Reduction is a chemical reaction in which a substance gains electrons. The term originally referred to any chemical process in which a substance either combines with hydrogen or loses oxygen. Reduction is the opposite of oxidation, a chemical reaction in which a substance gives up electrons. Reduction and oxidation always occur together. These two combined reactions are known as redox reactions. See Oxidation.
Many kinds of processes involve reduction. For example, metal plating occurs when metal ions (electrically charged atoms) in a solution are reduced to form neutral atoms. When a piece of copper is placed in a solution containing silver ions, it slowly becomes coated with silver. In this process, each positively charged silver ion gains an electron given up by a copper atom and becomes electrically neutral.
An example of the original meaning of reduction is the combining of nitrogen and hydrogen gases in the production of ammonia. Another example is the removal of oxygen from zinc oxide to form metallic zinc. Zinc may be extracted from its ore in this manner.
See also Corrosion; Electrolysis.
