9.08.2 Simple Covalent Substances (Molecules)

Syllabus

Substances that consist of small molecules are usually gases or liquids that have low melting points and boiling points.
These substances have only weak forces between the molecules (intermolecular forces).
It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils.
The intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points.
These substances do not conduct electricity because the molecules do not have an overall electric charge.
Students should be able to use the idea that intermolecular forces are weak compared with covalent bonds to explain the bulk properties of molecular substances.

What does this mean?

Intermolecular forces and melting point

Covalent bonds are very strong, so they take a lot of energy to break.

It would take a very high temperature to break all the covalent bonds in a fire-extinguisher full of Carbon Dioxide.

But Carbon Dioxide exists as tiny CO2 molecules.

The forces that hold one molecule to its neighbours are much weaker.

So it takes a far lower temperature to provide enough energy to break these weak intermolecular forces.

This is why Carbon Dioxide becomes gas at a relatively low temperature.

It also explains why so many other covalent substances are gases and liquids at room temperature, and why most covalent substances we think of as solids melt at fairly low temperatures.

But very long molecules have stronger intermolecular forces and so may be solids, like Bitumen (road tar).