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 Fe
2+. 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.