Ammonium salts are prepared by dissolving ammonia in the appropriate acid or neutralising aqueous ammonia with the appropriate acid. The ammonium salts are then separated out of the solution by crystallisation and not by evaporation because they are heat-liable. e.g.
Ammonium trioxocarbonate (IV) is prepared by heating a mixture of ammonium tetraoxosulphate (VI)and calcium trioxocarbonate (IV).
The ammonium trioxocarbonate (IV) sublimes as a white solid in the cooler parts of the beaker.
Ammonium salts are electrovalent compounds which contain ammonium ion, NH4+ as the cation.
Properties of ammonium salts
1) Solubility: All ammonium salts are white crystalline solids which dissolve readily in water. They ionise completely in solution and are strong electrolytes.
2) Action of heat: All ammonium salts decomposes when heated. Some however sublime when heated.
(a) Ammonium tetraoxosulphate( VI) decomposes on heating to liberate ammonia and tetraoxosulphate (VI) acid.
(b) Ammonium dioxonitrate (III) decomposes on heating to produce nitrogen and water.
(c) Ammonium trioxonitrate (V) decomposes on heating to dinitrogen (1) oxide and water.
(d) Ammonium chloride sublimes on heating and dissociates to form ammonia and hydrogen chloride which recombine on cooling.
(e) Ammonium trioxocarbonate(IV) is an unstable compound. It decomposes very readily to yield ammonia, carbon (IV) oxide and water.
3) Reaction with bases: All ammonium salts liberate ammonia when heated with bases or alkalis.
Test for ammonium salts
Heat some of the unknown substance with sodium hydroxide solution. If the unknown substance is an ammonium salt, ammonia gas will be evolved. The gas is recognised by its smell and its action on red litmus paper.