# Class 8 Science NCERT Solutions for Chapter – 6 Combustion and Flame

## Combustion and Flame

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

Conditions under which combustion can take place are as follows :

1. The presence of air (oxygen).
2. The ignition temperature should be reached.
3. A source of inflammable substance.

Question 2.
Fill in the blanks.

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

(b) A liquid fuel, used in homes is ______
Kerosene

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

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

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

CNG is a clean fuel. It produces harmful products in a very small amount. That is why pollution in our cities has reduced.

Question 4.
Compare LPG and wood as fuels.

LPG burns easily and produces more heat in comparison to wood. Besides, it is a clean fuel i.e., it does not produce fumes and ashes as wood do.

Question 5.
Give reasons :

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

Water is not used to control fires produced by .electrical equipment because water is a conductor of electricity and may cause electrocution or electric shock.

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

LPG is a substance which is readily available. It is cheaper and bums easily in the air at a moderate rate. It produces a large amount of heat and does not leave behind any undesirable substance.

On the other hand, wood does not bum so easily and produces a large amount of smoke which is very harmful to human beings. So, LPG is not a better fuel than wood.

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

Paper catches fire easily, but when it is wrapped around an aluminium pipe and heat ” is supplied than in the presence of aluminium pipe, the ignition temperature of the paper does not reach, because heat given to it is passed to the aluminium pipe which lowers the temperature of the paper. That is why the paper does not catch fire.

Question 6.
Make a labelled diagram of a candle flame.

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

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

Question 8.
Explain how COA is able to control fires.

CO2, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen gets cut off, the fire is controlled.

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

Green leaves contain a lot of water. So, when we try to burn green leaves, water present in the leaves cools the combustible materials (leaves), so that their temperature is brought below their ignition temperature. This prevents the burning of green leaves.

On the other hand, dry leaves do not contain any water. So when a burning process starts, their temperature is raised drastically above their ignition temperature and the leaves catch fire easily.

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

The goldsmith uses the outermost zone of a flame with a metallic blow-pipe for melting gold and silver. It is so because the flame in the outermost zone has the highest temperature which is sufficient to melt gold and silver.

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

The Calorific value of fuels

Here, the mass of fuel = 4.5 kg.

The heat produced = 180,000 kJ.

∴ Calorific value of fuel = $\frac{180,000 \mathrm{~kJ}}{4.5 \mathrm{~kg}}$

= 40,000 kJ/kg.

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

Yes, the process of rusting can be called combustion, in fact, slow combustion because rusting also takes place in the air (O2) in the presence of humidity.

Question 13.
Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?

The water of Ramesh’s beaker will get heated in a shorter time because the outermost part of the flame is the hottest.

Question 1.
Can you name a few fuels used in our homes?

• Cow dung cakes
• Wood
• Coal
• Charcoal
• Kerosene and LPG are a few fuels used in our homes.

Question 2.
Name a few fuels used in trade and industry.

• LPG
• Coal
• Petrol
• Diesel and nuclear fuels are used in trade and industry.

Question 3.
What fuels are used for running automobiles?

• Diesel
• Petrol
• CNG and LPG are used for running automobiles.

Question 4.
What is the difference between the burning of a candle and the burning of fuel like coal?

Candle bums with flame whereas coal does not.

6.1 What is Combustion?

Question 5.
Hold the piece with a pair of tongs and bring it near the flame of a candle or a Bunsen burner. What do you observe?

We see that charcoal burns in air producing carbon dioxide, heat and light.

Activity 6.1

Question 1.
If combustion takes place to mark the material combustible, otherwise mark it non-combustible.

Question 6.
Can you name some more substances which are combustible?

• Clothes
• Rubber
• Plastic
• Coal
• Bamboo, etc., are some of the other substances which are combustible.

Activity 6.2

Question 1.
What happens in the three cases?

The candle bums unaffected in case

• (a) when air can enter the chimney from below.
• In case (b), when air does not enter the chimney from below, the flame flickers produces smoke and goes off.
• In case (c), the flame goes off because the air is not available.

Question 2.
Can you infer anything at all about the role played by air in the process of burning?

It is clear from the activity that air is necessary for burning.

Activity 6.3

Question 1.
Does the charcoal stop burning after sometimes? Can you think of the reason why it stops burning?

Yes, the piece of burning wood/ charcoal stops burning after sometime when it is covered with a glass jar or a tumbler. The piece of burning wood/charcoal keeps on burning till there is air available inside the glass jar/tumbler.

As soon as the available air gets consumed it stops burning as the jar/ tumbler stops the supply of air to the wood/ charcoal.

