Copper(I) oxide or cuprous oxide is an inorganic compound with the chemical formula Cu2O. It is one of the main oxides of copper, the other being copper(II) oxide or copper oxide (CuO). This red solid is a component of some antifouling paints. The compound may be yellow or red, depending on the size of the particles. [3] Copper(I) oxide is found as the reddish mineral cuprite.

Copper(I) oxide can be produced by several methods. [4] Most directly, it is produced by the oxidation of copper metal:

4 Cu + O2 → 2 Cu2O

Additives such as water and acids affect the rate of this process and further oxidation to copper(II) oxide. It can also be produced commercially by reducing a copper(II) solution with sulfur dioxide.

Aqueous cuprous chloride reacts with base to form the same substance. In all cases, color is highly sensitive to procedural details.

The formation of copper(I) oxide is the basis of Fehling's test and Benedict's reducing sugar test. These sugars reduce alkaline solutions of copper(II) salts to produce bright red Cu2O precipitates.

It can form on silver-plated copper parts exposed to moisture when the silver layer is porous or damaged. This corruption is known as the Red Plague.

There is little evidence that copper(I) hydroxide, CuOH, dehydrates rapidly. A similar situation applies to the hydroxides of gold(I) and silver(I).

Solids are diamagnetic. As far as its coordination layer is concerned, the copper center is two-coordinated and the oxide is tetrahedral. Thus, the structure is in a sense similar to the major polymorphs of SiO2, and both structures have interpenetrating lattices.

Copper (I) oxide dissolves in concentrated ammonia solution to form a colorless complex [Cu(NH3)2]+, which is easily oxidized to blue [Cu(NH3)4(H2O)2]2+ in air. Dissolved in hydrochloric acid to give CuCl−

2. Dilute sulfuric acid and nitric acid produce copper(II) sulfate and copper(II) nitrate, respectively. [5]