Oxidation and reduction:Changes in oxidation state

Oxidation is an increase in oxidation state(number) during a reaction.
Reduction is a decrease in oxidation state (number during a reaction.
The concept of oxidation state is used in several reactions are given below.

1) Reaction of metals with dilute acids
Consider the reaction of zinc with dilute sulfuric acid:

Zn(s) +H2SO4 --->ZnSO4(aq) +H2(g)
 0              +1               +2             0

The oxidation of zinc increase from 0 to 2 in ZnSO4 and is oxidised. The oxidation state of hydrogen decreases from +1 in H2SO4 to 0 and so is reduced, NOTE THAT THE SULFATE ION DOES NOT CHANGE. THEREFORE THE OXIDATION NUMBER OF THE S(+6) AND O2(-2) in the ion DO NOT CHANGE.

2)Halide displacement reactions
Chlorides react with KI solution to give an ionic equation...

Cl2(aq) + 2I(ag) --> 2Cl + I2
Chlorine is reduced (oxidation number decreases) whereas the iodide ions are oxidised. (THE OPPOSITE)