IGCSE Science Paper 6 Practice Test

When sulfite ions (SO3^2-) meet hydrochloric acid, the acid provides H+ to form sulfurous acid (H2SO3). This unstable acid breaks down on warming to sulfur dioxide gas and water: H2SO3 → SO2 + H2O. So the gas released is sulfur dioxide. Carbon dioxide would only be produced from carbonate-like species reacting with acid (for example, CO3^2- + 2 H+ → CO2 + H2O).