When bromine water is added to a sample containing an alkene, what happens to the color of the bromine water?

• A. Decolorizes from orange-brown to colorless
• B. Turns blue
• C. Turns green
• D. Becomes more orange

Answer: (A)

Alkenes react with bromine water by adding bromine across the carbon–carbon double bond. Bromine water contains Br2, which gives an orange-brown color. When the double bond reacts, Br2 is consumed and forms a vic-dibromide, so there is no free bromine left to stain the solution. The result is decolourization—the solution changes from orange-brown to colorless. If no double bond is present, the bromine water would stay orange-brown.