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Since most salts are extensively ionised in solution, the reactions between them usually occur between ions. On adding a solution of sodium chloride to one of silver nitrate, the silver ion and chloride ion form insoluble silver chloride, which is precipitated, whilst the sodium and nitrate ions take no part in the reaction. Instead of the equation: AgNO3 + NaCl = AgCl + NaNO3, therefore, a more correct representation is: Ag˙ + NO3' + Na˙ + Cl' = AgCl + Na˙ + NO3'; or, since the sodium and nitrate ions are not concerned, the ionic equation: Ag˙ +Cl' = AgCl. A necessary condition for the correctness of an ionic equation is that the sum of the positive and negative ionic charges shall be the same on both sides:
Zn + Cu˙˙ = Zn˙˙ + Cu;
2Fe(CN)6'''' + Cl2 = 2Fe(CN)6''' + 2Cl';
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