Copper(I) oxide is a red solid. It can also be yellow, but not commonly. It does not dissolve in water. It turns into copper(II) oxide when in air. It dissolves in ammonia to make a colorless complex. This is oxidized to a blue complex of copper(II). It dissolves in concentrated hydrochloric acid. It also dissolves in concentrated sulfuric acid to make copper(II) sulfate and nitric acid to make copper(II) nitrate.