Copper
Copper is a well known element, already known from ancient
times. It is a red lustrous metal. Copper, together with gold, is quite
remarkable among the metals because of its distinctive color. Copper is a
fairly noble metal, which is used for coins and tubing. Dilute non-oxidizing
acids do not attack the metal. Nitric acid, however, quickly dissolves the
metal, especially, when the acid is moderately to strongly concentrated.
A good source of pure copper is electricity wire. Especially high quality wires, used for loudspeakers, contain remarkably pure copper. This copper metal can be used as a source for copper compounds. Very pure powdered copper metal sometimes is available on eBay. The picture shows some copper wire and some very pure copper powder.
In its compounds, copper has oxidation states +1, +2 and to a little extent +3. For practical purposes, only the +1 and +2 oxidation states are relevant. Copper (I) ion is quite stable, but only in the solid state, or when it is coordinated to certain ligands. Aqueous copper (I), without other ligands besides water, is not possible. This will disproportionate to metallic copper and copper (II) ions at once. Copper (II) compounds are most stable in aqueous solution. These solutions can have colors, ranging from yellow-brown to deep blue, depending on which ligands are present. Copper has a remarkably rich aqueous chemistry and this is one of the most interesting elements to experiment with. Unfortunately, the richness of copper chemistry hardly is covered in common text books on chemistry. On the other hand, this leaves something new to be discovered by means of simple home experiments.
Copper compounds are moderately toxic, both to humans and for the environment. It is best to bring waste of copper experiments to a proper waste processing facility.
Commercially available copper compounds, accessible for the public, are the following:
- copper sulfate, CuSO4·5H2O
- copper (II) oxide, CuO
- copper (I) oxide, Cu2O
- copper (II) chloride dihydrate, CuCl2·2H2O
- copper (I) chloride, CuCl
- basic copper carbonate, Cu(OH)2·CuCO3
Copper sulfate is the most common compound of copper. It is a bright blue
crystalline solid, which dissolves in water easily. Its solutions are sky-blue.
On standing, these solutions become slightly turbid, due to hydrolysis of copper
(II), forming hydrated copper hydroxide and making the solutions slightly
acidic. Copper sulfate can be purchased at ceramics supply shops, photography
raw chemical suppliers and chemical supply houses. It is cheap and it is a very
convenient chemical for studying copper chemistry. This is a compound, which
should not be missing in a home lab.
Copper (II) oxide is available from ceramics suppliers. It is a black powder,
which can be dissolved in acid with some heating. This also is a good source of
copper compounds, but it is not really necessary to have this compound at hand,
when copper sulfate is available.
Copper (I) oxide is available from chemical supply houses and possibly from
ceramics suppliers. It is a red powder, which can be dissolved in hydrochloric
acid. It forms a colorless solution, when absolutely no oxygen is present,
otherwise a dark brown solution is formed, containing a dark mixed valence
copper (I)/copper (II) complex. The exact nature of this dark compound only is
understood partially, see
riddle on copper (I) and copper (II). Copper (I) oxide is an interesting
chemical for the home lab, but performing experiments with this is really
difficult, due to the extreme sensitivity of aqueous copper (I) compounds to
oxygen from the air.
Copper (II) chloride, dihydrate, is a green/cyan crystalline solid, which
dissolves in water very well. Solutions of this compound can have colors between
yellow/green for concentrated solutions and light blue for dilute solutions. The
color-range is due to formation of the green/yellow tetrachlorocuprate (II)
complex, which on dilution dissociates to chloride and plain aqueous copper (II)
ions, which are sky-blue. Copper (II) chloride is available from photography raw
chemical suppliers and chemical supply houses. Having this compound at hand is
not really necessary. Most interesting coordination experiments with copper,
involving chloride, can also be done very well with combined solutions of copper
sulfate and sodium chloride or hydrochloric acid.
Pure copper (I) chloride is a snow-white crystalline solid. This compound,
however, is quite sensitive to aerial oxidation and the commercial product
usually is light brown. Copper (I) chloride does not dissolve in water, but it
can be dissolved in hydrochloric acid or a concentrated solution of sodium
chloride. Such solutions are oxidized by air very easily. On standing, they
quickly turn dark brown, due to formation of copper (II), which forms a dark
complex with copper (I) and chloride, see
riddle on copper (I) and
copper (II). Having copper (I) chloride at hand is not really necessary. A
colorless solution of copper (I) chloride in hydrochloric acid can be prepared
by dissolving some copper sulfate or copper chloride in concentrated
hydrochloric acid and adding excess copper wire to this. This should be allowed
to stand, tightly stoppered for a day or two.
Basic copper (II) carbonate is available from ceramics suppliers. It is a cyan
powder, which can be dissolved in acid with some heating. This, like copper (II)
oxide, also is a good source of copper compounds, but it is not really necessary
to have this compound at hand, when copper sulfate is available.