Sulfur dioxide (also sulphur dioxide) is the chemical compound with the formula SO. It is a gas. It smells like burnt matches. It also smells suffocating. Sulfur dioxide is produced by volcanoes and in various industrial processes. It is also used to protect wine from oxygen and bacteria. It can be produced by burning sulfur. It dissolves in water to produce sulfurous acid. It can be oxidized to trioxide, which is dissolved in sulfuric acid to make more sulfuric acid. It is used to make sulfites.