Air and Combustion

Composition of Air

Air is a mixture of gases surrounding the Earth. By volume it is roughly 78% nitrogen, 21% oxygen, and about 1% other gases including argon, carbon dioxide and water vapour.

Uses of the Components of Air

• Oxygen – supports respiration and combustion
• Nitrogen – used in making ammonia and fertilisers, and to provide an inert atmosphere
• Carbon dioxide – used by plants in photosynthesis and in fire extinguishers

Meaning of Combustion

Combustion is a chemical reaction in which a substance (the fuel) combines with oxygen, releasing heat and usually light. It is commonly called burning.

Conditions Necessary for Combustion

• Presence of a fuel (combustible material)
• Presence of oxygen (or air)
• A source of heat to reach the ignition temperature

These three requirements are often shown as the “fire triangle”; removing any one of them puts out a fire.

Types of Combustion

• Complete combustion – occurs with enough oxygen, producing carbon dioxide and water, and releasing more energy
• Incomplete combustion – occurs with limited oxygen, producing carbon monoxide and soot (carbon), and releasing less energy

Everyday Examples and Safety

Combustion occurs in candles, cooking gas, wood and petrol engines. Incomplete combustion is dangerous because carbon monoxide is a poisonous, colourless and odourless gas, so rooms where fuels are burned should always be well ventilated.

Summary

Combustion is an oxygen-dependent chemical reaction that releases energy, and understanding it helps us use fuels efficiently and safely.