Sulfite ions react with hydrochloric acid and gentle warming to evolve which gas?

• A. Carbon dioxide is evolved.
• B. Hydrogen is evolved.
• C. Oxygen is evolved.
• D. Chlorine is evolved.

Answer: (A)

When sulfite ions meet an acid, the sulfite is oxidized to sulfur dioxide as water forms. The reaction can be written as: SO3^2− + 2 H+ → SO2 + H2O. Gentle warming helps drive the sulfur dioxide gas out of the solution. This is why the gas evolved in this reaction is sulfur dioxide. The other gases listed would come from different reactions (carbon dioxide from carbonate with acid, hydrogen from metal-acid reactions, oxygen from certain decompositions, chlorine from strong oxidizers with chloride), not from sulfite reacting with hydrochloric acid.