A strong acid is an acid which completely dissociates (gives up its protons) in water. Some example strong acids include hydrochloric acid (HCl) and hydrobromic acid (HBr). Sulfuric acid is also a strong acid but it is a special case; it is diprotic, but only the first hydrogen completely dissociates in water. The highly reactive nature of such acids make them useful in the etching of materials, particularly metals. Strong acids are also used in tanning.