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When a compound burns it reacts with oxygen. Because of this we can predict what the products of the reaction will be. The oxygen will combine with each individual element in the compound.

Since hydrocarbon fuels only contain two elements, we always obtain the same two products when they burn. In the equation below methane (CH₄) is being burned. The oxygen will combine with the carbon and the hydrogen in the methane molecule to produce carbon dioxide (CO₂) and water (H₂O).

The products from the combustion of hydrocarbon fuels can be identified with the following set up in the lab.

Carbon dioxide gas turns limewater from colourless to milky white. In the above experiment, the limewater in the boiling tube changes colour indicating carbon dioxide is produced by combustion of the hydrocarbon fuel.

If water is produced it will condense in the u tube cooled by the ice water. This results in cobalt chloride paper changing colour from blue to pink.

Incomplete combustion

While carbon dioxide and water are produced when hydrocarbons burn in a plentiful supply of oxygen, complete combustion is not always possible. If the fuel is burned with a limited supply of oxygen, incomplete combustion can occur and the following pollutants can be formed:

• carbon monoxide (CO) – a poisonous gas
• soot - unburned carbon which leaves as dirty particles

Air pollution from the burning of hydrocarbons can be reduced by special exhaust systems. Car exhaust systems have catalytic converters. Expensive transition metal catalysts are used to convert the pollutant gases into less harmful gases:

carbon monoxide + nitrogen monoxide → nitrogen + carbon dioxide