Copper, Cu²⁺

Most common oxidation states: +1, +2

M.P. 1083^o

B.P. 2582^o

Density 8.92 g/cm³

Characteristics: Reddish-yellow, rather inactive metal. Dissolves readily in HNO₃ and in hot, concentrated H₂SO₄.

Characteristic reactions of Cu²⁺:

The +2 oxidation state is more common than the +1. Copper(II) is commonly found as the blue hydrated ion, [Cu(H₂O)₄]²⁺.

Aqueous Ammonia:

Copper(II) ion reacts with stoichiometric quantities of aqueous ammonia to precipitate light blue Cu(OH)₂. Some basic salts may also form.

The precipitate dissolves in excess ammonia to form a dark blue complex ion:

Sodium Hydroxide:

Sodium hydroxide precipitates copper(II) hydroxide:

The precipitate does not dissolve in excess sodium hydroxide unless the NaOH solution is very concentrated. However, the precipitate will dissolve upon addition of concentrated ammonia solution.

 Potassium Ferrocyanide:

Potassium ferrocyanide precipitates red-brown copper(II) ferrocyanide from Cu²⁺ solutions:

This test is very sensitive. The precipitate is soluble in aqueous ammonia.

Note: Many metal ions form ferrocyanide precipitates, so potassium ferrocyanide is not a good reagent for separating metal ions. It is used more commonly as a confirmatory test.

No Reaction:

Cl^-, SO₄^2-

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