When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds. Simple molecular substances are gases, liquids or solids with low melting and boiling points.The animation shows how the weak intermolecular forces between water molecules break down during boiling or melting: Your web browser does not have JavaScript switched on at the moment.imple molecular substances are gases, liquids or solids with low melting and boiling points. The animation shows how the weak intermolecular forces between water molecules break down during boiling or melting: Your web browser does not have JavaScript switched on at the moment.