# Copper

## Copper is a metal that conducts heat and electricity; its most common oxidation states, +1 and +2, are often called cuprous and cupric.

#### Terms

• A reddish-brown, malleable, ductile metallic element with high electrical and thermal conductivity. Its symbol is Cu and its atomic number is 29.

#### Figures

1. ##### Copper(I) Oxide

Copper (I) has a red color.

2. ##### Tetramminecopper(II) sulfate

Copper(II) acquires a deep blue coloration in the presence of ammonia ligands.

Copper is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; a freshly exposed surface has a reddish-orange color (Figure 1). It is used as a conductor of heat and electricity, a building material, and a constituent of various metal alloys. Its compounds are commonly encountered as copper(II) salts, which often impart blue or green colors (Figure 2) to minerals such as turquoise, and have been widely used as pigments. Copper(II) ions are water-soluble, meaning they function at low concentration as bacteriostatic substances, fungicides, and wood preservatives. In sufficient amounts, they are poisonous to higher organisms; at lower concentrations, they are an essential trace nutrient to all higher plant and animal life. In animals copper is mainly found in the liver, muscles, and bones.

Copper forms a rich variety of compounds with oxidation states +1 and +2, which are often called cuprous and cupric, respectively. It does not react with water, but reacts slowly with atmospheric oxygen, forming a layer of brown-black copper oxide. In contrast to the oxidation of iron by wet air, this oxide layer stops the further, bulk corrosion. Hydrogen sulfides and sulfides react with copper to form various copper sulfides on the surface. In the latter case, the copper corrodes, as is seen when copper is exposed to air containing sulfur compounds. Oxygen-containing ammonia solutions yield water-soluble complexes with copper, as do oxygen and hydrochloric acid, which form copper chlorides, and acidified hydrogen peroxide, which form copper(II) salts. Copper(II) chloride and copper combine to form copper(I) chloride.

The simplest compounds of copper are binary compounds, i.e. those containing only two elements. The principal compounds are the oxides, sulfides, and halides. Both cuprous and cupric oxides are known. Among the numerous copper sulfides, important examples include copper(I) sulfide and copper(II) sulfide. The cuprous halides with chlorine, bromine, and iodine are well known, as are the cupric halides with fluorine, chlorine, and bromine. Attempts to prepare copper(II) iodide yield cuprous iodide and iodine.

Copper, like all metals, forms coordination complexes with ligands. In aqueous solutions, copper(II) exists as [Cu(H2O)6]2+. This complex exhibits the fastest water exchange rate (speed of water ligands attaching and detaching) of any transition-metal-aquo complex. Adding aqueous sodium hydroxide causes the precipitation of light blue solid copper(II) hydroxide. A simplified equation follows:

$Cu^{2+} + 2 OH^{-} \rightarrow Cu(OH)_{2}$

Aqueous ammonia results in the same precipitate. Upon adding excess ammonia, the precipitate dissolves, forming tetraamminecopper(II):${ Cu({ H }_{ 2 }O) }^{ 4 }{ (OH) }_{ 2 }+4{ NH }_{ 3 }→{ [{ Cu({ H }_{ 2 }O) }_{ 2 }{ ({ NH }_{ 3 }) }_{ 4 }] }^{ 2+ }+2{ H }_{ 2 }O+2O{ H }^{ - }$

Many other oxyanions form complexes: these include copper(II) acetate, copper(II) nitrate, and copper(II) carbonate. Copper(II) sulfate forms a blue crystalline pentahydrate, which is the most familiar copper compound in the laboratory. It is used in a fungicide called the Bordeaux mixture. Polyols, compounds containing more than one alcohol functional group, generally interact with cupric salts. For example, copper salts are used to test for reducing sugars. Specifically, using Benedict's reagent and Fehling's solution, the presence of the sugar is signaled by a color change from blue copper(II) to reddish copper(I) oxide. Schweizer's reagent and related complexes with ethylenediamine and other amines dissolve cellulose. Amino acids form very stable chelate complexes with copper(II). Many wet-chemical tests for copper ions exist, one, for example, involving potassium ferrocyanide, which yields a brown precipitate with copper(II) salts.

#### Key Term Glossary

acid
an electron pair acceptor; generally capable of donating hydrogen ions
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alcohol
class of organic compounds containing a hydroxyl functional group
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alloy
A metal that is a combination of two or more elements, at least one of which is a metal.
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amine
organic compounds or functional group that contains a basic nitrogen atom with a lone pair
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amino acid
any of the 20 naturally occurring α-amino acids (having the amino and carboxylic acid groups on the same carbon atom) and a variety of side chains that combine, via peptide bonds, to form proteins
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aqueous
Consisting mostly of water.
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binary compound
a chemical substance that contains exactly two elements
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carbonate
Any salt or ester of carbonic acid
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compound
A substance made from any combination elements.
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concentration
the proportion of a substance in a mixture
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conductor
something that can transmit electricity, heat, light, or sound
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coordination
The reaction of one or more ligands with a metal ion to form a coordination compound.
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copper
A reddish-brown, malleable, ductile metallic element with high electrical and thermal conductivity. Its symbol is Cu and its atomic number is 29.
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corrosion
Erosion by chemical action, especially oxidation.
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crystalline
having a regular three-dimensional molecular structure
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ductile
Capable of being pulled or stretched into thin wire by mechanical force without breaking.
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element
Any one of the simplest chemical substances that cannot be decomposed in a chemical reaction or by any chemical means, and are made up of atoms all having the same number of protons.
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functional group
a specific grouping of elements that is characteristic of a class of compounds, and determines some properties and reactions of that class
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group
A vertical column in the periodic table, which signifies the number of valence shell electrons in an element's atom.
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halide
a salt of any halogen acid
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heat
Heat is defined as the energy transferred from one system to another by thermal interaction.
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hydroxide
An univalent anion (OH-1) based on the hydroxyl functional group.
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ion
An atom or group of atoms bearing an electrical charge, such as the sodium and chlorine atoms in a salt solution.
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ligand
An ion, molecule, or functional group that binds to another chemical entity to form a larger complex.
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malleable
able to be hammered into thin sheets; capable of being extended or shaped by beating with a hammer or by the pressure of rollers.
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metal
Any of a number of chemical elements in the periodic table that form a metallic bond with other metal atoms; generally shiny, somewhat malleable and hard, often a conductor of heat and electricity
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mixture
Something that consists of diverse elements
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oxidation
a reaction in which the atoms of an element lose electrons and the valence of the element increases
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Oxidation
Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion.
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oxidation state
the state of an atom having a particular oxidation number.
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Oxidation State
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound.
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oxide
a binary chemical compound of oxygen with another chemical element
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oxygen
a chemical element (symbol O) with an atomic number of 8 and relative atomic mass of 15.9994
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precipitate
a solid that exits the liquid phase of a solution
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precipitation
A reaction that leads to the formation of a heavier solid in a lighter liquid; the precipitate so formed at the bottom of the container.
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salt
An ionic compound that is composed of cations and anions The product is electrically neutral. The two ions are held together by ionic bonds, not covalent bonds.
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solid
A substance in the fundamental state of matter that retains its size and shape without need of a container (as opposed to a liquid or gas).
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solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
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Solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
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state
The physical property of matter as solid, liquid, gas or plasma
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substance
Physical matter; material.
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thermal
pertaining to heat or temperature.