The rusting of an iron gate is considered a chemical change because a new substance, iron oxide (rust), is formed. This new substance has different properties than the original iron. For instance, rust is reddish-brown and brittle, unlike the strong, silvery-grey iron. This change is irreversible; once iron rusts, it cannot be easily converted back to its original metallic form. The two essential conditions required for rusting to occur are the presence of both oxygen (from air) and water (or water vapour). If either of these is absent, rusting will not take place.