How can you distinguish between alkanes and alkenes using bromine water?

• A. Alkenes decolourise bromine water to colourless; alkanes do not react.
• B. Alkanes decolourise bromine water; alkenes do not.
• C. Both decolourise bromine water equally.
• D. Bromine water turns blue with alkanes.

Answer: (A)

Bromine water tests for unsaturation. Alkenes have a carbon–carbon double bond, and bromine adds across that double bond in a reaction that uses up Br2, turning the orange‑brown bromine solution colorless as the dibromo compound forms. Alkanes, being saturated with no double bonds, don’t react with bromine water under normal conditions, so the solution stays the same color. So decolorisation indicates the presence of an alkene, while no change indicates an alkane.