Combustion and Flame

Answer of Questions Asked in different section

Question In the First paragraph

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

Answer LPG, Kerosene, Coal, wood.

These days in most of the households LPG is used as domestic fuel. However even today kerosene, coal and wood are used in some parts of the country especially in the rural areas.

Question (2) What fuels are used for running automobiles?

Answer Petrol, Diesel and CNG are used for running automobiles. These days some automobiles are run on electric also.

Section 6.1

Question (1) Hold the piece (of charcoal) with a pair of tongs and bring it near the flame of a candle or a Bunsen burner. What do you observe?

Answer Piece of charcoal starts burning and produce heat.

Question (2) You might have heard that when the clothes of a person catch fire, the person is covered with a blanket to extinguish fire. Can you guess why?

Answer

A blanket made of wool act as insulator and cut off the supply of oxygen which extinguish the fire. This is the cause that when the clothes of a person catch fire, the person is covered with a blanket to extinguish fire.

Question (3) Does a matchstick burn by itself? How does it burn?

Answer No a matchstick does not burn by itself.

A matchstick burns only when it is rubbed against the side of matchbox which is meant to rub the matchstick to ignite it. And when a matchstick is brought near the burning object matchstick starts burning.

Question (4) You must have had an experience of burning a piece of paper. Does it burn when a burning matchstick is brought near it?

Answer Yes. A piece of paper catches fire when a burning matchstick is brought near it.

Question (5) Can you burn a piece of wood by bringing a lighted matchstick near it?

Answer No. When a lighted matchstick is brought near a piece of wood it does not start burning because the ignition temperature of wood is higher than a piece of paper.

Question (6) Why do you have to use paper or kerosene oil to start fire in wood or coal?

Answer The ignition temperature of wood or coal is higher and it require more heat to start burning. This is the cause that to start fire in wood or coal paper or kerosene oil is used as the ignition temperature of paper or kerosene oil is lower than that of wood or coal.

Question (7) Have you heard of forest fire?

Answer Yes. Many times fire is broken out in forest in the summer. Because of more heat dry hay are caught fire which spreads to trees also. However sometimes due to negligence of people also fire is broken out in forest.

Question (8) Do these experiences tell you that different substances catch fire at different temperature?

Answer Yes. Different substances have different ignition temperature. Thus different substances catch fire at different temperature.

Question (9) Can you tell now why a matchstick does not catch fire on its own at room temperature?

Answer Matchstick has more ignition temperature than room temperature. This is the cause that a matchstick does not catch fire on its own at room temperature.

Question (10) Why does the match stick start burning on rubbing it on the side of the matchbox?

Answer The heads of the safety match contains antimony trisulphide and potaasium chlorate. And the side of matchstick, which is rubbing surface, contains powdered glass and a little of red phosphorous. When a matchstick is rubbed on the side of matchbox, some red phosphorous gets converted into white phosphorous. This immediately reacts with potassium chlorate in the matchstick head to produce enough heat to ignite antimony trisulphide and start the combustion.

Question (11) Have you ever seen cooking oil catching fire when a frying pan is kept for long on a burning stove?

Answer Yes. When a frying pan containing cooking oil is kept for long on a burning stove, oil in the pan starts vaporizing. When vapour coming out from the frying pan spread, its catches fire.

Question (12) Does it mean that ignition temperature of kerosene oil is lower than that of wood?

Answer Yes. The ignition temperature of kerosene oil is lower than that of wood.

Question (13) Does it mean that we need to take special care in storing kerosene oil?

Answer Yes. It is necessary to take special care in storing kerosene oil. Because it kerosene oil is not stored carefully, it may be fallen on the floor or other substance. And when a fire would be brought near the spread kerosene oil or fallen over it accidently, it can catch fire.

Question (14) Can you list some more inflammable substances?

Answer Apart from petrol, alcohol, LPG some more inflammable substances are Diesel, kerosene oil, CNG, etc.

Section 6.2

Question (1) Does you city/town have a fire brigade station?

Answer Yes, my city has a fire brigade station. In India almost all the district headquarter has a fire brigade station.

Question (2) When a fire brigade arrives, what does it do?

Answer Fire brigade starts pouring water on the fire to extinguish fire. However if fire involving electricity or oil, fire brigade spread carbon dioxide gas over it to extinguish fire. As water is good conductor of electricity thus pouring of water to fire involving electricity may leads to electrocution.

Question (3) You have learnt that there are three essential requirements for producing fire. Can you list these requirements?

Answer The three essential for producing fire are fuel, air (to supply oxygen) and heat. Heat raises the temperature of the fuel beyond the ignition temperature.

Section 6.3

Question (1) Bring a burning matchstick or a gas lighter near a gas stove in the kitchen. Turn on the knob of the gas stove. What do you observe?

Answer Gas coming out of burner catches fire when a burning matchstick is brought near it after turning knob of gas stove on.

Section 6.4

Observe an LPG flame. Can you tell the colour of the flame. What is the colour of a candle flame?

Answer

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

Table 6.2

Some items are given in Table 6.2. Record your observations and mention whether on burning the material forms a flame or not.

Answer

Section 6.5 Structure of a Flame

Question (1) In Activity 6.5, could the vapours of wax coming out of the glass tube be the cause of the flame produced?

Answer Yes. The vapours of wax coming out of the glass tube is the cause of the flame produced.

Question (2) When the candle flame is steady, introduce a clean glass plate/slide into the luminous zone of the flame (Figure). Hold it there with a pair of tongs for about 10 seconds. Then remove it. What do you observe?

Answer

A black spot forms on the glass plate. This black spot on glass plate is formed because unburnt carbon particles coming out of wax are get deposited over it.

Question (3) Hold a thin long copper wire just inside the flame for about 30 seconds.

Notice that the portion of the copper wire just outside the flame gets red hot. Does it indicate that the non luminous zone of the flame has a high temperature?

Answer Yes. The non luminous zone of a candle flame has highest temperature and it is called the outer zone of complete combustion. It is the hottest part of the flame.

Question (4) Goldsmiths blow the outermost zone of a flame with a metallic blow pipe for melting gold and silver. Why do they use the outermost zone of the flame?

Answer The outermost zone of the flame is the zone of complete combustion and is the hottest part of a flame. This is the cause that goldsmiths blow the outermost of a flame with a metallic blow pipe for melting gold and silver.

Section 6.7 Fuel Efficiency

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.

Answer

I will prefer LPG to boil the given water.

This is because LPG has more calorific value than those of cow dung and coal. Thus, LPG will boil water more quickly.

Table 6.3 Types of Fuels

Answer

8-science-home

---

Reference: