Distinguish between the following pairs of compounds using a reagent as a chemical test:
(i) Calcium nitrate and Zinc nitrate solution.
(ii) Ammonium sulphate crystals and Sodium sulphate crystals.
(iii) Magnesium chloride and Magnesium nitrate solution.
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(i) When ammonium hydroxide (NH4OH) is added to zinc nitrate solution [Zn(NO3)2], a gelatinous white ppt of zinc hydroxide [Zn(OH)2] is obtained which is soluble in excess of NH4OH.
$\underset{\text{colourless solution}}{{\text{ZnSO}_4}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white gelatinous ppt}}{\text{Zn(OH)}_2↓} + \underset{\text{colourless in solution}}{(\text{NH}_4)_2\text{SO}_4}$
(With excess NH4OH ppt. dissolves)
$\text{Zn(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 + 2\text{NH}_4\text{OH} \longrightarrow \underset{\underset{\text{zinc (II) sulphate}}{\text{Tetraammine}}}{[(\text{Zn(NH}_3)_4]\text{SO}_4} + 4\text{H}_2\text{O}$
On the other hand, calcium nitrate solution [Ca(NO3)2] does not give any ppt. even when excess of ammonium hydroxide is added.
(ii) Ammonium salts react with alkalis on heating to liberate Ammonia gas. Hence, we can use NaOH as reagent to distinguish between Ammonium Sulphate crystals and Sodium Sulphate crystals. Below is the reaction for the same:
(NH4)2SO4 + 2NaOH ⟶ Na2SO4 + 2H2O + 2NH3 [g]
(iii) Add silver nitrate soln. to the given solns., Magnesium chloride reacts to form a white ppt. which is soluble in NH4OH and insoluble in dil. HNO3. The other solution is Magnesium nitrate.
MgCl2 + 2AgNO3 ⟶ 2AgCl ↓ [white ppt.] + Mg(NO3)2
Mg(NO3)2 + AgNO3 ⟶ no white ppt.