CBSE Class 10 Science 2019 · Paper 31/2/3 · Question 5(b)

Unsaturated carbon compounds are more reactive than saturated ones, because their double or triple bond allows addition reactions. Test (bromine water test): bubble each gas through reddish-brown bromine water. Ethene decolourises it at once, because bromine adds across its double bond, ${\text{CH}}_2={\text{CH}}_2+{\text{Br}}_2\to {\text{CH}}_2\text{Br}-{\text{CH}}_2\text{Br}$ (colourless); ethane, being saturated, does not react, so the bromine water stays reddish-brown. (Also accepted: ethene decolourises purple alkaline ${\text{KMnO}}_4$ (Baeyer's reagent) and burns with a yellow, sooty flame, while ethane does neither and burns with a clean flame.)