Question: do alkynes decolourise bromine water?
Answer: when reacted with bromine water, the bromine breaks its bonds with water and forms bonds with the akynes, so the alkyne can replace the triple bod with a double bond. the bromine will now be a part of the alkyne and hence wont show its brown colour. Yes, Alkynes decolorize Bromine water from reddish-brown to colorless.