Corrosion of iron demonstration. Iron nails in sealed test tubes containing (from left to right): water, anhydrous calcium chloride and boiled water, showing different levels of corrosion (rust, orange). Rust is an iron oxide formed by the reaction of iron and oxygen in the presence of water. Normal water (left) contains a high proportion of oxygen so this nail rusts the most. Anhydrous calcium chloride acts as a desiccant, removing moisture from the air and preventing the middle nail from rusting. Boiling water (right) removes some of the oxygen meaning that this nail does rust but does so much slower than the left tube.