Concept:Bromine water is decolourised by unsaturated hydrocarbons because bromine adds across their multiple bonds.
Explanation:Bromine water has an orange-brown colour due to dissolved
Br2.
When an unsaturated compound is added, the bromine reacts and the solution becomes colourless.
Propyne,
CH3C≡CH, contains a carbon-carbon triple bond, so it is unsaturated.
Hence, propyne readily decolourises bromine water.
Propane is a saturated alkane and does not react with bromine water under normal conditions.
Propanoic acid and propanol also lack a carbon-carbon multiple bond, so they will not decolourise bromine water.
Therefore, substance X is most likely propyne.
Answer:C. propyne