Why cannot oxidation occur without reduction

In a chemical reaction, the species which loses electrons is said to be oxidised and the species which gains electrons is said to be reduced. Since there is no net loss or gain of electrons in a chemical reaction, it means that a substance can gain electron only if another substance that can lose electron is also present in the system. Similarly, a substance can lose electrons only if another substance which can gain electrons is also present in the system. This means oxidation can take place only if reduction also takes place simultaneously and vice versa. In other words, oxidation-reduction reactions are electron transfer processes and always occur side by side. Such reactions in which oxidation and reduction take place simultaneously are called oxidation-reduction or simply redox reactions. For example, consider the following reaction:


This reaction consists of two distinct simultaneous processes which take place in such a way that 

No. of electrons lost  = No. of electrons gained
(i) Oxidation half-reaction:

Here each atom of zinc loses two electrons and thus gets oxidised to Zn²⁺ ions.
(ii)  Reduction half-reaction:


Thus, a redox reaction may also be defined as a reaction in which electrons are transferred from one reactant to the other.