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Question

Explain salt hydrolysis. Name two salts which are (a) acidic (b) basic (c) neutral, when dissolved in water.

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Solution

The reaction between a salt and water to produce an acid and a base is called salt hydrolysis.

(1) Salts of strong acid and weak base: On hydrolysis, these salts form acidic solution with pH value less than 7. Examples include aqueous solutions of copper sulphate and ammonium chloride.

(2) Salts of strong base and weak acid: On hydrolysis, these salts form alkaline solution with pH value greater than 7. Examples include aqueous solutions of sodium carbonate and sodium acetate.

(3) Salts of weak acid and weak base: On hydrolysis, these salts form acidic, alkaline or neutral solutions. Examples include aqueous solutions of ammonium acetate and ammonium carbonate.

(4) Salts formed from strong acids and strong bases cannot be hydrolysed. Also, their aqueous solutions are neutral and have the pH value 7. Examples include aqueous solutions of sodium chloride and potassium nitrate.

Disclaimer: Any two salts mentioned under each category will be sufficient to answer this question.

(a) Salts that are acidic when dissolved in water are ammonium sulphate(NH4SO4), ammonium chloride(NH4Cl), calcium chloride(CaCl2), sodium hydrogen sulphate(NaHSO4), sodium hydrogen phosphate(NaHPO4), sodium dihydrogen phosphate(NaH2PO4) etc.
(b) Salts that are basic when dissolved in water are sodium carbonate(Na2CO3),
sodium acetate(CH3COONa), potassium carbonate(K2CO3), lead hydroxide(Pb(OH)2), magnesium hydroxide(Mg(OH)2), basic lead chloride(Pb(OH)Cl), basic copper chloride(Cu(OH)Cl), basic copper nitrate(Cu(OH)NO3) etc.
(c) Salts that are neutral when dissolved in water are sodium chloride(NaCl),
potassium nitrate(KNO3), sodium sulphate(Na2SO4), sodium phosphate(Na3PO4), potassium sulphate(K2SO4) etc.

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