Draw the chemical structures of butane, butene and butyne.

I would then go through with the student and explain that an alkane is a hydrocarbon made up of single carbon-carbon bonds only. This is why butane ends in -ane, it begins with but- due to the number of carbons in the chain (4). Then expand to explain what alkenes and alkynes are and how to name them. (Perhaps get onto the numbering of such molecules but I think this is A level rather than GCSE).