WHY沸點較高之物質，熔點不一定較高

Take water as an example , it has hi BP but rather low MP . This is because of its polar structure . Take CH4 , it has low BP (-162 degree C) & low MP(-183) . CH4 is unpolar molecule . Also take C4H10 a higher member of alkane series , it has low MP (-130) & hi BP(36) . Because of more atoms , the forces betweem each atom is more complicated , more energy must be applied to separate each molecule , hence higher boiling point . The geometric structure , polar or unpolar , the number of atoms ... & many other factors also have to consider , will affect the MP & BP of the molecule .