Sulphur burning in air. Sulphur being burnt on a spoon, producing a blue flame. Sulphur is the tenth most abundant element in the universe. At room temperature it is a pale yellow, odourless solid. When burnt in air it reacts with oxygen to produce sulphur dioxide (SO2). Sulphur dioxide is the main component of acid rain. It forms in the atmosphere when fossil fuels contaminated with sulphur are burnt.