CBSE – 8th Standard Science


Combustion and Flame

• A chemical process in which a substance reacts with oxygen to give off heat is called combustion. The substance that undergoes combustion is a fuel.


• The fuel may be solid, liquid or gas. Sometimes, light is also given off during combustion, either as a flame or as a glow.


• A combustible substance cannot catch fire or burn as long as its temperature is lower than its ignition temperature.

• The lowest temperature at which a substance catches fire is called its ignition temperature.

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

• Examples of inflammable substances are petrol, alcohol, Liquified Petroleum Gas (LPG),

• Water cools the combustible material so that its temperature is brought below its ignition temperature. Water vapours also surround the combustible material, helping in cutting off the supply of air. So, the fire is extinguished.

• For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO2) is the best extinguisher. CO2, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. The added advantage of CO2 is that in most cases it does not harm the electrical equipment.

Types of Combustion


• There are three different zones of a flame – dark zone, luminous zone and non luminous zone

• Non-luminous zone of the flame has a high Temperature this part of the flame is the hottest part


Fuel Efficiency

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

Burning of Fuels Leads to Harmful Products

• Carbon fuels like wood, coal, petroleum release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory disease, such as asthma.

• Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of carbon dioxide in the air is believed to cause global warming.

• Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil.

• The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas), because CNG produces the harmful products in very small amounts. CNG is a cleaner fuel.

Fire control

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