Non-metals react with oxygen in the air to produce non-metal oxides. Here are two examples for the non-metals carbon and sulphur.
Non-metallic oxides in water forms acidic solution. For example, carbon dioxide in water forms carbonic acid. We can prove it because aqueous solution of carbon dioxide turns blue litmus red. Further aqueous solution of carbon dioxide is neutralized by a base, calcium hydroxide [Ca(OH)2], to form salt and water.
CO2 + Ca(OH)2 + H2O → CaCO3 + 2H2O
