Copper(II) complexes and precipitates

Solutions and precipitates of different color are obtained when blue solution is mixed with colorless solutions.

Scientific name: Copper ions combine easily with different ligands to form coordination complexes. Precipitates of different color are produced in reactions of copper sulfate with sodium salts.


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Reaction formula

[Cu(H2O)6]2+ + 4NH3 → [Cu(H2O)2(NH3)4]2+ + 4H2O (blue)

[Cu(H2O)6]2+ + 2C2H8N2 → [Cu(H2O)2(C2H8N2)2]2+ + 4H2O (dark blue)

[Cu(H2O)6]2+ + C10H12N2O84- → [Cu(C10H12N2O8)]2- + 6H2O (bright blue)

[Cu(H2O)6]2+ + 4Br- → [Cu(Br)4]2- + 6H2O (violet)

[Cu(H2O)6]2+ + 4Cl- → [Cu(Cl)4]2- + 6H2O (green)

CuSO4 + 2NaOH → Na2SO4 + Cu(OH)2 (blue precipitate)

CuSO4 + Na2S → Na2SO4 + CuS2 (dark-brown precipitate)

CuSO4 + Na2CO3 → Na2SO4 + CuCO3 (light-blue precipitate)

CuSO4 + 2C7H5NaO2 → Na2SO4 + Cu(C7H5O2)2 (milky-blue precipitate)

Cu(OH)2 + NaClO + NaCl → CuO + NaCl + H2O (black precipitate)

Step-by-step instruction

  1. In order to perform the first part of this experiment (with soluble complex compounds) one should prepare CuSO4 solution, C2H8N2 (ethilenediamine) solution, C10H16N2O8 (ethylenediaminetetraacetic acid) solution, NH3 solution, diluted HCl, diluted HBr. All these solutions except CuSO4 and HBr are colorless. The CuSO4 solution is light-blue. The HBr solution is light-yellow.
  2. Take six glasses.
  3. Leave the first glass empty.
  4. Separately pour solutions of NH3, C2H8N2, C10H16N2O8, HCl and HBr to each glasses.
  5. Add the solution of CuSO4 to all glasses.
  6. When the CuSO4 solution is added to the second component the formation of a colored complex compound is observed.
  7. To illustrate the relative stability of the complexes one can add the solutions of chelating agents to CuSO4 solution in a sequence: NH3 --> C2H8N2 --> C10H16N2O8.
  8. The solution will first turn blue then dark blue and finally bright blue.
  9. For the second part of this experiment (with precipitates) one needs to have the following solutions: NaOH, Na2S, Na2CO3, C7H5NaO2, NaOH mixed with NaClO.
  10. Pour CuSO4 solution to the five test tubes.
  11. Add NaOH solution to the first one, N2S solution to the second one and so on.
  12. The formation of differently colored precipitates will be observed.

Scientific background

The first part of this experiment is based on a very important phenomenon: the ability of metals to form chemical structures called coordination compounds. In such structures the central metal atom is bounded with one or more ligands. A ligand is an atom or a molecule which contains one or more electron pairs that can be shared with a metal ion. Copper ions are well known to combine easily with different ligands to form coordination complexes. These compounds have very symmetrical, octahedral geometry. Depending on the ligand type and coordination number (the number of neighbors near a metal) complexes have different colors. The water solutions in the beakers in the considered experiment contain various ligand ions which form well defined colored complexes.

Glass 1. Cooper sulfate dissociates in the water solution and gives hydrated Cu2+ ions (Cu(H2O)2+) and SO42- ions:

CuSO4 + 6H2O → Cu(H2O)62+ + SO42-(light blue)

When it comes into contact with solutions in the glasses, ligand ions (molecules) replace water molecules from the Cu coordination shell.

Stability of complex compounds depends on the number of bonds between the metal ion and ligand. The more chelating ligands give more stable complexes. In the row NH3 → C2H8N2 → C10H16N2O8 the chelating ability increases and so each next substance added replaces previously bounded molecules.

The second part of this experiment is devoted to the ability of cooper ions to form precipitates. It easily reacts with NaOH, Na2S, Na2CO3, C7H5NaO2, NaOH mixed with NaClO producing differently colored insoluble substances.


Published on 04 April 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst