Question

What causes water's low vapor pressure?

Answer 1

The vapour pressure of water is depressed by the degree of intermolecular interaction (intermolecular force) present in liquid water.

Explanation:

There is a great deal of intermolecular interaction present in liquid water. This interaction arises from hydrogen bonding, in which electronegative oxygen atom polarizes electron density from the `H-O` bonds, and acquires a partial negative charge. The (now) electropositive hydrogen atoms are attracted to the oxygen atom of an adjacent water molecule, and such interactions propagate through the bulk phase.

The end result is a liquid with fairly strong intermolecular interaction (NOTE that intermolecular interaction is distinct from intramolecular interaction!). The vapour pressure of water is REDUCED and its BOILING POINT is thus increased in comparison to similar molecules.