Lesson: Combustion and Flame
Topic 1: Combustion and Flame

Question: 1

What is produced during combustion?

Solution:

Heat and light (flame or glow)

Question: 2

What is combustion?

Solution:

The chemical process in which a substance reacts with oxygen to produce heat and light is called combustion. For example, magnesium burns in air to form magnesium oxide and produces heat and light.

Question: 3

What are the different types of combustion?

Solution:

There are three types of combustions:

a)      Rapid combustion

b)      Spontaneous combustion

c)      Explosion

Question: 4

What is ignition temperature?

Solution:

The lowest temperature at which a combustible substance catches fire without the need for an external flame is called its ignition temperature.

Question: 5

What are the conditions required for combustion to take place?

Solution:

For combustion to occur the following conditions are necessary:

a)      Presence of a combustible substance

b)      Presence of oxygen

c)      Enough heat to raise the temperature of the combustible substance to its ignition temperature

Question: 6

Goldsmiths use which zone of a flame for melting gold and silver to make jewellery? Give reasons for your answer.

Solution:

The outermost zone or non-luminous zone is the hottest part of a flame. Due to this reason, goldsmiths use this zone for melting gold and silver.

Question: 7

Why is carbon dioxide used to extinguish fires?

Solution:

Carbon dioxide being heavier than oxygen forms a blanket around the burning substance, thereby cutting off the oxygen supply from the burning material and extinguishing the fire.

Question: 8

Why is it more difficult to burn a heap of green leaves as compared to dry leaves?

Solution:

Unlike dry leaves, green leaves contain water which absorbs heat and makes it more difficult for the ignition temperature to be reached.

Question: 9

An empty paper cup catches fire immediately, but one having water in it does not. Give reasons for this observation.

Solution:

The water conducts heat away from the paper cup. This heat raises the temperature of the water to its boiling point and only after all the water has been converted into vapour can the ignition temperature of the paper cup be reached.

Question: 10

Identify whether the given statement is true or false - The process of rusting can be called combustion.

Solution:

In rusting, though heat is produced there is no flame or glow. Hence it cannot be considered as combustion.

Question: 11

What are combustible and non-combustible substances?

Solution:

Substances that burn in air to produce heat and light are called combustible substances. For example - wood, coal, kerosene, petrol, cardboard, straw etc.

Substances that do not burn in air are called non-combustible substances. For example - stone, cement, glass, iron nails etc.

Question: 12

Neha and Rahul were performing an experiment in which water was to be boiled in a beaker. Neha kept the beaker in the luminous zone of the candle flame whereas Rahul kept the beaker in the outermost zone of the flame. Whose water will boil first?

Solution:

The water in Rahul’s beaker will boil first because he has kept the beaker in the outermost zone which is the hottest part of the candle flame.

Question: 13

When a person’s clothes catch fire, the person is wrapped with a blanket to extinguish the fire. Can you explain why?

Solution:

For a substance to keep burning, a constant supply of air or oxygen is required. A blanket wrapped around a burning object cuts off the supply of air or oxygen, thereby extinguishing the fire.

Question: 14

How does carbon dioxide help in fire-fighting?

Solution:

Carbon dioxide is the best fire extinguisher to put out fires caused by flammable materials such as petrol. Being heavier than air, it covers the fire cutting off the supply of oxygen and putting out the flames.

Question: 15

Why should special care be taken while handling kerosene oil?

Solution:

Kerosene oil is a highly flammable substance with a low ignition temperature. Due to this it easily catches fire on being heated and poses a fire risk.

Question: 16

How does water help to extinguish fire?

Solution:

When water is poured over a burning material, it absorbs heat from the material. As a result, the temperature of the burning material falls below its ignition temperature and put outs the fire.

Question: 17

Why does a candle stop burning when covered with a glass tumbler?

Solution:

When a candle is covered with a glass tumbler, the carbon dioxide formed descends on the candle flame and extinguishes it.

Question: 18

Name the different techniques used for extinguishing fires.

Solution:

In general, any fire extinguisher uses any or all of the following methods.

a)      Removing the combustible substance

b)      Cooling the burning substance to below its ignition temperature.

c)      Cutting off the supply of air/oxygen.

Question: 19

What are different types of combustion?

Solution:

There are three different types of combustion as explained below:

a)      Rapid Combustion: In this type of combustion, the combustible substance burns rapidly and produces both heat and light. This is called rapid combustion. For example, LPG burning on a kitchen stove.

b)      Spontaneous Combustion: Spontaneous combustion occurs due to internal exothermic reactions such as fermentation, which bring the fuel to its ignition point. For example, stacks of hay or compost can self-ignite through heat produced by bacterial fermentation.

c)      Explosion: When combustion takes place with a sudden release of heat, light, sound and a large, rapidly expanding volume of gas, it is called explosion. Examples are crackers and bombs.

