COMBUSTION AND FLAME
1. What is combustion?
A: A chemical process in which a substance reacts with oxygen or air with the
liberation of heat and light is called combustion.
Substances can be classified as combustible or non-combustible substances depending
on whether or not they can be burnt.
2. What are fuels?
A: Substances that undergo combustion with release of energy are called fuels.
3. Define ignition temperature?
A: The lowest temperature at which a substance catches fire is known as ignition
temperature.
● The ignition temperature of phosphorus is 35 degrees celsius. It means
that phosphorus will not catch fire unless heated above 35 degrees
celsius.
4. What are inflammable substances?
A: Substances which have a low ignition temperature are called inflammable
substances.
● Eg: alcohol, petrol, acetone (nail polish remover) and perfume etc.
5. What are the different types of combustion?
A: Combustion can be divided into three types;
Rapid, Spontaneous,Explosion
Rapid : A combustion in which the fuel burns rapidly and produces heat and light.
Eg:LPG.
Spontaneous : A combustion in which a material suddenly bursts into flames, without
the application of any apparent cause is called spontaneous combustion.Eg: Forest
fires, sodium,Phosphorus and potassium.
Explosion : When a substance is ignited, a sudden reaction takes place with evolution
of light, heat and sound. A large amount of gas is formed in the reaction. Such a
reaction is called an explosion.Eg: Bursting of crackers, blast of cylinders, bombs.
6. What are the conditions necessary for combustion?
A:The conditions required for combustion are as follows
★ Presence of a combustible substance(fuel)
★ Presence of a supporter of combustion(oxygen)
★ Heat to raise the temperature of the fuel beyond the ignition temperature.
7. Explain how CO2 is able to control fire.
A: CO2 being heavier than oxygen, covers the fire like a blanket . Since the contact
between the fuel and oxygen is cut off , the fire is controlled.
● It can also cool the burning substance since it expands enormously in volume
when released from the cylinder.
8. Explain the different zones of a flame with the help of a diagram.
A:
I. The Outermost Zone:(Non luminousZone)
● Hottest part of the flame
● Wax vapours undergo complete combustion.
● It burns with a blue flame.
II. The Middle Zone:(Luminous zone)
● Moderately hot.
● Wax vapours undergo partial combustion.
● Burns with a yellow flame.
III. The Innermost Zone:(Dark zone)
● Least hot
● Wax vapours remain unburnt as no oxygen is available.
● Completely black.
9. Define calorific value. What is its unit?
A: The amount of heat energy produced on complete combustion
of 1kg of a fuel is called its calorific value.
The calorific value of a fuel is expressed in a unit called; Kilo
Joule per Kilogram (KJ/kg).
10. In an experiment, 4.5kg of fuel was completely burned. The
heat produced was measured to be 180,000 KJ. Calculate the
calorific value of the fuel.
A: Calorific Value = Amount of heat produced
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Quantity of fuel
● Quantity of fuel =4.5 kg ; amount of heat produced =
180,000 KJ
= 180,000 KJ
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4.5 Kg
= 40,000 KJ/kg.
11. Write a short note on :
1) Global Warming
● It is the rise in temperature of the atmosphere of the
Earth.
● Combustion of most fuels releases CO₂ in the
environment. Increased concentration of carbon dioxide
in the air is believed to cause global warming.
● This results in the melting of the polar ice caps, which
lead to rising sea levels, causing floods in low lying
areas.
2) Acid Rain
● Burning of coal and diesel releases sulphur dioxide gas.
It is an extremely suffocating and corrosive gas.
● Oxides of sulphur and nitrogen dissolve in rainwater
and form acids. Such rain is called acid rain.It affects
buildings,crops and soil
12. What are the characteristics of a good fuel ?
● Low ignition temperature
● Moderate rate of combustion
● Easily available
● No residue after burning
● Pollution free
● High calorific value
● Safe to handle,store and transport
13. LPG is a better domestic fuel than firewood.Give reason.
● LPG does not give out any harmful residue on burning
● It has a high calorific value (fuel efficiency)
● It is easy to store and transport
● It does not cause any respiratory problems
