combustion and flame class-8-NCERT Solutions-Chapter 6

Combustion

Combustion is a process of burning of a substance in the presence of air or oxygen along with the evolution of heat and light.

Ex:- When charcoal burns in air, it gives carbon dioxide, heat and light

charcoal + air → carbon dioxide + heat + light

C + O₂ → CO₂ + heat + light

combustible substance

The substance that undergoes combustion is called combustible substance.

Ex:- wood, paper, kerosene, charcoal etc.

non-combustible substance

A substance that doesnot undergo combustion is said to be non-combustible substance.

Ex:- bricks, stone, metal, soil etc.

fuel

Any combustible substance that is used to produce heat at a reasonable cost is called fuel.

Ex:- cow dung, wood, paper, kerosene, charcoal etc

types of fuel

There are three types of fuel.

1- solid fuel ( Ex:- wood, paper, charcoal, cow dung etc. )

2- liquid fuel (Ex:- kerosene, petrol, LPG etc. )

3- gas fuel ( Ex:- biogas etc. )

complete combustion

A combustion is said to be a complete combustion when a fuel burns in sufficient amount of air.

Ex:- When methane completely burns in air produces carbon dioxide, heat and water

Methane + oxygen → carbon dioxide + water + heat

CH₄ + O₂ → CO₂ + 2H₂O + heat

incomplete combustion

A combustion is said to be a incomplete combustion when a fuel burns in insufficient amount of air.

Ex:- When methane burns in insufficient amount of air produces carbon monoxide, water and very lesser amount of heat

Methane + oxygen → carbon monoxide + water + heat

2CH₄ + 3O₂ → 2CO + 4H₂O + heat

conditions that is necessary for combustion

The three conditions that is necessary for combustion are

1- Presence of a combustible substance.

2- Presence of a supporter of combustion

3-Attainment of ignition temperature of the combustible substance.

supporter of combustion

Those substances that help the combustible substance in burning is known as supporter of combustion.

EX:- oxygen or air

Why is a person covered with wool blanket when he catches fire?

Combustion takes place in the presence of oxygen or air. A person covered with wool blanket when he catches fire because wool blanket cuts the supply of air and the fire gets extinguished.

ignition temperature

The lowest temperature at which a substance catches fire or starts burning is called its ignition temperature. Ignition temperature is also called kindling temperature.

Why do paper or match stick not catch fire on their own?

paper or match stick do not catch fire on their own because their temperature is less then their ignition temperature.

inflammable substances

The substances which have very low ignition temperature and can catch fire easily with a flame are called inflammable substances.

Ex:- petrol, alcohol, LPG etc.

How does water help to put out the fire?

Water brings down the temperature of the combustible material below its ignition temperature. This prevent the fire from spreading. In this process water vapour is also produced which surround the combustible material and cuts off the supply of air. Hence fire gets extinguished.

Which types of fire can water not extinguish? Explain.

Water can not extinguish the fire caused by electric short circuit because water may conduct electricity and give shock to the person trying to extinguish the fire.

Water also can not extinguish the fire caused by oil and petrol because oil and petrol are lighter than water, it keeps burning on surface of water.

How does Carbon dioxide help to put out the fire?

Carbon dioxide is heavier than oxygen. It forms a blanket around the fire and cutting the supply of air. Hence the fire is controlled.

Which types of fire can Carbon dioxide extinguish? Explain.

Carbon dioxide can extinguish the fire caused by electrical short circuit, oil, kerosene, petrol etc.

fire extinguisher

A fire extinguisher is a device used to extinguish small fires due to short circuits, petrol, oil etc.

How does a fire extinguisher work?

In a fire extinguisher cylinder, carbon dioxide gas is filled and stored under high pressure in liquid form. When carbon dioxide released from the cylinder, carbon dioxide expands enormously in volume and forms a blanket around the fire. It also decreases the temperature of the fuel below its ignition temperature. Hence the fire is controlled.

Rapid combustion

The combustion reaction in which a large amount of heat and light are produced in a short time is called rapid combustion. It needs external heat to occur.

Ex:- burning of cooking gas, burning of kerosene

spontaneous combustion

The combustion reaction which occur on its own without the help of any external heat is called Spontaneous combustion. Spontaneous combustion takes place at room temperature.

Ex:- burning of white phosphorus at room temperature, forest fires due to heat of sun.

explosion

A very fast combustion reaction in which a large amount of heat, light and sound are produced is called explosion.

Ex:- burning of crackers etc.

Which types of combustible material produces flame?

Those combustible material which vaporise on burning produces flames.

Ex:- candle produces flame due to vaporization of wax

Earthen lamp produces flame due to vaporization of ghee.

Why does candle produce flame during burning?

Candle produces flame during burning because candle has wicks, molten wax rises through the wicks and get vapourise.

Why does charcoal not produce flame during burning?

Charcoal does not produce a flame during burning because It does not vapourise on burning.

structure of flame

The flame is generally spindle in shape. A flame has three main zones.

1- The outer non-luminous zone

2- The middle luminous zone

3- The innermost dark zone

1- The outer non-luminous zone- This is the hottest part of the flame. it is the complete combustion zone due to sufficient supply of air.The flame of this zone is blue in colour.

2- The middle luminous zone- this part of the flame is moderately hot and it is a zone of incomplete combution since the supply of air is not sufficient . A black residue is left on the object placed in this part of the flame. The colour of the flame is yellow in this zone.

3- The innermost dark zone- This is the least hot part of the flame ,near the wick of the candle. No combution takes place here due to lack of air. This zone contains unburnt wax vapours and so appears black incolour.

features of a good fuel

A fuel is considered good if

👉 It burns easily in air.

👉 It produces a large amount of heat.

👉 It does not leave unwanted residues.

👉 It is easy to store and transport.

👉It is cheap and easily available.

fuel efficiency

The amount of energy released per unit mass of the fuel is called fuel efficiency.

calorific value

The amount of heat energy produced by the combution of one kg of a fuel is called its calorific value. It is expressed in the unit of kilojoule per kg (kj/kg).

Hydrogen has highest calorific value

Why does hydrogen not use as domestic fuel?

Hydrogen is not used as domestic fuel because its calorific value is very high. Therefore, it is not easy to handle.

Liquid hydrogen is used as spacecraft fuel.

Harmful empects of burning fuels

The five empect of burning fuels are

1- Burning of fuels like wood,coal, petrolium produces unburnt carbon particles . These particles are very fine and can cause respiratory diseases, like asthma.

2- Combution of fuels add carbon dioxide to the atmosphere. Which leads to greenhouse effect and increase global warming.

3- Incomplete combution produces carbon monoxide piosonous gas and can cause death.

4- Burning of coal and diesel produce sulphur dioxide gas. This gas is highly poisonous and corrosive in nature. It dissolves in rain water to form acid. It destroys, crops, buildings, soil, etc.

5-Fumes from engines of vehicles contain oxides of nitrogen. These oxides also dissolve in rainwater to produce acid rain.

combustion and flame class 8-NCERT Solutions-Chapter 6