Hydrogenation - to treat with hydrogen - is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst.
The process is commonly employed to reduce or saturate organic compounds.
The largest scale application of hydrogenation is for the processing of vegetable oils (fats to give margarine and related spreads and shortenings).
Typical vegetable oils are derived from polyunsaturated fatty acids (containing more than one carbon-carbon double bonds).
Their partial hydrogenation reduces most but not all, of these carbon-carbon double bonds.
Hydrogenation converts liquid vegetable oils into solid or semi-solid fats, such as those present in margarine.