Ionic substances are made up from positive metal ions (cations) and negative non-metal ions (anions).

Billions of these ions are held together in a giant ionic lattice by the strong force of electrostatic attraction between oppositely-charged ions.

The intermolecular forces we have talked about in the 'States of Matter' section are - in the case of ionic substances - the ionic bond itself: that strong force of electrostatic attraction.

So... to melt ionic substances requires us to weaken the ionic bond. This takes a lot of energy and leads to a high melting point

To turn ionic substances into a gas requires us to BREAK the ionic bond - this takes a very large amount of energy and leads to a very high boiling point.

Here's Dad to explain: