9.11C - Why alkanes are saturated

Which elements are present?

The word only matters. $\ce{C3H8}$ contains just C and H, so propane is a hydrocarbon. A compound containing C, H and O is not a hydrocarbon because it also contains oxygen.

Methane, ethane, propane and butane all contain only carbon and hydrogen. That establishes that they are hydrocarbons, but says nothing by itself about whether they are saturated. To decide that, inspect the bonds.

Why single bonds mean saturated

A saturated hydrocarbon contains only single covalent bonds. In an alkane with more than one carbon atom, every carbon–carbon bond is single. There is no carbon–carbon double bond, $\ce{C=C}$.

Carbon has four bonds in these molecules. Using only single bonds between carbons leaves the maximum number of bonds available to hydrogen for that carbon skeleton. Ethane, for example, has one C–C bond, leaving three C–H bonds on each carbon: $\ce{CH3-CH3}$.

Methane has no C–C bond because it has just one carbon atom. It is still saturated: all four bonds are single C–H bonds.