CBSE Class 10 Science 2017 · Paper 31/1/C · Question 21

(a) Bromine water test: add a few drops of bromine water (orange-brown) to the hydrocarbon and shake. An unsaturated hydrocarbon, with a $\text{C}=\text{C}$ or $\text{C}\equiv \text{C}$ bond, decolourises the bromine water, because bromine adds across the multiple bond; for example, ${\text{CH}}_2={\text{CH}}_2+{\text{Br}}_2\to {\text{CH}}_2\text{Br}-{\text{CH}}_2\text{Br}$ (colourless). A saturated hydrocarbon leaves the orange-brown colour unchanged (in the absence of bright sunlight). (b) Ethane burns in air to form carbon dioxide and water, releasing heat and light: $2{\text{C}}_2{\text{H}}_6+7{\text{O}}_2\to 4{\text{CO}}_2+6{\text{H}}_2\text{O}+\text{heat}+\text{light}$. (c) ${\text{CH}}_4+{\text{Cl}}_2\stackrel{\text{sunlight}}{\to }{\text{CH}}_3\text{Cl}+\text{HCl}$. It is a substitution reaction because a hydrogen atom of methane is replaced by a chlorine atom: one type of atom takes the place of another.