Explain why Magnesium ions form precipitate with dilute ammonia solution, but no precipitates is formed if ammonium chloride is added to magnesium ions prior to ammonia solution 

Ammonia hydrolyses in water to hydroxyl ions

NH₃ (aq) + H₂O (l) → NH₄⁺(aq) + OH^–(aq)

Hydroxyl ions react with magnesium ions to form white ppt. of magnesium hydroxide

Mg²⁺(aq) + 2OH^–(aq) → Mg(OH)₂(s)

In presence of ammonium chloride magnesium ions do not form ppt. the hydroxyl ions are neutralized by ammonium ions

NH₄⁺ (aq) + OH^– (aq) → NH₃ (aq) + H₂O(l)

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Dr. Bbosa Science