The Many Colors of Copper
Difficulty: Easy–Medium | Time: 45–60 min | Visual Impact: Very High
Historical Context
No metal has left a more colorful mark on human history than copper. The brilliant blue of Egyptian faience beads, the vivid green of malachite used as eye paint in ancient Kohl, the turquoise patina of bronze statues—all are copper compounds. Medieval painters ground malachite and azurite (both natural copper carbonates) into pigments. Verdigris, made by exposing copper to vinegar fumes, was the brightest green available to European artists for centuries. The Statue of Liberty’s famous blue-green skin is nothing more than copper reacting slowly with air, rain, and sea salt.
In 1848, Hermann von Fehling developed his glucose test around copper chemistry: an alkaline copper solution turns brick-red when it reacts with glucose. Generations of chemists and doctors used Fehling’s test to diagnose diabetes before modern blood-sugar meters existed.
All these colors come from the same element. This experiment recreates eight of them in an afternoon.
Materials
| What | How much |
|---|---|
| Copper sulfate (CuSO₄·5H₂O) | 15 g |
| Cupric chloride (CuCl₂) | 3 g |
| Sodium carbonate | 4 g |
| Dextrose (glucose) | 3 g |
| Copper wire, coins, or scraps | a small piece |
| Acetic acid / white vinegar | 30 mL |
| Water | ~300 mL |
| Test tubes or small glass jars | 6–8 |
| Hotplate or candle | for heating |
| Aluminum foil | small sheet |
| Safety glasses and gloves |
Colour Guide
Here is what you are building toward:
| # | Colour | Compound | Copper oxidation state |
|---|---|---|---|
| 1 | Reddish-orange | Cu metal | Cu⁰ |
| 2 | Sky blue | CuSO₄ solution | Cu²⁺ with water ligands |
| 3 | Blue-green | CuCl₂ solution | Cu²⁺ with chloride ligands |
| 4 | Powder blue | Cu(OH)₂ precipitate | Cu²⁺ |
| 5 | Vivid green | Cu₂(OH)₂CO₃ (synthetic malachite) | Cu²⁺ |
| 6 | White | CuSO₄ anhydrous | Cu²⁺ (no water) |
| 7 | Brick red | Cu₂O (cuprous oxide) | Cu⁺ |
| 8 | Black | CuO (cupric oxide) | Cu²⁺ |
Procedure
Work through the steps in order. Line up your samples as you go—by the end you will have a full colour series on the bench.
Sample 1 — Reddish-orange metal
- Hold a copper coin or a short length of copper wire in your hand. This is Cu⁰. The reddish-orange metallic lustre is copper’s ground state. Set it aside as your reference.
Sample 2 — Sky blue sulfate solution
- Dissolve 5 g of copper sulfate crystals in 50 mL of water. The vivid sky blue comes from Cu²⁺ ions surrounded by six water molecules. Pour into a test tube and label it.
Sample 3 — Blue-green chloride solution
- Dissolve 3 g of cupric chloride in 30 mL of water. Notice the distinctly different blue-green (teal) colour compared to Sample 2—same Cu²⁺ ion, different ligands, different colour. Place beside Sample 2 and compare.
Sample 4 — Powder blue hydroxide
- Take 20 mL of your copper sulfate solution (Sample 2). Add sodium carbonate solution (1 g in 10 mL water) drop by drop with stirring. A powder-blue precipitate forms immediately—this is copper hydroxide, Cu(OH)₂. Stop when roughly half the blue has precipitated. Label and set aside.
Sample 5 — Vivid green malachite
- To a fresh 20 mL portion of copper sulfate solution, add the sodium carbonate solution more generously—2 g in 15 mL water, all at once. The precipitate is now distinctly greener: basic copper carbonate, Cu₂(OH)₂CO₃, the same mineral as malachite. Compare it directly with the powder-blue Sample 4.
Sample 6 — White anhydrous sulfate
- Place a small heap of copper sulfate crystals (about 2 g) on a piece of aluminum foil. Heat gently over a hotplate or candle, moving constantly. The crystals lose their water of crystallisation and turn white or very pale grey. Once cool, add a drop or two of water and watch the blue return—this is one of chemistry’s most satisfying reversible reactions.
Sample 7 — Brick red cuprous oxide (Fehling’s)
- In a test tube, mix 5 mL of copper sulfate solution with 1 g sodium carbonate (powder-blue Cu(OH)₂ forms). Add 1 g of dextrose (glucose). Heat the test tube gently in a water bath at ~80 °C, or hold it carefully over a candle. Within 2–5 minutes, a brick-red precipitate appears—cuprous oxide, Cu₂O. This is Fehling’s test: glucose donates electrons to Cu²⁺, reducing it to Cu⁺. The medical laboratory test for sugar in urine uses exactly this reaction.
Sample 8 — Black copper oxide
- Hold a piece of copper wire or coin in a candle flame or over a hotplate for 30–60 seconds. The surface turns black: Cu²⁺ in CuO. This is the same reaction used to blacken copper jewellery and to make black ceramic glazes. Let it cool—if you then dip it briefly in the copper sulfate solution, the black oxide dissolves back and the copper surface reappears.
