What happens when bromine water is added to an alkene?

• A. The colour changes from brown to colourless as the double bond is broken.
• B. The colour changes from colourless to brown.
• C. A solid polymer forms with no colour change.
• D. No reaction occurs and the colour remains brown.

Answer: (A)

Bromine water is orange-brown, and alkenes have a carbon–carbon double bond that readily undergoes electrophilic addition with bromine. In the reaction, the Br–Br bond breaks and each bromine atom adds to one carbon of the double bond, converting the C=C into a single bond and forming a vicinal dibromide. Because the bromine is now part of the saturated product, there’s no free Br2 left in solution, so the orange-brown color disappears and the solution becomes colourless. This decolorization indicates that a reaction has occurred.

The other possibilities don’t fit: turning colorless back to brown would require generating Br2, which doesn’t happen in this test; a solid polymer isn’t the typical product of bromine adding to an alkene under these conditions; and no reaction would leave the brown color unchanged.