3. All are hydrocarbons and all are in the eth- group i.e. all have two carbon atoms. Ethane is an alkane, ethene is an alkene and ethyne is an alkyne.
4. No C=C double bonds, only single C-C bonds. Saturated with hydrogen atoms (as many C-H bonds as possible).
5. As branching increases, boiling points decrease. (Molecules exert intermolecular forces of attraction between each other and these are stronger when they are closer together. Branching makes it harder for the hydrocarbon molecules to pack closely together therefore these forces are weaker and it takes lesser energy to break them.)
