Using oxidation states to identify what's been oxidised and what's been reduced
This is easily the most common use of oxidation states.
Remember:
Oxidation involves an increase in oxidation state
Reduction involves a decrease in oxidation state
In each of the following examples, we have to decide whether the reaction involves redox, and if so what has been oxidised and what reduced.
Example 1:
This is the reaction between magnesium and hydrochloric acid or hydrogen chloride gas: