Describe how bromine water can be used to distinguish between an alkane and an alkene, and explain why the two substances give different results.

Bromine water is a dilute solution of Br₂ in water, giving an orange–brown colour. When an alkane is added, the solution remains orange–brown because alkanes are saturated hydrocarbons and do not react with Br₂. In contrast, when an alkene is added the orange–brown colour disappears and the solution becomes colourless. This decolourisation occurs because the C=C double bond in the alkene undergoes electrophilic addition with Br₂, forming a dibrominated alkane and consuming the bromine.

The different results are due to the presence of a carbon–carbon double bond in the alkene, which is reactive towards bromine, whereas the alkane has only single C–C bonds and is inert to bromine under these conditions.