1. The problem asks: What decolourises bromine water? 2. Bromine water is an orange-brown solution of bromine (Br2) in water. 3. Decolourisation means the orange-brown color disappears. 4. This happens when bromine reacts with substances that can add bromine atoms or reduce bromine. 5. Common substances that decolourise bromine water are unsaturated compounds like alkenes and alkynes, which have carbon-carbon double or triple bonds. 6. The reaction is an addition reaction where bromine adds across the double or triple bond, removing the color. 7. For example, ethene (C2H4) reacts with bromine water as: $$\mathrm{C_2H_4 + Br_2 \rightarrow C_2H_4Br_2}$$ 8. The bromine atoms add to the carbons, and the solution loses its color. 9. Saturated compounds (alkanes) and many other substances do not decolourise bromine water. 10. Therefore, bromine water is used as a test for unsaturation in organic compounds. Final answer: Bromine water is decolourised by unsaturated compounds such as alkenes and alkynes due to addition reactions with bromine.