Water is a liquid at room temperature. What is this due to?

Water is a molecule of fairly negligible mass: #18.01*g*mol^-1#. This is less than ammonia, or dioxygen, or dinitrogen, a little more than methane, but still less than ethane, and propane. And yet all these other molecules are GASES at room temperature, and have normal boiling points below that of water.

We know, or should know, that as atoms, and molecules get bigger, there should be a greater degree of dispersion forces that operate between particles, yet it seems that the boiling point of water is anomalously high. And so in fact it is.

The difference between water and these other molecules that another intermolecular force operates, and that is hydrogen bonding. Oxygen is electronegative with respect to hydrogen, and tends to polarize electron density towards itself. The result is that we may represent the water molecule as #""^(+delta)H-stackrel(delta-)O-H^(delta+)#, i.e. as a dipole in which charge is strongly separated.

Given the strength of this dipole, and the small size of the water molecule, this interaction between the dipoles of adjacent molecules acts as a potent intermolecular force that elevates water's boiling point.