Made of copper-based materials, the statue and dome gradually changed color due to oxidation.
Oxygen can combine with copper and copper can combine in different ways to form two types of compounds: copper(I) oxide, which is usually a reddish powder, and copper(II) oxide, which is usually a black powder. They occur naturally as minerals in crystal form. Both forms of copper oxide are used in the production of pigments, but they have several other different uses.
As an efficient electrical conductor, copper oxide is used in photoelectric cells.
There are two oxides of copper because it can bond to nonmetals by donating one or two electrons. Oxygen forms compounds with metals gaining two electrons, so it can accept one electron from each of two copper atoms – forming copper(I) oxide – or two electrons from one atom – forming copper(II) oxide. The numbers “I” and “II” represent the number of electrons that the metal has provided; this is known as its oxidation number. The chemical formulas of these compounds are Cu 2 O and CuO, respectively.
Copper oxide (I)
Occurrence and Production
Copper oxide is an active component of many fungicides and pesticides.
This compound, also known as cuprous oxide, occurs naturally in some parts of the world as the mineral cuprite. Most of the compost for industrial use, however, has already been manufactured. It can be industrially produced by heating metallic copper to a high temperature, by electrolysis of salt solutions using copper electrodes, and by mixing some other copper compounds with a reducing agent. Some sugars, such as glucose, act as reducing agents and this reaction, in which Cu 2 O forms as a bright red powder, is a very sensitive lab test for these types of sugars.
If inhaled, copper oxide can cause shortness of breath.
Cu 2 O is a red powder or crystalline material that melts at 2250°F (1232°C). It tends to slowly oxidize to copper(II) oxide in moist air. Although it does not dissolve in water or any organic solvent, it does react with strong acids such as hydrochloric, nitric and sulfuric acid to form salts. It will also react with strong alkalis, such as sodium and potassium hydroxide, to form compounds known as cuprates.
When ingested, copper oxide can cause flu-like symptoms.
This compound was one of the first semiconductors to be discovered. It also demonstrates the photoelectric effect, where exposure to light causes an electrical current to flow. Therefore, it is used in photoelectric cells and light detectors. A ceramic material made of it acts as a superconductor at relatively high temperatures. Superconductors that do not require extreme cooling to operate are still an area of active research.
One of the biggest uses of copper(I) oxide is in agriculture. It is an ingredient in many fungicides used to protect a wide variety of cultivated plants against fungal diseases. The compound is also the active component in many antifouling paints, which are used to protect ships and underwater structures from becoming encrusted with marine plants and animals. It is also used as a pigment in the manufacture of some types of glass and in ceramic glazes, giving them a red color, and as a catalyst in some chemical processes.
The use of Cu 2 O pesticides has raised concerns about their effects on the environment. Soluble copper compounds derived from these products are toxic to some aquatic life forms. This is thought to be a bigger problem in freshwater, where compounds can build up from fungicides washed off the plants. The presence of salt in the water tends to reduce the uptake of metals by marine life.
Occurrence, Production and Properties
This compound – also known as cupric oxide (CuO) – occurs naturally as the black or gray mineral tenorite. Like Cu 2 O, it can be made by heating copper, but at a lower temperature. This method gives an impure form, however, and is best prepared by heating some oxygen-containing copper compounds, such as carbonate, hydroxide, or nitrate. It is a black solid that melts above 2192°F (1200°C). At this temperature, it loses some oxygen, leaving small amounts of copper(I) oxide. Like the other form, copper(II) oxide is insoluble in most solvents, but reacts with acids to form salts.
CuO is sometimes added to clay glazes as a pigment. Several colors, including red, blue, and green, can be derived from it, depending on how it is used. It is a precursor in the production of cuprammonium hydroxide, which is used in the manufacture of rayon. Sometimes, the compound is added in small amounts to animal feed to protect against copper deficiency. It is also used as an abrasive for polishing lenses and other optical components.
Although copper is an essential element for mammals, many of its compounds, including both forms of copper oxide, are toxic in all but small doses. If inhaled, copper(I) oxide can cause shortness of breath, coughing, and damage to the respiratory tract. Swallowing this compound can cause irritation of the gastrointestinal tract, vomiting, stomach pain, and diarrhea. Copper(II) oxide can cause similar symptoms if swallowed, as well as vision problems and skin discoloration. Both compounds can cause metal fume fever, a condition that produces flu-like symptoms and is a hazard in occupations that involves strong heating of copper structures or wire.