H2SO3, known as sulfurous acid, is a chemical compound formed when sulfur dioxide (SO2) dissolves in water. It is a weak acid and is often used in various industrial processes, including the production of sulfites and as a preservative in food and beverages. Sulfurous acid can exist in equilibrium with its gaseous form, sulfur dioxide, and is important in environmental chemistry, particularly in the formation of acid rain. It is also used in laboratories for its reducing properties and as a reagent in organic synthesis.