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

An alkene will make its double bond into a single bond, to bond to two bromines. Bromine is added to the molecule. The product made is colourless. When alkenes are put in bromine water it turns from brown to colourless (a good way of testing for alkenes.)
For example:

C₂H₄(g) + Br₂ (aq) → C₂H₄Br₂ (aq)