When ethene is treated with bromine water, what change is observed?

• A. It turns blue
• B. The bromine water loses its orange/brown color
• C. It forms a white precipitate
• D. It releases chlorine gas

Answer: (B)

The idea being tested is how alkenes react with bromine water. An alkene like ethene has a carbon–carbon double bond that readily undergoes electrophilic addition with Br2. The bromine molecule is polarized by the electron-rich double bond and adds across it, giving 1,2-dibromoethane. Because bromine in solution is orange-brown, adding it across the double bond consumes the Br2 and removes its color, so the bromine water becomes colorless. This decolorization is a classic test for unsaturation.