Experiment 2: Copper Flame Test

A PDF Experiment Sheet is available for this experiment.

date | aim | method | materials & equipment | results | conclusion | extra

Date

27 October 2009 by Quest 2, to link in with Chapter 2 of our book.

Aim

To use a flame test to check for the presence of copper in various substances.

Method

Use a meths burner to produce a (nearly) colourless flame (which means let it burn for a little while before starting to use it).

Use pliers or a spatula to hold the following over the flame:

1. A piece of fine copper wire.
2. A piece of thick copper wire.
3. A piece of copper sheet.
4. A copper coin.
5. A copper plated steel (ie, magnetic) coin.
6. Copper sulphate (CuSO₄) crystals. Place crystal(s) in test tube, add water, gently heat over meths burner. Pour into shallow container and soak a piece of newsprint in the solution.
7. Copper sulphate (CuSO₄) solution.
8. Copper carbonate (CuCO₃) powder. Use spatula.
9. Copper carbonate (CuCO₃) solution. Place powder in test tube, add water, gently heat over meths burner. Pour into shallow container and soak a piece of newsprint in the solution.
10. Junk mail with blue and/or green ink.
11. Junk mail with red and/or orange ink.
12. Unprinted junk mail (ie, from a margin).

Materials & Equipment

• Fine copper wire.
• Thick copper wire.
• Copper sheet.
• Copper coin.
• Copper plated coin.
• Copper sulphate crystals - CuSO₄.
• Copper carbonate powder - CuCO₃.
• Junk mail with a variety of inks.
• Water.

• Meths burner, lighter.
• Metal spatula.
• Pliers.
• Magnet.
• Test tubes.
• Rubber bungs.

Results/Observations

Specimen #	Specimen	Experimenter	Green Flame?	Observations
1	Fine copper wire	John		The fine copper wire was initially black. It melted and sometimes burned with a green flame.
2	Thick copper wire	John		There was some continous colour change on the surface of the wire - sometimes shiney, sometimes dark - but no green flame.
3	Copper sheet	James		Initial burst of green then the surface of the copper sheet continually changed colour.
4	Copper coin	James		Took a while to get going but then produced a green flame.
5	Copper plated coin	Hamish		Coin was tested as being magnetic. It was initially shiney and turned dark and had colour changing surface but no green flame.
6	Copper sulphate (CuSO4) crystals	Hamish		Intense green flame. Blue crystal turned white around the edges.
6	Copper sulphate (CuSO4) crystals	James		Strong green flame. Blue crystal turned white and fragile. When it was accidentally crushed there was a large burst of green flame.
7	Copper sulphate (CuSO4) solution	Kent		The CuSO4 crystal easily dissolved, making a pale blue solution. The wet paper held in the flame made an intermittent strong green flame. The remaining paper/ash was brown.
8	Copper carbonate (CuCO3) powder	Mr Mander		Powder turned black. No green flame.
9	Copper carbonate (CuCO3) solution	James		CuCO3 didn't dissolve. Paper just burned, no green flame.
10	Junk mail with blue and/or green ink	John		Paper burned well with green flame, leaving white paper/ash behind.
10	Junk mail with blue and/or green ink	Kent		Green flame, very noticeable.
11	Junk mail with red and/or orange ink	Hamish		No green flame.
12	Unprinted junk mail	Kent		Paper burned slowly but no green flame.

The pliers got noticeably hot during testing.

Conclusion

• A flame test for detecting copper works extremely well for some substances and OK for others, but not at all for some copper samples.
• The flame test worked best for testing CuSO₄.
• Copper sulphate dissolves easily in water, especially after heating the water.
• Copper carbonate does not dissolve easily in water.
• The copper sheet ended up more flexible, probably because it's thinner (no thickness measurements were taken).
• Pliers should be kept out of the flame as much as possible to help keep them cool.

Extra

When the copper sheet and coins were heated a dark coating was formed on the surface. When the CuCO₃ was heated it converted into a black powder. The black substance is copper(II) oxide, also called cupric oxide. Its formula is CuO and it's made from heating copper in air, or from heating CuCO₃, a reaction which gives off carbon dioxide.

CuCO₃ (s) → CuO (s) + CO₂ (g)

Copper(II) oxide is an irritant. Handling copper(II) oxide should be done in well ventilated area, and care should be taken to avoid contact with the skin or eyes. If handled, wash hands thoroughly.

CuCO₃ is one of two compounds that makes up the green patina on copper roofs and statues, the other being copper(II) hydroxide, Cu(OH)₂. They are present in patina in a 1:1 ratio.

After the experiment James raised the possibility of using CuSO₄ with iron filings to make green sparklers. A nice idea. How could small crystals of CuSO₄ be grown on the iron filings?

For other flame tests see the Flame Tests page.