The reaction in which the reactants lose oxygen atoms or gain hydrogen atoms is called a reduction reaction.The reaction in which the reactants gain oxygen atoms or lose hydrogen atoms is called an oxidation reaction.The reaction in which one reactant gets oxidized and the other reactant gets reduced is known as a redox reaction.For example, in this reaction, chlorine combines with hydrogen, thus undergoing reduction.The hydrogen sulfide loses a hydrogen atom, thus undergoing oxidation.Thus, in a redox reaction, the oxidation of one reactant is accompanied by the reduction of another reactant.The reaction between copper oxide and hydrogen is also an example of a redox reaction.The copper oxide loses oxygen and thus undergoes reduction.When hydrogen combines with oxygen, it undergoes oxidation.