Reactions
Samples 4 & 5 — Precipitation
\[\ce{CuSO4(aq) + Na2CO3(aq) + H2O -> Cu(OH)2(s) + Na2SO4(aq) + CO2(g)}\]
With excess carbonate, the hydroxide converts to basic copper carbonate (synthetic malachite):
\[\ce{2Cu(OH)2(s) + CO2(g) -> Cu2(OH)2CO3(s) + H2O}\]
Sample 6 — Reversible dehydration
\[\ce{CuSO4 * 5H2O(s) <=>[\Delta][\text{+ H}_2\text{O}] CuSO4(s) + 5H2O(g)}\]
Sample 7 — Fehling’s reduction
\[\ce{2Cu^{2+}(aq) + C6H12O6(aq) + 4OH^-(aq) ->[\Delta] Cu2O(s) + C6H12O7(aq) + 2H2O(l)}\]
Copper(II) is reduced to copper(I); glucose is oxidised to gluconic acid.
Sample 8 — Oxidation
\[\ce{2Cu(s) + O2(g) ->[\Delta] 2CuO(s)}\]
The Science
Copper is a transition metal, meaning its electrons fill the d orbitals—the same orbitals responsible for the brilliant colours of most transition metal compounds.
The key idea is that Cu²⁺ absorbs visible light to shuffle electrons between d-orbital energy levels. Which wavelength gets absorbed—and therefore what colour you see—depends on exactly which molecules or ions are bonded to the copper atom. These bonded species are called ligands.
Different ligands split the d-orbital energies by different amounts. Chloride ions (Cl⁻) are weak-field ligands: they cause a small energy split, so the copper absorbs red-orange light and transmits blue-green → teal. Water molecules cause a medium split, absorbing orange light and transmitting blue → sky blue. Hydroxide and carbonate are stronger still, shifting the absorption toward shorter wavelengths → paler blue and green.
This explains the table:
| Ligand | Field strength | Absorption | Transmitted colour |
|---|---|---|---|
| Cl⁻ | Weak | Red-orange | Blue-green (teal) |
| H₂O | Medium | Orange | Sky blue |
| OH⁻ | Stronger | Yellow-orange | Pale blue |
| CO₃²⁻ | Stronger still | Yellow | Green |
The white anhydrous sulfate seems to break the pattern—but it fits perfectly: with no water ligands, the d-orbital geometry collapses and the d-d transition shifts into the ultraviolet, out of visible range altogether. No visible absorption → white.
Cu⁺ (brick red Cu₂O) and Cu⁰ (metal) follow different rules. Cu⁺ is a d¹⁰ ion with fully filled d orbitals—no d-d transitions are possible, so Cu₂O’s red colour comes from a different mechanism (charge-transfer between O²⁻ and Cu⁺). Cu⁰ has delocalised metallic electrons that absorb most light except orange-red wavelengths.
Explore Further
A ninth color — deep royal blue: Add ammonia solution drop by drop to a portion of your copper sulfate (Sample 2). First a pale blue precipitate of Cu(OH)₂ forms; keep adding and it dissolves completely into a striking deep royal blue-violet solution — the tetraamine copper(II) complex, [Cu(NH₃)₄]²⁺. This color is distinctly more saturated and violet than any of your eight samples. NH₃ is a much stronger-field ligand than H₂O, shifting the d-orbital energy gap and changing the absorbed wavelength. Add it to your color series.
Make real verdigris — place a copper coin over a small dish of vinegar in a sealed jar without letting the coin touch the liquid. After 24–48 hours, blue-green verdigris crystals of copper acetate hydroxide grow on the surface. This is how medieval artists made the pigment.
Concentration gradient — dissolve different amounts of copper sulfate in the same volume of water and line up the test tubes: pale blue (0.1 g), medium blue (1 g), deep blue (5 g), intense blue (10 g). Concentration directly changes how deeply the colour saturates without changing its hue.
Reconvert the oxides — drop your black CuO piece into copper sulfate solution: the acid slowly dissolves the oxide back. Or add a drop of vinegar: the black surface dissolves faster, giving a green copper acetate solution.
Connect to electroplating — use your deep blue copper sulfate solution (Sample 2) as the electrolyte in the Electroplating experiment. The same Cu²⁺ → Cu⁰ reduction that deposits red copper on the nail is a controlled version of the Fehling’s reduction.
Chemicals used in this experiment:
- Copper Sulfate — blue aqueous solution, white anhydrous, malachite and hydroxide precipitation
- Cupric Chloride — blue-green teal solution
- Sodium Carbonate — precipitating agent for hydroxide and carbonate
- Dextrose — reducing agent in Fehling’s test (Sample 7)
- Acetic Acid — white vinegar for verdigris formation (Samples 1 & Explore Further)
- Ammonia — copper tetraamine complex [Cu(NH₃)₄]²⁺ deep royal blue (Explore Further)