3.8 Describe the addition reaction of alkenes with bromine, including the decolourising of bromine water as a test for alkenes

When an alkene reacts with bromine, it turns one of its double (unsaturated) bonds into a single bond so it can bond with the bromine atoms. For example, ethane (shown below as ethylene) will bond with bromine to form 1,2 dibromoethane. (Br is on molecules 1 and 2, there are 2 of them, and the rest is ethane. That's the naming process aha)

Because of this, Br water will decolorize if you mix an alkane with it. 

It will not decolorize immediately with alkenes because they are saturated.

This is a good way to test for alkenes, since there is always a colour change (bromine water turns from brown to colourless) whenever alkenes are put in it.

Also, to bond to two bromines, the alkene must make its double bond a single bond, hence the name addition reaction