Class 8 Science

Combustion

Combustion is a chemical process in which a substance reacts with oxygen to give off heat. Some substances may also give light along with heat during combustion.

Combustible Substance: A substance which can undergo combustion is called a combustible substance. A combustible substance is also called fuel. It can be solid, liquid or gas.

Non-combustible Substance: A substance which cannot undergo combustion is called a non-combustible substance.

Factors Required for Combustion:

  • A combustible substance
  • Oxygen
  • Ignition temperature: 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 is can easily catch fire is called an inflammable substance, e.g. 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 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 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 firefighters.

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.


Types of Combustion

There are following types of combustion:

Rapid Combustion: When combustion happens at a faster rate, it is called rapid combustion. Petrol and gas show rapid combustion.

Spontaneous Combustion: When combustion starts on its own; without an apparent cause; it is called spontaneous combustion. For example; coal dust in coal mines can start burning on its own. Forest fires is spontaneous in most of the cases.

Explosion: When combustion reaction is so sudden that it releases a large amount of heat, light and sound, it is called explosion. Firecrackers explode because of this type of combustion.