Mixed iron and sulphur powders. Image 2 of 3. Iron (black) is a dense magnetic metal while sulphur (yellow) is a less dense, non-magnetic non-metal. These elements can be used to demonstrate the difference between a physical and a chemical change. Mixing the two powders, as here, is a purely physical change: the elements retain their properties and can be easily separated. A magnet could be used to pull away the iron, for instance. If the elements are heated together, however, a chemical change occurs and they react to form iron sulphide (see A500/595). This compound's properties are unlike those of its consituent elements. See A500/593 for the separate powders.