is believed

to cause global warming.

(e) True

(f) False; the outer zone is the hottest zone of a flame.

(g) True

Question. 29 Cracker on ignition produces sound. Why?

Answer. When a cracker is ignited, a sudden reaction (very rapid) takes

place with the evolution of heat, light, and sound, and a large amount of
gas is liberated. Such a reaction is called an explosion. The gases
produced are heated by the heat evolved in the reaction.
The hot gases expand rapidly and cause an explosion (producing a loud
sound).
Question. 30 What do you understand by fuel efficiency?

Answer. Fuel efficiency is determined by its calorific value, the amount of

heat energy produced on the combustion of 1 kg of fuel. The calorific
value of a fuel is expressed in kJ/kg. A good fuel should have a high
calorific value. That means it should produce a large amount of heat per
unit mass.

More Resources – NCERT Solutions for Class 8

NCERT Solutions for Class 8 Science NCERT Solutions for Class 8 Soci
Science

NCERT Solutions for Class 8 English NCERT Solutions for Class 8 Math

Long Answer Type Questions

Question. 31 You are given three watch glasses containing milk,

petrol, and mustard oil. Suppose you bring a burning candle near
these materials one by one; which material(s) will catch fire
instantly and why?

Answer. The watch glass containing petrol will catch fire instantly
because its ignition temperature is very low. Also, petrol is an inflammable
substance, i.e., it can easily catch fire with a flame.

Question. 32 Manu was heating oil to fry potato chips. The

cooking oil suddenly caught fire; he poured water to extinguish
the fire. Do you think this action was suitable? If yes, why? If not,
why not? In such a condition, what should Manu have done?

Answer. Pouring water to extinguish the fire due to oil was not suitable
action. It is because oil is lighter than water. So, water will settle down
below the oil. The oil floats on the water and continues to burn.

In such conditions, sand or soil should be used. They cut off the supply of
air to the fire. Thus, fire can be controlled. But sand or soil will not be
available at that place.
So, Manu should have switched off the flame of the burner and put a lid on
the frying pan. By doing this, the contact between fuel and oxygen is cut
off, and the flame will go off,

Question. 33 What are the three essential requirements to

produce fire? How is a fire extinguisher helpful in controlling fire?

Answer. Three essential requirements to produce fire are as follows:

(i) Fuel (combustible substance),

(ii) Air (or oxygen) and

(iii) Heat to acquire the ignition temperature.

The job of a fire extinguisher is to cut off the supply of air, bring down the
temperature of the fuel, or both. The most common fire extinguisher is
water. But water works only when things like wood and paper are on fire.
Water extinguishes fire by cooling the burning substance.

For fires involving electrical equipment and inflammable materials like

petrol, carbon dioxide (C02)is the best extinguisher. C02 being heavier
than oxygen covers the fire like a blanket. Since the contact between the
fuel and oxygen is cut off, the fire can be controlled. The added advantage
of C02 is that it does not harm the electrical equipment in most cases.

Question. 34 Give two examples of a solid, liquid, and gaseous

fuel, along with some important uses.

Answer. Solid fuels Examples are wood and coal. These are used for
cooking food in homes. Coal is also used in industries. Liquid fuels
Examples are kerosene and petrol. Kerosene is used in stoves to cook food
and lamps, and petrol is used in automobiles. Gaseous fuels Examples are
natural gas and petroleum gas. These are used in. industries; CNG is used
to run automobiles.

Question. 35 The calorific values of petrol and CNG are 45000

kJ/kg and 50000 kJ/kg, respectively. If you have a vehicle that can
run on petrol and CNG, which fuel will you prefer and why?

Answer. We will prefer CNG (Compressed Natural Gas) because the

calorific value of CNG is higher than that of petrol. CNG will produce a
large amount of heat energy than petrol.
At the same time, it produces the least air pollutants. CNG will be more
economical.

Question. 36 Although wood has a very high calorific value, we

still discourage its use as a fuel. Explain.

Answer. Burning wood has several disadvantages. These are as follows:

(i) Wood burning produces a lot of smoke which causes respiratory

diseases.

(ii) The cutting down of trees to obtain wood fuel leads to deforestation,
which is very harmful to the environment.
(iii) Trees provide us with many-useful substances. To obtain fuel wood,
when trees are cut down, all useful substances obtained from trees are
lost.

Question. 37 Forest fire produces a lot of air pollution. Write in

brief about the reasons for forest fires.

Answer. The reasons for forest fires are :

(i) At high temperatures, sometimes dry grass catches fire which spreads
throughout the forest.

(ii) Campfire may also be a reason.

(iii) Due to the spark of lightning from the sky.

(iv) The use of fires by villagers to ward off wild animals.

(v) Fire lit intentionally by people living around forests for recreation.

(vi) Fires started accidentally by careless visitors to forests when they

throw away lighted cigarettes in the forest.

(vii) The friction of bamboo due to high wind velocity and rolling stones.

Question. 38 Complete the crossword with the help of the clues.

Across

1. Non-metal, which catches fire if exposed to air. (10)

3. The lowest temperature at which a substance catches fire is its
…………. temperature. (8)
5. The most common fire extinguisher. (5)
Down
2. A chemical process in which a substance reacts with oxygen to
give off heat. (10)
4. Petrol is used as a ………….. in automobiles. (4)
6. It is as hard as stone and black. (4)

Answer. Across

1. Phosphorus 3. Ignition 5. Water

Down

2. Combustion 4, Fuel 6. Coal