# What determines the phase of matter?

Nov 14, 2016

Clearly, the phase of the matter, whether it's a solid, a liquid, or a gas, is a function of intermolecular force.........

#### Explanation:

Non-polar covalent molecules clearly have a negligible degree of intermolecular attraction. Consider the boiling points of dihydrogen, dixoygen, dinitrogen. On the other hand, polar molecules have some degree of intermolecular attraction in that the dipoles can align along electrostatic lines.

You can illustrate this tendency by considering the boiling points of the bimolecular halides, ${X}_{2}$. Iodine (${I}_{2}$) is a molecular solid with a boiling point of $184.3$ ""^@C; chlorine is a molecular gas with a boiling point of $- 34$ ""^@C. On the other hand, the interhalogen $I - C l$ has a boiling point of $27.2$ ""^@C.

Because the $X - X '$ bond has become somewhat polarized in the interhalogen (i.e. ""^(delta+)X-X^(delta-)), this is an extra intermolecular force that raises the boiling point/melting points. This is in addition to the dispersion force that always operates in many electron molecules.

Non-molecular species, which have no discrete molecular boundaries, tend to have high melting and boiling points. You will have to find the diagrams yourself. But consideration of the degree of $\text{intermolecular force}$ is the key.