NOTES PREPARED BY
ASHAQ HUSSAIN BHAT
TEACHER SCHOOL
EDUCATION DEPARTMENT
JAMMU AND KASHMIR
CHAPTER 7
'Combustion and Flame'
KEYWORDS
Combustion: A chemical process, in which a substance reacts with oxygen to give off heat.
Fuel: A substance, which gives heat or energy on combustion.
Ignition Temperature: The lowest temperature, at which a substance catches fire.
Inflammable Substances : Substances having very low ignition temperature, easily catch fire with flame.
Explosion: A reaction (sudden), which give large amount of heat, light and gases.
Fuel Efficiency: The amount of heat energy produced on one complete combustion of heat, calorific value.
Acid Rain: Combustion of fuel produces nitrogen oxide, sulphur dioxide gases, which dissolve in rain water and form acids.
TEXT BOOK QUESTIONS
Q.1. List conditions under which combustion can take place.
Ans. The conditions of combustion are as follows:
(a) There must be proper supply of oxygen.
(b) The substance must be heated upto its ignition temperature before combustion.
Q.2. Fill in the blanks :
(i) Burning of wood and coal causes.........of air
(ii) A liquid fuel, used in homes is...............
(iii) Fuel must be heated to its.................before it starts burning.
(iv) Fire produced by oil cannot be controlled by..............
Ans. (i)- Pollution; (ii) - Liquefied Petroleum Gas (LPG); (iii) - Ignition temperature; (iv) - Water
Q.3. Explain how the use of CNG in automobiles has reduced pollution in our cities
Ans. The combustion of CNG (Compressed Natural Gas) does not
produce any residue and hazardous gases, like; nitrogen oxide,
sulphur dioxide. These gases are main cause of air pollution.
Therefore, use of CNG in automobiles helps in reducing of pollution.
Q.4. Compare LPG and wood as fuels.
Ans.
LPG
(i) It is gaseous fuel.
(i)It does not produce smoke.
(iii) Its calorific value is more (55000kJ/kg).
(iv) It is easily stored in cylinders.
(v) It does not cause any pollution.
helps in reducing of
Wood
(i) It is solid fuel.
(ii) It produces smoke.
(iii) Its calorific value is less (17000kJ/kg).
(iv)It requires more space to store.
(v) It causes more pollution.
Q.5. Give reasons:
(a) Water is not used to control the fire involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminum pipe does not.
Ans.
(a) In case of fires produced by electric shock or electric equipments, water is not used at all because water is a good not conductor of electricity and may result in electrocution or electric shock.
(b) LPG is liquefied petroleum gas. It has high calorific value of 50 KJ/g. It is neat and clean fuel. It burns with a smokeless flame and does not produce poisonous gases. While wood has less
calorific value of 17 KJ/g. It gives out smoke and poisonous gases on burning. So, LPG is better fuel than wood.
(c) When paper wrapped around an aluminium pipe is brought near a flame, it does not burn because the heat gets transferred to aluminium pipe and the ignition temperature of paper is nor
achieved.
Q.6. Make a labelled diagram of a candle flame.
Ans.
Hottest part .................Combustion (Blue)
Moderately hot............Middle zone of partial
combustion (yellow)
Least hot-.............Inner zone of unburnt
wax vapours (black)
Fig. Different Zones of Candle flame
Q.7. Name the unit in which the calorific value of a fuel is expressed?
Ans. The calorific value of a fuel is expressed in terms of Kilojoules
per Kg. (KJ/Kg.).
Q.8. Explain how CO, is able to control fires.
Ans. (1) CO₂ forms a blanket around fire due to which supply of air is stopped.
(ii) CO₂ also brings down the temperature of the fuel.
Q.9. It is difficult to burn a heap of green leaves, but dry leaves catch fire easily. Explain.
Ans. The green leaves contain some water dew with which the ignition temperature of leaves increases and they do not catch fire easily
while dry leaves have no water so they catch fire easily.
Q.10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Ans. The blue, non-luminous zone (outer zone) because this point of the flame has the high temperature, so it is used for melting gold
and silver.
Q.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 calorific value of the fuel.
Ans. Total mass of fuel = 4.5 kg
Total heat produced = 1,80,000 KJ
Heat produce by burning 1 kg of fuel
= 1,80,000 KJ / 4.5 kg
= 40,000 kJ/kg
So, the calorific value of fuel = 40,000 KJ/Kg.
Q.12. Can the process of rusting be called combustion ? Discuss.
Ans. The process of rusting cannot be called as combustion because
neither release of energy nor heat and light are produced during
it while in combustion-release of energy takes place with heat
and light.
Q.13.Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Arun kept the beaker near the wick in the yellow part of the candle flame. Rakesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?
Ans. As outermost part of flame is more hot than the middle part or yellow flame. So water of Rakesh will be heated in a shorter time.
