14.2.3—Of aq. bromine to show the presence of a C=C

Bromine water is orange-brown. An alkene decolourises it because the electron-rich C=C reacts with Br₂ in an electrophilic addition reaction.

The test is evidence for a reactive C=C under the stated conditions, not a universal test for every unsaturated substance. Keep the reagent and observation together.

Ethene + Br₂ → 1,2-dibromoethane, so the orange colour disappears. An alkane does not rapidly decolourise bromine water at room temperature without radical conditions.

Decolourisation is not proof that a molecule contains only one double bond, and bromine water is not the same as bromide solution.