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reaction endothermic reaction between nitric acid and metals

The video shows four flasks containing about 50 ml of nitric acid which is deposited gold, magnesium and zinc.

Gold does not react with nitric acid to dissolve gold if we have to use aqua regia, a mixture of nitric acid and hydrochloric acid in 1:3 ratio. The copper and magnesium turnings react quickly releasing a brown gas. The zinc reacts after a brief moment, in the same way.

nitric acid attacks most metals, even the silver and copper. The only exceptions are the noble metals: gold, platinum and iridium. Some metals such as iron, chromium and aluminum are not attacked because they are passive, ie it forms a thin oxide layer that protects the metal from acid attack. With zinc same thing happens, but eventually the oxide layer is attacked by nitric acid.

In all cases occur redox reactions: nitric acid is a strong oxidant:
3 Mg + 2 NO3-+8 H3O + -> 3 Mg 2 + + 2NO + 12 H2O
The metal is oxidized and the nitrogen is reduced. Nitrogen monoxide is colorless, but reacts with oxygen in the air to form NO2 brown.