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)

3.7 draw displayed formulae for alkenes with up to four carbon atoms in a molecule, and name the straight-chain isomers

In every alkene there is one double bond between two carbons. Bearing in mind that carbons can only make four bonds, then the double bonded carbons will either be joined to: another carbon and a hydrogen; or two hydrogens.

3.6 recall that alkenes have the general formula CnH2n

All compounds in the homologous group alkenes have the general formula CnH2n.