A student investigates two hydrocarbons: ethane and ethene. Ethene is an unsaturated hydrocarbon, whereas ethane is a saturated hydrocarbon. Explain how the student could use bromine water to distinguish between ethane and ethene, and explain why the two hydrocarbons react differently with bromine water.

Add a small amount of bromine water to each hydrocarbon (or bubble each gas through bromine water). The orange–brown colour of the bromine water disappears when it is bubbled through ethene, showing that ethene decolourises the solution. In contrast, the colour of the bromine water remains unchanged when ethene is bubbled through ethane, indicating that ethane does not decolourise the solution.

Ethene is unsaturated and contains a C=C double bond. The double bond opens during an addition reaction, allowing Br₂ to add across the bond and form 1,2‑dibromoethane, which removes the bromine colour. Ethane is saturated and has only C–H and C–C single bonds; it cannot undergo addition reactions, so bromine remains in solution and the colour is retained.