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

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 is hydrolysed to some extent, forming sodium hydroxide and carbonic acid.

\[\ce{NaCl_{(s)} + H2O_{(l)} -> NaOH_{(aq)} + HCl_{(aq)}}\]

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, making the solution basic.

\[\ce{Na2CO3_{(s)} + 2H2O_{(l)} -> 2NaOH_{(aq)} + H2CO3_{(aq)}}\]