Covalent bonds are strong but intermolecular forces are weak
The atoms within covalent molecules are held together by covalent bonds while the molecules in a covalent substance are attracted to each other by intermolecular forces.
Graph showing the increase in boiling point as the molecular size increases
The plastic coating around electrical wires is made from covalent substances that do not allow a flow of charge
When a covalent molecule melts or boils the covalent bonds do not break, only the intermolecular forces. If you think about it, when you boil a kettle full of water you are not generating large volumes of hydrogen and oxygen gas in your kitchen – this might give you an interesting unwanted chemical reaction ! Boom !