Iron and sulphur powders on watch-glasses with derivatives of these chemical elements. Iron (Fe, grey, upper left) is a metal that is extensively used in construction and manufacturing. Sulphur (S, yellow, upper centre) is a reactive element that is mainly used in the production of sulphuric acid for the chemical industry. An iron-sulphur mixture (lower left) may be heated to form the chemical compound iron (II) sulphide (FeS, black, lower right). This shows how chemical reactions produce compound materials that have different properties, such as the change in appearance from the original components and the mixture.