Properties of simple molecular substances

The physical properties of simple molecular substances can be explained by thinking about their structure and bonding.

Low melting points and boiling points

Simple molecular substances generally have melting points and boiling points and are often liquids or gases at room temperature. Melting and boiling points depend partly on the relative formula mass (Mr) of substances, so some molecules with a high Mr may be solids at room temperature.

Substance M_{\textup{r}}Melts atBoils atState at 20°C
Methane, CH416-182°C-162°CGas
Hexane, C6H1486-95°C69°CLiquid
Eicosane, C20H4228237°C343°CSolid

Melting and boiling

Melting and boiling are changes of state.

Energy is transferred to a substance to melt or boil it. This energy is needed to overcome the forces of attraction between the particles in the substance:

  • some forces of attraction are overcome during melting, allowing molecules to move over each other
  • more of the forces of attraction are overcome during boiling, allowing the molecules to move freely away from each other

The more energy needed, the higher the melting point or boiling point.

Explanation

There are intermolecular forces between simple molecules. Intermolecular forces are much weaker than the strong covalent bonds in molecules. When simple molecular substances melt or boil, it is these weak intermolecular forces that are overcome. The covalent bonds are not broken. Relatively little energy is needed to overcome the intermolecular forces, so simple molecular substances usually have low melting and boiling points.

Question

Why is carbon dioxide a gas at room temperature?

Carbon dioxide is a simple covalent substance. There are weak intermolecular forces between carbon dioxide molecules. Only a little energy is needed to overcome these forces, so the melting point and boiling point of carbon dioxide are very low.

Conduction of electricity

A substance can conduct electricity if:

  • it contains charged particles, and
  • these particles are free to move from place to place

Simple molecules have no overall charge, or charged particles that can separate, so simple molecular substances cannot conduct electricity, even when liquid or dissolved in water. There are some exceptions, because some simple covalent molecules react with water to form a solution containing ions. For example, hydrogen chloride gas (HCl) is a simple covalent molecule, but it reacts with water to form hydrochloric acid. The solution of the acid contains H+ ions and Cl- ions, so it will conduct electricity.

curriculum-key-fact
When simple molecular substances melt or boil, their weak intermolecular forces are overcome, not the strong covalent bonds.

For example, in a kettle, water is heated up and will boil. The liquid water becomes water vapour or steam - it does not break up into hydrogen and oxygen. The atoms in the water molecules are still covalently bonded together, it is only the forces between water molecules that have been overcome.