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Plant oils and food additives

Hardening vegetable oils

Unsaturated vegetable oils tend to be liquid at room temperature, but they can also be 'hardened', through a chemical process called hydrogenation, to make them solid at room temperature.

Testing for unsaturation

The carbon-carbon double bonds in unsaturated oils can be detected using the elements bromine or iodine. These elements react with the double bonds in the oils, and the more double bonds there are, the more bromine or iodine is used up.

You can check for unsaturated fats using a simple test with bromine water. The test is similar to one used to differentiate alkenes from alkanes.

Bromine water is a dilute solution of bromine, which is normally orange-brown in colour. It becomes colourless when shaken with an alkene, or with unsaturated fats. When shaken with alkanes or saturated fats, its colour remains the same.


During hydrogenation, vegetable oils are hardened by reacting them with hydrogen gas at about 60ºC. A nickel catalyst is used to speed up the reaction. The double bonds are converted to single bonds by the hydrogenation. In this way unsaturated fats can be made into saturated fats.

hydrogen adds to the double bond to make a single bond

The structure of part of a fatty acid

Saturated vegetable oils are solid at room temperature, and have a higher melting point than unsaturated oils. This makes them suitable for making margarine, or for commercial use in the making of cakes and pastry.

Back to Oils, Earth and atmosphere index

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