|MadSci Network: Chemistry|
Much depends on the type of nail you are using. Many nails today contain a polymer or oil coating. I suspect that is the initial purple color you may be seeing.
The ferrous ion is greenish in solution, while the ferric ion is a light violet. The distinct colors of iron compounds are due to the d-electrons, which can interact with light in many interesting ways. Ferrous oxide, as a mineral, is called hematite ("blood-stone") and is usually almost black. It leaves a red streak on unglazed porcelain, and in its usually finely- divided condition is a characteristic bright earthy red. In pure form, it is called rouge, good for reddening cheeks and polishing glass, or Venetian red, a red pigment. The red stains on concrete are ferric oxide, usually from reinforcement rods that are not properly protected from the weather. The red of Mars is ferric oxide, showing that the atmosphere once contained oxygen, the oxygen that is only liberated by life.
Ferrous sulphate, FeSO4 forms pale green crystals of the heptahydrate when a solution is evaporated. In air, these crystals will attract moisture and oxygen, and slowly become ferric. This salt is called copperas or green vitriol.
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