Question 7.
You might have heard that when the clothes of a person catch fire, the person is covered with a blanket to extinguish The fire. Can you guess why?

It is so because blanket stops the supply of air (oxygen) to the fire and thus the fire is extinguished immediately.

Question 8.
Does a matchstick burn by itself? How does it burn?

No. A matchstick does not bum on its own. When it is rubbed against the side of a matchbox then it bums.

Question 9.
You must have had an experience of burning a piece of paper. Does it burn when a burning matchstick is brought near it?

Yes, it burhs.

Question 10.
Can you burn a piece of wood by bringing a lighted matchstick near it?

No, because the wood has high ignition temperature and it requires to be heated for a longer time to start its burning. Lighted matchstick extinguishes before the wood can reach its ignition temperature.

Question 11.
Why do you have to use paper or kerosene oil to start a fire in wood or coat?

As wood or coal has high ignition temperature, it requires continuous heating for a lot of time before its burning can take place; That is why paper or kerosene oil which has lower ignition temperature is burnt with wood to start the fire.

Question 12.
Have you heard of a forest fire? How does it occur?

Yes, I have heard of it.
Reasons for forest fire :

1. Spontaneous burning of dry grasses due to the extreme heat of summer.
2. Lightning.
3. Human activities like throwing away of burning pieces of bidis and cigarettes and bonfires left unextinguished, etc.

Question 13.
Can you tell now why a matchstick does not catch fire on its own at room temperature? Why does the matchstick start burning on rubbing it on the side of the matchbox?

A Combustible substance cannot catch fire or bum as long as its temperature is lower than its ignition temperature. So, when the matchstick is rubbed on the side of the matchbox, due to friction it reaches its ignition temperature and starts burning.

Question 14.
Have you ever seen cooking oil catching fire when a frying pan is kept for long on a burning stove? Why it is so?

Yes, though cooking oil does not catch fire on its own at room temperature it is kept on the burning stove for long, it reaches its ignition temperature and starts burning.

Question 15.
Does it mean that the ignition temperature of kerosene oil is lower than that of wood? Does it mean that we need to take special care in storing kerosene oil?

Yes, kerosene oil has a lower ignition temperature than wood. So, we need to take special care in storing it.

Activity 6.4

Question 1.
Make two paper cups by folding a sheet of paper. Pour about 50 mL of water in one of the cups. Heat both the cups separately with a candle. What do you observe?

The paper cup in which water was taken does not bum. However the other cup bums.

Question 2.
What happens to the empty paper cup? What happens to the paper cup with water? Does water in this cup becomes hot?

The empty cup when heated will start burning within a few seconds. On the other hand, paper cup with water does not burn. Water in this cup becomes hot. The paper cup containing water does not bum.

Question 3.
Can you think of an explanation for this phenomenon?

Here, the heat supplied to the paper cup is transferred to the water. Further, the water lowers the temperature of the paper. In the presence of water, the ignition temperature of paper is not reached. Hence, it does not bum.

6.2 How do we Control Fire?

Question 16.

Yes, my city has a fire brigade station.

6.4 Flame

Question 17.
Can you tell the colour of the flame? What is the colour of a candle flame?

The colour of the LPG flame is bluish. The colour of a candle flame is yellowish.

Question 18.
Record your observations and mention whether on burning the material forms a flame or not.

6.5 Structure of a Flame

Activity 6.5

Question 1.
Do you see a flame caught at this end of the glass tube after a while? If so, what is it that produces a flame?

Yes, the substances which vapourise during burning, produce flame, e.g., Molten wax, kerosene oil, etc.

Question 19.
Activity 6.5, could the vapours of wax coming out of the glass tube be the cause of the flame produced?

Yes, it is so.

Question 20.
Does it indicate that the non-luminous zone of the flame has a high temperature?

Yes, it has.

Question 21.
Goldsmiths blow the outermost zone of a flame with a metallic blow-pipe for melting gold and silver (Fig. 6.14, textbook p 72). Why do they use the outermost zone of the flame?

It is because the outermost part of the flame is the hottest.

6.6 What is Fuel?

Question 22.
Make a list of fuels familiar to you. Group them as solid, liquid and gaseous fuels as in Table 6.3 (Textbook P. 72).

6.7 Fuel Efficiency

Question 23.
Suppose you were asked to boil a given quantity of water using cow dung, coal and LPG as fuel. Which fuel would you prefer? Give your reason. Do these three fuels produce the same amount of heat?

I would prefer LPG because

1. It burns easily.
2. It produces more heat in comparison to Cow dung and coal, and
3. It is a clean fuel, i.e., it does not produce smoke and ash.
4. No, these three fuels do not produce the same amount of heat.