Bromine water does not react with alkanes under standard conditions. Bromine water is used to tell the difference between alkanes and alkenes, as it reacts with alkenes to produce a brominated hydrocarbon and to turn the bromine water from yellow-brown to colorless.
Alkanes react with bromine water if ultraviolet light is present. In this case, they produce a brominated hydrocarbon with hydrogen bromide. If the alkane in question has more than two carbon atoms, a variety of compounds is formed. Alkynes, hydrocarbons with at least one carbon-carbon triple bond, also undergo bromination, so this method cannot be used to determine the difference between them and alkenes.