How can you test for the presence of an alkene in a hydrocarbon sample?

• A. Add bromine water; the orange color remains unchanged if an alkene is present.
• B. Add bromine water; the orange color decolorizes if an alkene is present.
• C. Add bromine water; the orange color changes to blue if an alkene is present.
• D. Add bromine water; the orange-brown bromine decolorizes if an alkene is present.

Answer: (D)

Bromine water reacts with alkenes by adding Br2 across the carbon–carbon double bond, an electrophilic addition. This breaks the Br2 color, so the orange-brown solution becomes colorless. That decolorization directly signals the presence of a double bond in the sample. If the hydrocarbon is saturated, no addition occurs and the orange-brown color stays. The clearest description of the test result is that the orange-brown bromine decolorizes when an alkene is present.