The Strangest Molecule

Water (H2O) breaks almost every rule you would expect for a molecule its size. It should be a gas at room temperature — hydrogen sulfide, a similar molecule, boils at -60°C. It should get denser as it freezes. It should be a poor solvent.

The secret is hydrogen bonding. Oxygen is highly electronegative, pulling electron density away from the hydrogen atoms. This creates a molecule with a slightly negative oxygen end and slightly positive hydrogen ends.