Explain the following in terms of gain or loss of oxygen with two examples each:

(a) Oxidation: It is defined as a process which involves gain of oxygen. For example,
(i) 2Mg(s) + O₂(g)  ----->  2MgO(s)
Magnesium  Oxygen Magnesium oxide 

Here, Mg has gained oxygen to form MgO. Hence, Mg has been oxidised to MgO.
(ii) 2Cu(s) + O₂(g)  Heat  -----> 2CuO(s)
Copper           Oxygen          Copper oxide
In this reaction, Cu has gained oxygen to form CuO. Thus, Cu is oxidised to copper oxide (CuO).
(b) Reduction: It is defined as the process which involves loss of oxygen. For example,
(i) Cuo + H₂ +Heat ------>    Cu + H₂O
Copper oxide    Hydrogen             Copper  Water
In this reaction, copper oxide is losing oxygen. So, it is being reduced to copper.