Sulphur dioxide gas being passed into water, with the reaction revealed by the addition of universal indicator. In neutral water, the indicator is green. Sulphur dioxide is an acidic gas, dissolving in water to form sulphurous acid, which readily dissociates in solution. This makes it very acidic, turning the universal indicator red. This is one of the reactions that occurs in the atmosphere to form acid rain, as sulphur dioxide is a common industrial pollutant.