Bromine water is an orange solution of bromine in water. It is used to tell an alkane from an alkene.
To do the test, add a few drops of bromine water to the hydrocarbon and shake. If the hydrocarbon is an alkene, the orange colour disappears and the mixture becomes colourless. We say the bromine water is decolourised.
This happens because the alkene has a C=C double bond. The bromine takes part in an addition reaction across the double bond and is used up, so the colour is lost. The product, a dibromoalkane, is colourless.
An alkane is saturated and has no double bond. There is nothing for the bromine to add to, so the bromine water stays orange. A colour change therefore shows that the hydrocarbon is unsaturated.
