Copper in hydrochloric acid. This is an example of a metal-acid reaction. Normally, the reaction would produce a metal salt (here copper chloride) and hydrogen gas, but copper's low reactivity means that the reaction does not occur, or only occurs very slowly, at room temperature. Hydrochloric acid is more commonly used to clean oxides from the surface of copper. For examples of more vigorous reactions involving different metals, see A500/756 and A500/757.