Aqua regia

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Aqua regia (; from Latin, "regal water" or "royal water") is a mixture of nitric acid and hydrochloric acid, optimally in a molar mixing ratio of 1:3. Aqua regia is a fuming liquid. Freshly prepared aqua regia is colorless, but it turns yellow, orange, or red within seconds from the formation of nitrosyl chloride and nitrogen dioxide. It was so named by alchemists because it can dissolve noble metals, such as gold and platinum, while leaving many other metals unaffected. It has been used to process or conceal gold into the modern era.

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