Two hydrocarbons X and Y were treated with bromine water. X decolorized the solution and Y did not not. Which class of compound does Y belong? 

Correct answer: Alkanes.

Explanation: Bromine water is decolorized by unsaturated hydrocarbons via addition across C=C or C≡C bonds. So X (which decolorized) is an alkene/alkyne. Y did not decolorize, indicating no multiple bond—i.e., a saturated hydrocarbon: an alkane. (Alkanes only react with Br₂ under UV light via substitution, not in cold aqueous bromine.)

• Benzene – Aromatic; does not decolorize bromine water without a catalyst, but the standard inference in this test is “saturated hydrocarbon = alkane.”
• Alkynes/Alkenes – Unsaturated; they do decolorize bromine water.