Metallic antimony is a relatively stable compound. Under standard conditions, antimony will keep its luster because it does not readily oxidize. Even in moist air and elevated temperatures in the range of 100 - 250° C, it resists oxidization. However, at temperatures above its melting point, powered antimony ignites and burns with a white-green flame. This reaction forms the pure metal into antimony trioxide, Sb2O3. At 750 - 800° C, molten antimony can decompose steam to liberate hydrogen.
Antimony does not dissolve in water, and is relatively stable in dilute hydrochloric acid and concentrated hydrofluoric acid. It forms SbCl3 and Sb2(SO4)3 respectively when brought in contact with concentrated hydrochloric acid and hot concentrated sulfuric acid (90 - 95°:).
Concentrated nitric acid will oxidize antimony, forming Sb2O3 or Sb2O5. It is believed the reaction mechanism centers around the intermediary formation of nitrous acid. A layer of oxide material forms on the surface of the metal and subsequently blocks any further reaction.
To conclude, antimony will dissolve in aqua regia, a fresh mixture of concentrated nitric and hydrochloric acids, or in a mixed solution of nitric acid and tartaric acid. It sparingly dissolves in phosphoric acid and some organic acids.
