How bromine water can be used to test for an alkene?
Alkenes, or hydrocarbons with at least one double bond undergo
an addition reaction when combined with bromine (Br2). The general
reaction is H2C=CH2 --> H2BrC--CBrH2, and it occurs readily.
This reaction is a good way to identify alkenes because bromine has
a reddish color, while alkanes and alkenes are colorless. So if
bromine is added to an unknown hydrocarbon, the disappearance of
the color is an indication of the presence of a pi bond.