reduction

In chemistry, the gain of electrons, loss of oxygen, or gain of hydrogen by an atom, ion, or molecule during a chemical reaction.

Reduction may be brought about by reaction with another compound, which is simultaneously oxidized (reducing agent), or electrically at the cathode (negative electrode) of an electric cell. Examples include the reduction of iron(III) oxide to iron by carbon monoxide:

Fe₂O₃ + 3CO → 2Fe +3CO₂

the hydrogenation of ethene to ethane:

CH₂CH₂ + H₂ → CH₃CH₃

and the reduction of a sodium ion to sodium:

Na⁺ + e⁻ → Na

Photosynthesis - the series of chemical reactions inside chloroplasts - is essentially a mechanism for reducing carbon dioxide to carbohydrate.