Water, HF, NH3 have relatively hight boiling points?

Question

Like water, hydrogen fluoride (HF), and ammonia (NH3), have relatively high boiling points. Explain.

Answer 1

It's all about the hydrogen bonding.  Each of these molecules have bonds in them that involves H and something that is highly electronegative.  These, of course, give you polar bonds and hence a polar molecule.  But you get an added bonus.

Since you have hydrogens that are slighly positive and something else (N, O, or F) that is slightly negative, you end up getting interactions between molecules where hydrogens of one molecule are attracted to the N, O, or F of another molecule.  This is hydrogen bonding in a nutshell.

More or less, the process of boiling starts with molecules that are relatively close to each other ("the liquid state") and  ends with molecules that are much more spread out from each other ("the gaseous state").  For this to happen, there must be enough energy in the system that you are talking about to break up the interactions ("intermolecular forces") between molecules.  Hydrogen bonding is a relatively strong intermolecular force -- more energy is required for boiling to occur -- hence higher BPs.