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

Well, the melting point is set by the molecules of the substance. The bigger a molecule is, the higher the melting point, and the smaller the molecule is, the lower the melting point. The gas hydrogen (H^2) is very small, so its melting point is very low. It is actually the second lowest melting point in the universe! Helium has the lowest melting point of all. Likewise, oxygen molecules (O^2) contain only two atoms, but water molecules (H^2 O) contain three atoms, meaning its melting point is a bit higher.

Melting point also is changed by polarity. All hydrogen molecules are positively charged and repel each other, just like all oxygen molecules are negatively charged and repel each other. This means they want to break apart, and so the melting point of pure hydrogen and pure oxygen is very low. Water is different, because one end of each molecule is positively charged, and the other end is negatively charged. This means that water molecules attract each other. This leads to a lot of very unique properties in water, like how it forms droplets instead of spreading out. It also means that its solid form, ice, is very stable, and so its melting point is much higher than hydrogen or oxygen.