Why is the boiling point of ammonia, NH3, higher then that of phosphine, PH3? Whenever we are asked a question about boiling point, the first thing we have to take into consideration is intermolecular forces. Intermolecular forces are the forces of attraction between molecules that hold them together and are therefore directly related to the boiling points of substances. The stronger the intermolecular forces are, the harder it is to separate the molecules from one another an