Combustion and Flame - 8th science

Combustion: A chemical process in which a substance reacts with oxygen to give off heat is called combustion. For example burning of coal, burning of natural gas, burning of paper, etc.

Combustible: The substance that undergoes in combustion is said to be a combustible. For example wood, petrol, diesel, coal, etc.

Combustibles are known as fuel also. However all combustible are not fuel but all fuels are combustible.

Image of Large Bone Fire

Example Cloth undergoes into combustion and it is combustible but is not used as fuel.

Fuel may be solid, liquid or gas. Sometimes light is also given off during combustion along with heat, either as flame or as a glow.

Conditions Under Which Combustion Takes Place

There are three conditions required to combustion takes place. These conditions are

(a) Fuel (b) Air and (c) Ignition Temperature

(a) Fuel

The first condition to combustion takes place is fuel. This means fuel is must for combustion. Fuel is anything that goes under combustion.

(b) Air (Oxygen)

Air is another important condition for combustion of fuel. Air means supply of oxygen. Oxygen is not combustible but support combustion. Without availability of oxygen combustion will not take place.

Image: Burning of magnesium

(c) Ignition Temperature

The minimum temperature at which a substance cathes fire is called its ignition temperature. Thus it is necessary for combustion that substance must achieve its ignition temperature otherwise substance will not catch fire.

In the absence of any one of three conditions of combustion to be taken place, a substance cannot go under combustion. This means a combustible substance cannot catch fire or burn as long as its temperature is lower than its ignition temperature and in absence of air i.e. in absence of oxygen.

Inflammable Substances

Substances which have very low ignition temperature and can easily catch fire with a flame are called Inflammable Substances. For example Liquid petroleum Gas (LPG), Compressed Natural Gas (CNG), Natural gas, Petrol, alcohol, etc.

Why do kerosene oil and wood not catch fire at room temperature?

Kerosene oil does not catch fire at room temperature as its Ignition Temperature is more than that of room temperature. Similarly, wood also has ignition temperature more than that of room temperature, thus wood does not catch fire at room temperature.

The Ignition Temperature of wood is more than the ignition temperature of kerosene oil.

Match Box

Match Box is a device used to ignite candles, gas stoves, etc. in house hold. The match box is called Safety Match also.

Match box is a small box with many of small and thin sticks. Head of these all sticks have rounded red coloured material. The two sides of box have light reddish surfaces or reddish dotted surfaces.

When match stick is rubbed over the side of match box, the head of match stick catches fire consequently match stick starts burning with flame.

The head of the stick of safety match contains mixture of antimony and potassium chlorate. And the rubbing surface contains mixture of powdered glass and little red phosphorous.

When match stick is struck against the rubbing surface, some red phosphorous gets converted into white phosphorous. This immediately reacts with potassium chlorate in the matchstick head to produce enough heat to ignite antimony trisulphide and start the combustion.

History of Match Box and Match Stick

It is said that device to ignite fire was invented in prehistoric time. People used flint, a type of stone form of mineral quart, to produce fire. People rub two flints to produce fire.

But it takes long time to make chemical matches. It is said that people felt the need of small device to produce fire to light smoking tobacco. Prior to use of matches, fire were produced using a burning glass (convex lens) to focus the sun on tinder. But this device worked only on sunny days. And gradually match sticks were invented.

More than five thousand years ago small pieces of pinewood dipped in sulphur were used as match stick in ancient Egypt. After hundreds of years a mixture of antimony trisulphide, potassium chlorate and white phosphorous with some glue and starch was applied on the head of a stick of pinewood was made. When this match stick rubbed against a rough surface, white phosphorous got ignited due to heat of friction. This started combustion of the match stick. However use of white phosphorous proved very dangerous for workers involved in the manufacturing of matches and for users.

In order to see the dangerous effect, white phosphorous was banned in most of the country. And gradually after many attempts modern safety match was invented.

How Do We Control Fire?

Since three conditions, fuel, oxygen and reaching the minimum ignition temperature, are necessary to produce fire, so by preventing out any one of three conditions to produce fire, fire can be controlled.

This means fire can be controlled by cutting of supply of fuel, by cutting of supply of air (oxygen) or lowering the ignition temperature of combustible.

There is a department called "Fire Service Department" comprises of "Fire Brigades" takes care of situation of breaking of fire anywhere in the city.

Whenever a building or anything catches fire, fire brigade arrives and pours water on the place of fire. Water lowers the ignition temperature of combustible. On the other hand water vapour produces due to pouring of water on fire makes a layer around the fire. This cut the supply of air (oxygen) and fire is extinguished.

Thus, the job of fire extinguisher is to cut off the supply of air, or to bring down the temperature of fuel, or both.

Fire Extinguisher

Water is used as most common fire extinguisher as it is readily available and cheaper. But when a building catches fire due to short circuit of electricity, water does not work as it conducts electricity, and hence may proved fatal for those trying to douse fire. On the other hand water is not suitable to put off fires involving oil and petrol. As water is heavier than that of oil, so water cannot cover the oil and fire is not extinguished broken involving oil or petrol.

Fire extinguisher is a device which is used to control fire. Carbon dioxide is another and best fire extinguisher. Carbon dioxide is heavier than oxygen and covers fire like a blanket. It cut off the supply of oxygen to combustible and fire is extinguished.

Carbon Dioxide Filled Fire Extinguisher

Carbon dioxide can be stored in cylinders as liquid at high pressure. When knob of fire extinguisher containing carbon dioxide at high pressure, released, carbon dioxide expands enormously in volume and cools down. Carbon dioxide covered the combustible and cut off the supply of oxygen. Along with this carbon dioxide also brings down the temperature of the fuel.

Fire Extinguisher Containing Dry Power of Baking Soda or Potassium Bicarbonate

Dry power of Baking Soda (Sodium Carbonate) or Potassium Bicarbonate is filled into a cylinder. When lot of these powders released near the fire, these chemicals give off carbon dioxide. Carbon dioxide covers the combustible which cut off the supply of air (oxygen) along with putting down the temperature. This extinguished the fire.