describe: (a) the complete and incomplete combustion of alkanes (b) the free-radical substitution of alkanes by \( Cl_2 \) or \( Br_2 \) in the presence of ultraviolet light, as exemplified by the reactions of ethane

explain, including by use of an equation, the use of chlorine in water purification to include the production of the active species HOCI and CIO\(^-\) which kill bacteria