In ionic solids, e.g. sodium chloride, the ions are held together by their opposite electrical charges. Each cation (+) attracts several anions (-) and vice versa. The ions build up into a giant ionic lattice structure. In sodium chloride each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions which forms a cubic shaped lattice. This simple cubic shape is found in other Group 1 halides, e.g. KF and LiCl.
It is possible to fit two different sorts of cations into an anion lattice provided the + and - charges balance. An example is potassium magnesium chloride KCl.MgCl2.H2O which exists as a double salt. It is a single substance and not a mixture of two salts. The Cations are arranged in a regular pattern throughout the lattice. The water molecule in this salt is called water of crystallisation. The crystals are called hydrated salts.
Many ionic substances dissolve in water. When they do, the ions become surrounded by water molecules. They are said to be hydrated. The hydrated ions are spread throughout the water at random and they behave independently of each other.
Some ionic substances dissolve in water, but others do not, so what decides whether an ionic substance will dissolve?
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