Sulfurous acid is a chemical compound with the formula H2SO3. It is formed when sulfur dioxide (SO2) dissolves in water. This weak acid is colorless and has a pungent smell, often associated with burnt matches or volcanic gases. Sulfurous acid is used in various industrial processes, including the production of sulfites and as a preservative in food. In nature, sulfurous acid can be found in acid rain, which occurs when sulfur dioxide is released into the atmosphere from sources like volcanic eruptions and fossil fuel combustion. This acid can have harmful effects on the environment, including damage to plants and aquatic ecosystems.