Since ancient times gold has been treasured by man. For much of the ages gold has been used as a currency or a currency denominator. It is also used in jewelry and is a very good conductor. Gold also does not react with air, water, alkalis or any acids except aqua regia. It does, however, react with halogens and can form compounds.
Aqua regia is a mixture of hydrochloric acid and nitric acid and also called also called nitro-hydrochloric acid. It can dissolve gold into chloroauric acid. Nitric acid and hydrochloric acid cannot do the job on their own but when combined they perform different tasks which allows them to dissolve the gold into an acid.
Gold reacts very slowly with chlorine gas at room temperature producing gold chloride or AuCI3. The process can be reversed, however, by heating the gold chloride gently so it decomposes and releases the pure elements of gold and chlorine again.
Unusually for a metal gold can also form compounds with certain elements. In a gold compound — auride — the oxidation number is negative. Gold combines with cesium to form cesium auride which has nonmetallic properties and is a semiconductor.
In a similar way to cesium, gold can also react with rubidium to form the compound rubidium auride. This compound has nonmetallic properties and is a semiconductor.