Rust experiment. Image 2 of 2. Iron nails in test tubes under different conditions, from left to right: water & air, drying agent, distilled water topped with oil, and salty water. Rust (brown) is hydrated iron oxide, produced by a chemical reaction between iron metal and the water & oxygen in the air. This process can be accelerated by the presence of ions in solution, such as saltwater. The nails in the tubes exposed to water and oxygen (far left & right) have rusted. The nails exposed to oxygen but no water (left) and water but no oxygen (right) have not rusted. For the beginning of this experiment, see image T745/069.