NILERED / SCIENCE PHOTO LIBRARY NILERED / SCIENCE PHOTO LIBRARY
Fuming nitric acid added to copper coins. Fuming nitric acid is a highly concentrated form of nitric acid (86 percent versus the normal 68 percent). However, when the fuming nitric acid is added to the copper coins in the beaker, it reacts with the copper coins to form a protective layer of copper oxide that ends the reaction. This effect is called passivation. Water is then added to reduce the concentration, triggering a vigorous reaction. The resulting green colour is the copper nitrate (Cu[NO3]2) that is formed as the nitric acid (HNO3) reacts with the copper (Cu), also producing a dark orange vapour of nitrogen dioxide (NO2) along with water (H2O).
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