(O Level Chem) Are all salts neutral?

You may think that all salt solutions are neutral. Well, it's not really true for all salt solutions. Some salt solutions are acidic (turns blue litmus red), some are neutral (litmus is purple) and others are alkaline (turns red litmus blue).

Not all salt solutions are neutral...

How do you predict whether a salt solution is acidic, neutral or alkaline? Recall from your Chemistry course that a salt is formed from an acid and a base. Begin from this and decide whether the base and acid are strong or whether they dissociate completely.

Let's begin from HA + BOH --> BA + H₂O...

Then decide if acid HA and alkali BOH are strong...

Acidic salt	Neutral salt	Alkaline salt
Aluminium chloride 3HCl(aq) + Al(OH)3(s) = AlCl3(aq) + 3H2O(l) HCl(aq) is a strong acid: HCl(aq) --> H+(aq) + Cl-(aq) We know that Al(OH)3(s) is a weak alkali because it is an insoluble ppt: Al(OH)3(s) = Al3+(aq) + 3OH-(aq)	Potassium nitrate HNO3(aq) + KOH(aq) --> KNO3(aq) + H2O(l) Both nitric acid and potassium bromide are strong.	Sodium bicarbonate H2CO3(aq) + NaOH(aq) = Na2CO3(aq) + 2H2O(l) H2CO3(aq) is a weak acid: H2CO3(aq) = 2H+(aq) + CO32-(aq) NaOH(aq) is a strong alkali: NaOH(aq) --> Na+(aq) + OH-(aq)
Zinc sulfate H2SO4(aq) + Zn(OH)2(s) = ZnSO(aq) + H2O(l) H2SO4(aq) is a strong acid: H2SO4(aq) --> 2H+(aq) + SO42-(aq) We know that Zn(OH)2(s) is a weak alkali because it is an insoluble ppt: Zn(OH)2(s) = Zn2+(aq) + 2OH-(aq)	Calcium bromide 2HBr(aq) + Ca(OH)2(aq) --> CaBr2(aq) + 2H2O(l) Both HBr(aq) and Ca(OH)2(aq) are strong.	Rubidium fluoride HF(aq) + RbOH(aq) = RbF(aq) + H2O(l) HF is a weak acid: HF(aq) = H+(aq) + F-(aq) RbOH(aq) is a strong alkali: RbOH(aq) --> Rb+(aq) + OH-(aq)
Ammonium chloride HCl(aq) + NH3(aq) = NH4Cl(s) HCl(aq) is a strong acid: HCl(aq) --> H+(aq) + Cl-(aq) NH3(aq) is a weak alkali: NH3(aq) + H2O(l) --> NH4+(aq) + OH-(aq)		Sodium ethanoate (sodium acetate) CH3COOH(aq) + NaOH(aq) = CH3COONa(aq) + H2O(aq) CH3COOH(aq) is a weak acid: CH3COOH(aq) = CH3COO-(aq) + H+(aq) NaOH(aq) is a strong alkali.

Note: “-->” in “HA(aq) --> H⁺(aq) + OH^-(aq)” shows complete dissociation and so HA is a strong acid. “=” in “BOH(aq) = B⁺(aq) + OH^-” shows partial dissociation and so BOH is a weak alkali.  


Try these questions before you check with the answers below.

1. Predict whether iron(III) chloride is acid, neutral or alkaline. Explain your answer.
   
2. Explain why (a) barium nitrate (b) sodium sulfate solutions are neutral.
   

Answers: 1. 3HCl(aq) + Fe(OH)₃(s) = FeCl₃(aq) + 3H₂O(l). HCl(aq) is a strong acid. We know that Fe(OH)₃(s) is a weak base because it is an insoluble ppt. Therefore, FeCl₃(aq) is an acidic salt solution. 2. (a) 2HNO₃(aq) + Ba(OH)₂(aq) --> Ba(NO₃)₂(aq) + H₂O(l). Both HNO₃(aq) and Ba(OH)₂(aq) are strong. (b) H₂SO₄(aq) + NaOH(aq) --> Na₂SO₄(aq) + H₂O(l). Both H₂SO₄(aq) and NaOH(aq) are strong.