Question

 Explain why an aqueous solution of sodium chloride is neutral but an aqueous solution of sodium carbonate is basic (or alkaline). Write chemical equations of the reactions involved.

Answer 1

NaCl(s) + H₂O(l) →  NaOH(aq) + HCl(aq)

An aqueous solution of sodium carbonate (washing soda) is alkaline because it is a salt prepared from the reaction of a weak acid, carbonic acid (H₂CO₃), and a strong base, sodium hydroxide (NaOH). When sodium carbonate is dissolved in water, it gets hydrolysed to some extent and forms sodium hydroxide and carbonic acid.

Na₂CO₃(s) + 2H₂O(l)  →  2NaOH(aq) + H₂CO₃(aq)

Sodium hydroxide, being a strong base, is fully ionised to give a large amount of hydroxide ions (OH^-), and carbonic acid, being a weak acid, is slightly ionised to form a small amount of hydrogen (H⁺) ions. Therefore, an aqueous solution contains more hydroxide ions than hydrogen ions, and this makes the solution basic.