What is the main difference between saturated and unsaturated compounds?

Saturated compounds are characterized by having only single bonds between the carbon atoms in their molecular structure. This means that every carbon atom in a saturated hydrocarbon is bonded to as many hydrogen atoms as possible, without forming any double or triple bonds. This fully "saturated" state results in a compound that has the maximum number of hydrogen atoms for a given number of carbon atoms.

In contrast, unsaturated compounds contain at least one double or triple bond between carbon atoms, which allows for fewer hydrogen atoms to be attached. This fundamental difference in bonding leads to diverse chemical properties between saturated and unsaturated compounds.

The other choices illustrate common misconceptions. Saturated and unsaturated compounds can exist as solids, liquids, or gases; thus, the statement about unsaturated compounds always being gases is incorrect. Additionally, unsaturated compounds do not have only single bonds; the presence of double or triple bonds is what defines them as unsaturated.