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.
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.