Aqua regia, a mixture of nitric acid and hydrochloric acid, is one of the few materials that will dissolve gold. This material, translated to "royal water" in English, was named so because it could dissolve the royal metal gold. First noted in the fourteenth century, aqua regia could be used to help ascertain whether a particular material was actually gold or was some trickery of the alchemist. Nitric acid by itself will not dissolve gold, but will in combination with hydrochloric acid. The chemistry of the process is rather complex, with both acids reacting with the metal to form soluble gold compounds. The gold can be recovered from the solution, making the process useful for purification purposes.
Acids and bases have had many uses throughout history. In this chapter we want to explore the properties of acids and bases and the reactions in which they take part.