Which test can distinguish between an alkane and an alkene?

• A. Bromine water
• B. Benedict's solution
• C. Universal indicator
• D. Litmus paper

Answer: (A)

This tests whether a hydrocarbon is saturated or unsaturated by its reaction with a halogen. Alkenes have a carbon–carbon double bond that readily undergoes electrophilic addition with bromine. When bromine water is added to an alkene, the double bond breaks and bromine atoms attach to the adjacent carbons, causing the reddish-brown bromine water to lose its color and become colourless. Alkanes, which have only single bonds, do not react with bromine under these conditions, so the bromine water stays the same color. So observing decolorization of bromine water indicates an alkene, while no change points to an alkane.

The other tests don’t target this property: Benedict’s solution reacts with reducing substances (not hydrocarbons), and universal indicator or litmus paper only show acidity/basicity, not the presence of a carbon–carbon double bond.