(i) Saturated hydrocarbons undergo substitution reaction through a mechanism called “free radical mechanism”, where the initial step of reaction is the formation of free radicals. Generation of free radicals requires sunlight, without which the reaction does not get initiated. So, sunlight is necessary for the substitution reactions of saturated hydrocarbons.

(ii) Ethyne (HC≡CH) is an unsaturated compound, and unsaturated compounds produce yellow sooty flame with lots of black smokes. This is due to incomplete combustion of ethyne in air.