What is the initial color of bromine water before it reacts with an alkene?

• A. Orange
• B. Colourless
• C. Green
• D. Brown

Answer: (A)

Bromine water is used as a test for unsaturation because bromine (Br2) in water has a distinct color that fades when it reacts. Bromine is a reddish-brown liquid, and when it’s dissolved in water the solution is typically described as orange (often thought of as orange-brown). That color comes from Br2 in the aqueous solution absorbing certain wavelengths of light. When an alkene is present, the double bond adds across the Br2, consuming the bromine in the solution and turning it colorless. So the initial color you’d observe before any reaction is orange.