Question: 20

How is spontaneous combustion different from an explosion?

Solution:

Spontaneous combustion occurs due to internal exothermic reactions such as fermentation, which bring the fuel to its ignition point. For example, stacks of hay or compost can self-ignite through heat produced by bacterial fermentation.

In an explosion, the introduction of heat or pressure causes a substance to burn suddenly and expands with the generation of a large amount of heat, light and sound. For example, the explosion of crackers on applying heat or pressure. Spontaneous combustion can sometimes lead to an explosion.

Question: 21

Give reasons:

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

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

Solution:

(a)   Water is not used to control fires involving electrical equipment because water may conduct electricity and harm those trying to put out the fire.

(b)   Paper wrapped around an aluminium pipe does not catch fire on heating because it is unable to attain its ignition temperature due to the transfer of heat to the aluminium pipe, which is a good conductor of heat.

Question: 22

Why a matchstick does not catch fire on its own at room temperature?

Solution:

A substance does not catch fire until its temperature is raised to its ignition temperature. A matchstick does not catch fire on its own because its ignition temperature is higher than room temperature.

Question: 23

Draw a well labelled diagram describing the flame of a candle and answer the following questions:

(a) Which zone is luminous and why?

(b) Why is the innermost zone the coldest and black in colour?

(c) Which zone is called the zone of complete combustion and why?

Solution:

(a)   The middle zone is luminous due to partial combustion of wax and unburnt carbon vapour. These unburnt carbon particles impart a pale-yellow colour to the flame.

(b)   The innermost zone is black in colour and is the coldest zone because it contains wax that has been vaporized but is unburnt. No combustion takes place here as no oxygen is available.

(c)   The outermost zone is called the zone of complete combustion. Here, carbon and hydrogen are completely oxidized to carbon dioxide and water vapour.

Question: 24

When does a combustible substance burn with a blue flame and when does it burn with a yellow flame?

Solution:

Complete combustion of a combustible substance gives rise to a blue coloured flame, whereas partial combustion of a combustible substance gives rise to unburnt carbon particles which glow with heat and impart a yellow colour to the flame.

Question: 25

Why do forest fires occur during extremely hot summers?

Solution:

During extremely hot summers, the high temperature sometimes reaches the ignition temperature of dry grass in the forest, due to which the dry grass catches fire. The fire from burning grass spreads to bushes and trees and very soon the whole forest is on fire.

Question: 26

Does a piece of wood burn when a lighted matchstick is brought close to it?

Solution:

No, it does not burn. This is because wood has a high ignition temperature which is not reached by the small amount of heat in a matchstick flame.

Question: 27

Why do we use paper or kerosene oil to burn coal?

Solution:

Coal has a high ignition temperature which cannot be reached by the small amount of heat in a matchstick flame. It needs a steady supply of heat from burning a combustible substance like paper or kerosene for some time to raise the coal temperature to its ignition point.

Question: 28

Why do kerosene oil lamps give off a flame while burning, whereas charcoal does not produce one?

Solution:

Kerosene oil is a liquid that, upon being heated, vapourises and the burning of the vapours produce a flame; charcoal is a solid that does not vapourise on being heated and so does not produce a flame. It only glows during combustion.

Topic 2: Fuel

Question: 1

What is a fuel? What are its different forms? Give their examples.

Solution:

A substance that is burnt to produce heat is called a fuel. Fuels can be in the form of solids, liquids or gases. We use fuels in our daily lives for various applications where heat is needed. Examples of fuels are:

Solid fuels: Wood, coal, charcoal

Liquid fuels: Petrol, diesel, kerosene

Gaseous fuels: Natural gas, biogas, coal gas

Question: 2

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

Solution:

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

Question: 3

Name the gases that contribute to the formation of acid rain.

Solution:

Sulphur dioxide and nitrogen dioxide are the gases that dissolve in rain water to form acid rain.

Question: 4

What are the effects of global warming?

Solution:

Global warming leads to melting of polar glaciers, rise in sea level that can cause coastal flooding and changes in weather patterns.

Question: 5

What is the meaning of calorific value?

Solution:

The amount of heat energy produced on complete combustion of 1 kg of a fuel in the presence of oxygen is called its calorific value. The calorific value of a fuel is expressed in kilojoule per kg (kJ/kg).

Question: 6

Calorific value of wood is approx. 17000 kJ/kg -22000kJ/kg. What does this mean?

