If we take the reaction betweenhydrochloric acid and sodium hydroxide from the previous page,
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
We can write it in terms of ions,since both the acid and the alkali form ions in water.
H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) Na+(aq) + Cl-(aq) + H2O(l)
Chloride ions (Cl-(aq)), and sodium ions (Na+(aq))appear on both sides of the equation.They are spectator ions, they are not changed by the reaction,and so they may be left out of the equation.
This leaves the equation
hydrogen ion + hydroxide ion waterH+(aq) + OH-(aq) H2O(l)
Compare this reaction with the ionisation of water.
This is the reaction that always occurswhen an acid + alkali salt + water.The hydrogen ion of the acid + the hydroxide ion of the alkalicombine to form water,leaving the metal from the alkali and the non-metal from the acidto form a salt solution.
How much acid is needed to neutralise an alkali?
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