How can you distinguish between alkanes and alkenes?

• A. Use bromine water; the alkene decolourises to colourless, but alkane shows no change.
• B. React with litmus.
• C. Heat with copper oxide.
• D. Squeeze test with dye.

Answer: (A)

Bromine water is a classic test for unsaturation. An alkene has a carbon–carbon double bond that readily reacts with bromine across the double bond in an electrophilic addition, forming a 1,2-dibromide. This reaction removes the brown color of the bromine solution, turning it colorless. A saturated alkane, which has no double bond, does not undergo this addition with bromine under normal conditions, so the bromine water stays the same color. That clear color change directly indicates the presence of a C=C bond, making bromine water the best way to tell alkenes from alkanes.

Other tests aren’t reliable for this distinction. Litmus would not react with either hydrocarbon, so it won’t differentiate them. The copper oxide test and the dye squeeze idea aren’t standard, consistent methods for distinguishing alkanes from alkenes under typical conditions.