Alkanes and alkenes are two families of hydrocarbons. Describe how bromine water can be used to distinguish between an alkane and an alkene, and explain why each hydrocarbon produces the result it does.

Bromine water is a dilute solution of Br₂ in water, which is orange‑brown.
1. Add a few drops of bromine water to a sample of the hydrocarbon.
2. If the solution remains orange‑brown, the hydrocarbon is an alkane.
3. If the solution becomes colourless, the hydrocarbon is an alkene.

The colour change (or lack of it) is due to the chemical nature of the hydrocarbons:
- Alkanes are saturated, containing only single C–C bonds. They do not react with bromine water, so the Br₂ remains in solution and the orange colour is unchanged.
- Alkenes are unsaturated, containing a C=C double bond. The double bond acts as a nucleophile and undergoes addition with Br₂, forming a dibromide. The Br₂ is consumed, so the solution turns colourless.

Thus, bromine water can be used to distinguish alkanes from alkenes by observing whether the orange colour disappears.