If the melting temperature of hydrogen is -259°C and the melting temperature of oxygen is -218°C then why is the melting temperature of water (H2O) 0°C?

Question

Shouldn’t it be around -245°C?

In: Chemistry

Answer 1

Hydrogen (H2) and Oxygen (O2) are very small and symmetrical molecules. Two hydrogen mopecules aren’t attracted to each other very much. The more attraction, the higher the melting point.

For attraction to happen you need different charges. The hydrogen molecule itself has no charge. The cores of the atoms are positively charged, the hull is negatively charged. The exact distribution of the charge within the hull varies slightly, so one side might be a bit more negative than the other. This little bit of variance creates a little bit of attraction hence the melting point being -259 and not e.g. -270. Oxygen molecules are a bit bigger, so there is more room for variation, so it’s melting point is a bit higher.

Water (H2O) is not symmetrical. The oxygen atom has a very positively charged core, much more so than either of the hydrogen atoms. So the negative charge in the hull (the electrons they share to form the bond) gathers closer to the oxygen atom rather than the hydrogen cores. The two hydrogen atoms don’t attach opposite of another but at an angle of ~120°. So the side of the molecule where the apex of that angle is (the side of the oxygen atom) is significantly more negative than the side with the hydrogen atoms.

This difference makes the negative oxygen side attract to the positive hydrogen side of the next molecule over. And vice versa. Big attraction means high melting point.