Melting and boiling in small molecular substances

Small molecular substances consist of separate molecules. Each molecule has strong covalent bonds holding its atoms together, but there are only weak intermolecular forces between one molecule and another.

Because these intermolecular forces are weak, relatively little energy is needed to overcome them. Small molecular substances therefore usually have low melting and boiling points, and many are liquids or gases at room temperature.

What happens when small molecular substances melt or boil

When a solid melts, energy is used to overcome enough of the intermolecular forces for the molecules to move past one another as a liquid. When the liquid boils, energy is used to overcome the intermolecular forces so that the molecules can separate and move freely as a gas.

The covalent bonds within the molecules are not broken during melting or boiling. The molecules remain intact; it is the forces between them that are overcome.

Effect of molecular size

Larger molecules tend to have stronger intermolecular forces than smaller molecules. More energy is therefore needed to overcome the forces between them, so larger molecules tend to have higher melting and boiling points.

Electrical conductivity of small molecular substances

For a substance to conduct electricity, it must contain charged particles that can move through it and carry charge. Small molecular substances do not conduct electricity because they do not have such particles.

A small molecular substance is made of molecules with no overall charge. Even if the molecules can move, they cannot carry an electric current through the substance because they are neutral.

The atoms within each molecule are joined by covalent bonds, in which electrons are shared between atoms. These electrons are not free to move through the substance. There are also no mobile ions to carry charge, so an electric current cannot flow through the material.