SHORT ANSWER TYPE QUESTIONS
Q.1. What are inflammable substances ? Give its examples.
Ans. The substances, which have very low ignition temperature and can easily catch fire with a flame are called 'inflammable substances'. Examples of inflammable substances are petrol,
alcohol, Liquefied Petroleum Gas (LPG).
Q.2. Define ignition temperature.
Ans. The lowest temperature, at which a substance catches fire is called its ignition temperature.
Q.3. Define calorific value of a fuel. Also give its unit.
Ans. The amount of heat energy produced on complete combustion of 1kg of fuel is called its calorific value. The calorific value of fuel
is expressed in a unit called kilojoules per kg (kJ/kg)
Q.4. What is an explosion ?
Ans. When a cracker is ignited, a sudden reaction takes-place with the evolution of heat, light and a large amount of gas formed in the reaction is liberated. Such a reaction is called explosion. Explosion can also take place if pressure is applied on the Explosion
Q.5. What are the characteristics of a good fuel ?
Ans. Characteristics of a good fuel :
(i) It should be readily available.
(ii) It should be cheap.
(iii) It burns easily in the air at moderate rate.
on the cracker.
(iv) It produces a large amount of heat.
(v) It does not leave behind any undersirable substances.
Q.6. What is Global Warming? Write its effects.
Ans. Global Warming: The rise in temperature of the atmosphere of the earth, is called global warming. This causes melting of polar ice which leads to a rise in the sea level, causing floods in the coastal areas. Low lying coastal areas may even be, permanently, submerged under water.
Q.7. What is Acid Rain?
Ans. Acid Rain : Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocating and corrosive gas, Moreover petrol engines are responsible for the formation of nitrogen oxide gases. Sulphur dioxide and nitrogen oxides dissolve in rain water and form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil.
Q.8. How does forest fire starts? What is its effect?
Ans. During extreme heat of summer, at some places dry grasses catch fire. From grasses to trees, and very soon the whole forest is on
fire. It is difficult to control such fire. This affects the local environment and climate badly. No rainfall occurs and it leads to the drought and famine.
Q.9. What is done to extinguish fire due to burning of oil or petrol?
Ans. Fire produced by burning of oil or petrol cannot be controlled by throwing (spreading) water on it. Water being heavier than oil,
settles down below the oil and oil continues to burn. This fire can be controlled by throwing sand or soil over it.
Q.10. What would you do in case of fires produced by electric short circuit?
Ans. In the case of fire caused by electric short circuit, water is not used at all because it is a conductor of electricity and may result into electrocution or electric shock. The only solution is to cut off the supply of electricity and then use the spray of water on the flame to extinguish fires or use fire extinguQUESTIONS
LONG ANSWER TYPE QUESTIONS
Q.11. What do you understand by combustion ? Explain different types of combustion.
Ans. Combustion: It is the process of heating of a substance in the presence of oxygen with the evolution of heat and light. It is an oxidation reaction, in which substance (fuel) burns in the presence of oxygen, so, as to liberate heat and light.
Combustion is of three types:
(i) Rapid Combustion: When gases burn rapidly to produce heat and light it is called rapid combustion.
(ii) Spontaneous Combustions: When any material, like; phosphorus burns on its own without any apparent cause, it is called spontaneous combustion.
(iii) Explosion: When combustion takes place with sudden release of heat and light and a large amount of gas in form of bang, it is called explosion as in case of crackers and bombs.
Q.12. Describe the various zones of a candle flame.
Ans. The candle flame has three different zones. These zones can be distinguished on the basis of their colour.
(i) The Outer Zone: It is blue in colour and it has very high temperature. It is the hottest zone of the flame. In this zone the
wax vapours burn completely due to availability of enough oxygen; in turn carbon-dioxide and water vapours are produced.
The Middle Zone: In this zone, the flame burns with yellow colour and the wax vapours start burning here. The yellowish colour of the flame is due to the burning of carbon particles. These carbon particles are produced due to incomplete burning of the wax vapours due to less supply oxygen. Carbon-monoxide is also produced along the carbon particles. This zone is luminious
but with low temperature.
(iii) The Inner Zone: This zone appears black in colour as no combustion of wax vapour take place in this zone due to no supply of oxygen in this zone. It is the coolest zone of the flame having unburnt wax vapour.
Q.13. Discuss the construction and working of soda-acid fire extinguisher.
Ans. Soda-acid fire extinguisher is based on the principle of cooling the fire (as shown in Fig A And B.)
 |
| Soda fire extinguisher Fig A |
 |
| Fig B |
Carbon dioxide liberated by the reaction of sulphuric acid with sodium bicarbonate solution comes out with a
stream of liquid water at high pressure. Water puts off the fire by lowering the temperature
of the combustible material below its ignition temperature and carbon dioxide cuts off the
supply of air to the combustible substance.
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