定义Definition
还原是物质得到电子、使元素氧化数降低的过程，总是与氧化同时发生。被还原的物质是氧化剂。传统上失去氧或得到氢也称为还原，如氧化铜被氢气还原为铜。
Reduction is the gain of electrons by a substance, lowering the oxidation number of an element; it always accompanies oxidation. The substance reduced is the oxidising agent. Traditionally, losing oxygen or gaining hydrogen is also called reduction, as when hydrogen reduces copper(II) oxide to copper.
例子Examples
得电子：Cu²⁺ + 2e⁻ → Cu；氧化数降低：Cl₂ + 2e⁻ → 2Cl⁻ 中氯由 0 变为 −1；失氧：CuO + H₂ → Cu + H₂O；加氢：CH₂=CH₂ + H₂ → CH₃CH₃（镍催化）。
Gain of electrons: Cu²⁺ + 2e⁻ → Cu. Fall in oxidation number: in Cl₂ + 2e⁻ → 2Cl⁻ chlorine goes from 0 to −1. Loss of oxygen: CuO + H₂ → Cu + H₂O. Gain of hydrogen: CH₂=CH₂ + H₂ → CH₃CH₃ over nickel.
冶金中的还原Reduction in metal extraction
金属冶炼本质上是把金属化合物还原为金属：高炉中 CO 还原铁矿石，Fe₂O₃ + 3CO → 2Fe + 3CO₂；活泼金属如铝和钠则用电解还原，在阴极得到电子。
Extracting a metal means reducing its compounds: in the blast furnace CO reduces iron ore, Fe₂O₃ + 3CO → 2Fe + 3CO₂, while reactive metals such as aluminium and sodium are reduced by electrolysis, gaining electrons at the cathode.