COMBUSTION: -Those chemical processes in which a substance reacts with oxygen to give off heat is called combustion.

Combustible Substance:-Those substances which can be burned are called combustible                     substance.            Ex:-  paper, cloth, wood, kerosene etc.

A combustible substance is also called fuel. It can be solid, liquid or gas.

The material which produces heat energy on burning in air are called fuels.

Non-combustible Substance: Those substances which cannot be burned are called           non-combustible substance.    Ex:-  rock, sand, water, cement etc. 

Factors Required for Combustion:

A combustible substance

Oxygen(air)

Ignition temperature(Prajwalan Tap):-The minimum temperature at which           combustion starts in a substance is called ignition temperature of that substance.

Inflammable Substance: A substance which has very low ignition temperature so that it                                                can easily catch fire is called an inflammable substance.                                                                 Ex:-  petrol, alcohol, etc.

Fire Control:-

Fire is highly useful for human beings. But while controlled fire is beneficial for us, uncontrolled fire can be devastating (bhayanak) for us. Fire can be controlled by removing any or all of the factors of combustion, i.e. fuel, oxygen (air) and ignition temperature (by lowering the temperature).

The fire extinguisher tries to cut off the supply of air.

The burning substance or fuel cannot be removed in most of the cases of fire.

Different methods can be utilized to cut off the supply of air.

If a suitable substance is poured over the burning material, it can reduce the temperature and even bring down the temperature below ignition temperature.

 Some common fire extinguishers:

Water: Water is the most often used fire extinguisher. It helps in bringing down the                   temperature. When water is poured over a burning material then steam is created due         to heat. The layer of steam cuts off air supply. Thus, water helps in putting off fire.

Drawbacks of Water: Water cannot be used for fire because of oil because oil is lighter than         water. When water is poured over fire, oil comes on top and continues to burn. Water         should not be used for fire because of short circuit. We know that normal water is a good conductor of electricity because it contains many salts. This increases the risk of electric shock for fire fighters.

Blanket: If fire is at a small scale then a blanket can be very useful in controlling the fire. When the burning object is covered with blanket, it helps in cutting off oxygen supply. This helps in putting off fire.

Carbon Dioxide: This is the best fire extinguisher. Carbon dioxide creates a blanket over fire and cuts off air supply because it is heavier than air. Carbon dioxide expands quickly and brings down the temperature. This helps in putting off fire. For extinguishing fire, carbon dioxide can be supplied in any of the following ways:

Carbon dioxide is stored in pressurized cylinder and can be released through a nozzle.

In soda-acid fire extinguisher, carbon dioxide is evolved because of reaction between soda and acid.

Baking Soda (sodium bicarbonate) or potassium bicarbonate powder is sprayed on burning substance. The powder makes carbon dioxide because of heat.

Q. Why is the person caught in fire, is covered with a blanket?

Ans:-If the clothes of person catch fire, he is immediately wrapped in a thick blanket. The blanket cut of the supply of air i.e. the supporter of combustion and hence the fire is put off.

Q. Why does the fire goes off when water is poured over burning wood?

Ans:- It happens because water absorbs a large amount of heat energy, which results in fall in the temperature of wood below its ignition temperature and the fire goes off.

Types of Combustion

There are three types of combustion:

Rapid Combustion (Triv):-When combustion happens at a faster rate, it is called rapid            combustion.                   Ex:-Match Sticks, Petrol and LPG, Spirit.etc.

Spontaneous Combustion (swath):- When combustion starts on its own without an                          apparent cause it is called spontaneous combustion.             

Ex:- coal dust in coal mines can start burning on its own, burning of white phosphorus in air.

Explosion (Visphot):-  When combustion reaction is so sudden that it releases a large amount                  of heat, light and sound it is called explosion. Ex:- Firecrackers.

Flame:-A region of burning gases is called flame.      

Only those substances which vaporize on burning produce flame. If a substance does not get vaporized, it does not produce flame.

Zones of Flame: When we light a candle and observe its flame, we find that the flame has different coloured regions these regions are called zones of flame.

There are three zones in flame: outer zone, middle zone and inner zone.

Outer (non-luminous)Zone: 

This zone is thin and blue in colour.

 It is the hottest part of the flame, because complete combustion takes place in this zone.

The temperature of this zone is maximum around 18000 C.

Middle(luminous) Zone: 

This zone is yellow and orange in colour.

This zone is moderately hot because partial combustion takes place in this zone.

Inner (Dark)Zone:

This zone is dark in colour.

This is the coolest part of the flame because no combustion takes place in this zone.

 

 

 

Fuel:-          Those substances which produce heat on burning are called fuel.

                                Ex:-   LPG, CNG, wood, coal etc.

Q. Why is sulphur not used as a fuel even though it can burn in air to produce heat?

Ans:-Sulpher is easily available in nature and can burn in air to produce heat. However,it is not a fuel because on burning,it forms a poisonous gas,SO2, which can cause serious respiratory problem and can even be fetal.

Characteristics of a Good Fuel:

It is readily available.

It is cheap.

It is easy to store.

It burns easily in air.

It have moderate ignition temperature.

It burns at a moderate rate.

It produces a large amount of heat.

It do not produce gases which pollute the air.

It does not leave behind any undesirable substance.

There is probably no fuel which can be considered as ideal fuel.

Types of Fuels:-

There are three types of fuels: Solid fuels, Liquid fuels, Gaseous fuels

Solid fuels:-         Those fuels which occur in solid state at room temperature is called solid fuels.

                             Ex:    coal, wood, dung-cakes, agricultural residue,  etc.

Liquid fuels:-Those fuels which occur in liquid state at room temperature is called liquid fuels.

                                                Ex:    kerosene, petrol, diesel etc.

Gaseous fuels:-Those fuels which occur in gas state at room temperature is called gaseous fuels.

                             Ex:    coal gas, biogas/gobar gas, natural gas, water gas, CNG,  etc.

Fuel Efficiency

. The amount of heat energy produced on combustion of 1 kg of a fuel is called calorific value of that fuel. Calorific value gives the measure of fuel efficiency. The more is the calorific value of the fuel, more is the efficiency of the fuel. It is expressed as kilo Joule per kg (kJ/kg).

Calorific values of fuels
Fuel	Calorific value (kJ/kg)
Cowdung cake	6000 – 8000
Wood	17000 – 22000
Coal	25000 – 33000
Petrol	45000
Kerosene	45000
Diesel	45000
Methane	50000
CNG	50000
LPG	55000
Biogas	35000 – 40000
Hydrogen	150000

 

Harmful effects of Burning  Fuels

Burning of carbon fuels (wood, coal, and petroleum) results in release of unburnt carbon particles in air. These particles cause respiratory disease; like asthma.

Burning of most fuels releases carbon monoxide. It is a poisonous gas; even at low concentration. Hence, coal should not be burnt in a closed room.

Burning of most fuels results in release of carbon dioxide in atmosphere. Higher level of carbon dioxide in the atmosphere causes global warming.

Burning of coal and diesel results in release of sulphur dioxide in air. Burning of petrol produces oxides of nitrogen. Oxides of sulphur and nitrogen mix with rainwater; to cause acid rain. Acid rain is harmful for living beings, buildings, and for monuments.