= Physical properties of salts
Many <ionic crystals> have high <melting> <temperatures> compared with small molecular <solids>, because reorganizing their strongly interacting <ions> costs <energy>. But <melting> leaves a dense <liquid> with interactions; it does not perform the complete separation defining a <lattice energy>. At fixed pressure, the transition occurs when the <Gibbs free energies> of the <solid> and <liquid> match. The relevant <enthalpy> and <entropy> differences both matter, so <lattice energy> alone is not a universal ordering of <melting> <temperatures>. Strong <covalent bonds> in extended networks can also give high <melting> <temperatures>.<Chemistry 2e>[§10.5, “Ionic Solids” and “Covalent Network Solid”; §16.4, equations defining Gibbs free energy and its change]{ref}
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