(a) Give a chemical test to distinguish between saturated and unsaturated hydrocarbon.
(b) Name the products formed when ethane burns in air. Write the balanced chemical equatioin for the reaction showing the types of energies liberated.
(c) Why is reactiion between methane and chlorine in the presence of sunlight considered a substitution reaction ?

Explanation

(i) `Br_(2)` - water test: `Br_(2)` - water is a brown coloured liquid.
Unsaturated hydrocarbons give addition reaction with `Br_(2)`, so the colour of `Br_(2)` - water gets decolourised.
Saturated hydrocarbons do not react with `Br_(2)` - water, so the colour of `Br_(2)` - water does not get decolourised.
(ii) On burning ethane in air, the products obtained are carbon dioxide and water, along with heat and light.
`2C_(2)H_(6)(g)+7O_(2)(g) to 4CO_(2)(g) +6H_(2)O(l)+"Heat"+"Light" `
(iii) Methane reacts with chlorine in the presence of sunlight to form chloromethane and hydrogen chloride.
`underset("Methane")(CH_(4))+underset("Chlorine")(Cl_(2)) overset("Sunlight")to underset("Chloromethane")(CH_(3)Cl)+underset("Hydrogen chloride")(HCl)`
With the excess of chlorine, all the four hydrogen atoms of methane are replaced by chlorine atoms to form carbon tetrachloride `(C Cl_(4))`. This reaction is considered as substitution reaction because hydrogen of methane is substituted by chlorine.