Why do ionic substances have such high boiling points?
In order to move the ions further apart and so as to become a gas, a lot of energy is required as ionic bondings are strong. In the case of covalent bonding, generally, small molecules (e.g. oxygen, water, carbon dioxide) are formed. Although the covalent bonds are also very strong, we do not have to break them to change the state of the substance. We merely have to separate the molecules which are held together by weak Van der Waals forces. (Please note that some covalent substances have got high melting and boiling points too as they exist as giant macromolecular structures, e.g. sand and diamond).
