Alkanes and alkenes are both hydrocarbons produced during cracking. Describe how bromine water can be used to distinguish between an alkane and an alkene, and explain why the two types of hydrocarbon give different results in this test.

Add a few drops of bromine water to a sample of each hydrocarbon.
1. The alkene turns the orange bromine water colourless (decolourisation).
2. The alkane leaves the bromine water orange.
The alkene is unsaturated – it contains a C=C double bond. The double bond is electron‑rich and reacts with Br₂ to give a dibrominated product, consuming the bromine and removing the colour. The alkane is saturated; it has only C–H and C–C single bonds and does not react with Br₂ under these conditions, so the orange colour remains.