Redox Reaction is the reaction when both oxidation and reduction occur simultaneously. Now, take the example given here:
Pass hydrogen gas over black copper oxide and get a reddish coloured layer of copper.In this action, when the positive charge of an atom or an ion increases or the negative charge is decreased, then it is called oxidation. Likewise, when the positive charge is decreased or the negative is increased, then it is reduction. Take the equation, Cu0 + H2 → Cu + H20
In this reaction, when an oxygen atom is removed from CuO (copper oxide), then reduction of copper oxide takes place, whereas the hydrogen molecule takes up oxygen atom forming water (H2O). Here, oxidation of hydrogen takes place. In this reaction, oxidation and reduction takes place simultaneously. Here, the reductant is oxidized by the oxidant, while the reductant reduces the oxidant. Because of the characteristics of the reduction and oxidation reactions, the single term ‘redox’ is used as an alternative to two terms. Thus, Redox Reaction= Reduction + Oxidation.