The behaviour of metals towards the attack by acid solutions is shown, with a brief discussion based on their standard reduction potentials values. Metals with negative reduction potential (e.g. zinc) dissolve easily producing hydrogen gas, while metals with higher reduction potential (e.g. copper) do not. The latter can be dissolved in concentrated nitric acid, yielding nitrogen oxides. Metals which have still higher reduction potential (e.g. gold) cannot be dissolved even in nitric acid and require stronger oxidizing agents.