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.
Copper(I) oxide is found in the ground. It is a copper ore.
Copper(I) oxide is made by heating copper in air. Some copper(II) oxide is also made. It can also be made by reducing copper(II) oxide or other copper(II) compounds. It is the result of a test for sugars. Green copper(II) chloride is reduced to red copper(I) oxide when a sugar is present.