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||Melts at||Boils at||State at 20°C|
Melting and boiling are changes of state.
The more energy needed, the higher the melting point or boiling point.
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.
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.
A substance can conduct electricity if:
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.
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.