Identify the substances that are oxidised and the substances that are reduced in the following reactions.

$(i)$ $4 Na ( s )+ O _{2}( g ) \rightarrow 2 Na _{2} O ( s )$

$(ii)$ $CuO ( s )+ H _{2}( g ) \rightarrow Cu ( s )+ H _{2} O ( l )$

Why is respiration considered an exothermic reaction ? Explain.

Explain the "Oxidation'' in terms of gain or loss of oxygen with two examples.

Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

What does one mean by exothermic and endothermic reactions ? Give examples.