The red form of HgO can be made by heating Hg in oxygen at roughly 350 °C, or by pyrolysis of Hg(NO). The yellow form can be obtained by precipitation of aqueous Hg with alkali. The difference in color is due to particle size; both forms have the same structure consisting of near linear O-Hg-O units linked in zigzag chains with an Hg-O-Hg angle of 108°.