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

Step 1: Analyze the question Two hydrocarbons were tested with bromine water. X decolorised it (a positive test for unsaturation). Y did not, so Y is not a typical unsaturated hydrocarbon. We need to identify Y's class.

Step 2: The concept Bromine water (reddish-brown) is used to test for unsaturation:

• Alkenes (C=C) and alkynes (C≡C) undergo addition reactions with bromine, so the brown colour disappears (decolorised).
• Alkanes are saturated, with only single bonds. They do not undergo addition reactions with bromine water, so the colour stays. (They react with bromine only under UV light by substitution.)
• Benzene is also unreactive toward bromine water because of its stable delocalised ring. It needs a catalyst such as FeBr₃ to react.

Step 3: Evaluate the options

• A. Benzene: does not decolorise bromine water. This is technically consistent with the observation.
• B. Alkynes: decolorise bromine water. Eliminated.
• C. Alkenes: decolorise bromine water. Eliminated.
• D. Alkanes: do not decolorise bromine water. Consistent.

Step 4: Choosing between A and D Both benzene and alkanes fail to decolorise bromine water, so strictly the question is ambiguous. However, the question is built as a contrast between an unsaturated hydrocarbon (X) and a saturated one (Y), and the standard textbook answer is that the hydrocarbon that does not decolorise bromine water is a saturated one, an alkane.

Note: benzene is a defensible alternative on pure chemistry grounds, but in the JAMB context the expected answer is alkanes.

Exam tip: Remember "Bromine water test: Alkenes and alkynes decolorise it (unsaturated); alkanes do not (saturated)." When both alkanes and benzene appear, pick the one that best fits the question's framing. Here, the simple saturated vs unsaturated contrast points to alkanes.