(i) In this reaction, sodium (Na) is changed into sodium oxide (Na2O). This is the addition of oxygen to sodittm. Since addition of oxygen is called oxidation, therefore, the substance sodium (Na) is oxidised.
Oxygen (O2) is changed into Na2O. Here, the addition of metal to oxygen takes place. So, the substance reduced is oxygen.
(ii) Here, copper oxide is reduced to copper metal whereas hydrogen is oxidised to water.