Ferric refers to iron-containing materials or compounds. In chemistry the term is reserved for iron with an oxidation number of +3, also denoted iron(III) or Fe³⁺. On the other hand, ferrous refers to iron with oxidation number of +2, denoted iron(II) or Fe²⁺. Iron(III) is usually the most stable form of iron in air, as illustrated by the pervasiveness of rust, an insoluble iron(III)-containing material.