Which statement is true about bromine water reacting with alkanes?

Which statement is true about bromine water reacting with alkanes?

• A. A colour change to colourless is observed.
• B. No colour change is seen.
• C. Bromine water turns green.
• D. Bromine water ignites.

Answer: (B)

Bromine water is used as a test for unsaturation because bromine adds across carbon–carbon double bonds and the bromine color disappears as it is consumed. Alkanes are saturated, meaning they have only single bonds and no C=C to react with bromine. Under normal conditions, there’s no reaction with alkanes, so the bromine water keeps its original color and no colour change is seen. If you ever hear about a reaction under UV light, that would be a slow radical bromination of alkanes, but that doesn’t produce the visible decolorization characteristic of bromine reacting with alkenes, and it isn’t what’s observed in the typical bromine water test.

Bromine water is used as a test for unsaturation because bromine adds across carbon–carbon double bonds and the bromine color disappears as it is consumed. Alkanes are saturated, meaning they have only single bonds and no C=C to react with bromine. Under normal conditions, there’s no reaction with alkanes, so the bromine water keeps its original color and no colour change is seen.

If you ever hear about a reaction under UV light, that would be a slow radical bromination of alkanes, but that doesn’t produce the visible decolorization characteristic of bromine reacting with alkenes, and it isn’t what’s observed in the typical bromine water test.