Solution:

A calorific value of 17000 kJ/kg -22000kJ/kg means that when 1kg of wood is burnt completely in the presence of oxygen, it produces 17000-22000 kJ of heat energy.

Question: 7

In an experiment, 20 Kg of a fuel produced 400,000 KJ of heat. Calculate the calorific value of that fuel.

Solution:

The calorific value of a fuel 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 kilo joule per kg.

Thus, the calorific value of the fuel

Question: 8

Explain the circumstances that lead to acid rain. How does acid rain affect us?

Solution:

The burning of coal and diesel releases sulphur dioxide gas which is a suffocating and corrosive gas. Moreover, petrol engines give off gaseous oxides of nitrogen as well. When these oxides of nitrogen and sulphur dissolve in rain water, the rain water becomes acidic. Such rain is called acid rain. It is very harmful for crops, buildings, aquatic animals and even to human skin.

Question: 9

Why is LPG considered a better domestic fuel than wood?

Solution:

LPG is a better domestic fuel than wood due to the following reasons.

(a)   LPG has a higher calorific value than wood. This means that LPG produces more heat on burning than an equal mass of wood.

(b)   LPG burns without producing any smoke or leaving any solid residue. But when wood is burnt it produces a lot of smoke and leaves behind a solid residue called ash.

Question: 10

Explain why CNG is considered a better fuel than petrol and diesel.

Solution:

Petrol and diesel produce a lot of unburnt carbon particles and emit carbon dioxide and oxides of nitrogen and sulphur upon burning. These gases are poisonous and cause environmental hazards such as global warming and acid rain. In comparison, CNG is a clean fuel. It produces very small quantities of these harmful products. Since the burning of CNG does not cause air pollution, it is considered a better fuel than petrol and diesel.

Question: 11

In an experiment 2.5 kg of a fuel was completely burnt. The heat produced was measured to be 120,000 kJ. Calculate the calorific value of the fuel.

Solution:

Total mass of fuel burnt = 2.5 kg

Heat produced by burning the given mass of fuel = 120,000 kJ

We know that calorific value of a fuel

= Heat produced by burning the given mass of fuel/ Total mass of fuel burnt

= 120,000 kJ/2.5 kg

= 48,000 kJ/kg

Hence, the calorific value of the given fuel = 48,000 kJ/kg

Question: 12

What are the harmful effects of burning of fuels on the environment?

Solution:

The burning of fuel causes the following harmful effects on the environment:

(i)     The burning of fuels like wood, coal and petroleum releases unburnt carbon particles that cause respiratory diseases like asthma.

(ii)  Incomplete combustion of fuels releases carbon monoxide gas. This is a highly poisonous gas that can even be lethal to humans if inhaled in large quantities.

(iii)            Burning of most fuels release carbon dioxide gas. Excess of carbon dioxide in the atmosphere causes trapping of heat, leading to a rise leading to a rise in the earth’s temperature. This is called global warming. It causes melting of polar ice, rise in sea level and consequent flooding of coastal areas.

(iv) The burning of coal and diesel releases sulphur dioxide gas. Moreover, petrol engines give off gaseous oxides of nitrogen as well. When these oxides of nitrogen and sulphur dissolve in rain water, the rain water becomes acidic. This is called acid rain. It is very harmful for crops, buildings, aquatic animals and also our skin.

Question: 13

Why is it not advisable to burn coal in a closed room?

Solution:

Normally, carbon dioxide, which is a molecule with one atom of carbon bonded to two oxygen atoms, is formed from the combustion of coal. But with the limited oxygen in closed room, another oxide of carbon known as carbon monoxide begins to be formed; the carbon atoms in whose molecules are each bonded to a single oxygen atom. Carbon monoxide is a colourless, odourless, highly toxic gas which can cause tissue damage and even death if inhaled in large quantities. Because of the dangerous possibility, it is not advisable to burn coal in a closed room.

Question: 14

Why is petrol known as inflammable substance?

Solution:

Petrol has a low ignition temperature and catches fire easily. Therefore, it is known as inflammable substance.

Question: 15

Why is petrol not used as a domestic fuel?

Solution:

Petrol is not used as a domestic fuel because it has a very low ignition temperature. Due to this a can full of petrol can catch fire very easily on being lighted with a matchstick and can burn explosively, thereby increasing the chances of accidental fires in the household.

Question: 16

Suppose you are asked to boil a given quantity of water using wood, coal and LPG as a fuel, which fuel would you prefer? Give your reasons

Solution:

Among the three fuels, LPG is the most efficient fuel and wood is the least efficient one.  LPG has the highest calorific value compared to the other fuels. The higher the calorific value the higher is the fuel’s efficiency. Therefore, LPG should be preferred to boil water faster.