Copper foil is folded into the shape of an envelope before being heated in a Bunsen burner. On cooling, the foil can be opened and it can be seen that where there was no contact with oxygen the copper remained unreacted.

In this experiment, students fold a piece of copper foil into the shape of an envelope, before heating it using a Bunsen burner. When the foil has cooled, students can open the envelope and discover that where there was no contact with oxygen the copper remained unreacted.

Warn students that there can be sharp corners on the copper. The copper stays hot for some time and there is a risk of burns.

The experiment will take about 20–30 minutes.

To enable students to light their Bunsen burners they will need access to matches or lighters. Alternatively, light one or two Bunsen burners around the room and students can light their own using a splint.



  • Eye protection
  • Bunsen burner
  • Heat resistant mat
  • Tongs


  • Copper foil, 4 cm x 4 cm

Health, safety and technical notes


  1. Fold the copper foil into an envelope as shown in the diagram below.

A diagram illustrating the sequence of folds to make an envelope of copper foil

Source: Royal Society of Chemistry

Fold the copper foil in the steps shown (and remember that there can be sharp corners!)

  1. Wear eye protection and light the Bunsen burner.
  2. Hold the envelope in the tongs and heat strongly in the Bunsen flame for five minutes. You will need to have the air hole fully open.
  3. Place the envelope on the heat resistant mat and allow to cool. This will take a few minutes.
  4. Open the envelope and compare the inside to the outside surface.

Teaching notes

The outside of the envelope will react with oxygen in the air and will turn black. This can confuse students who think that it is soot which has coated the outside of the copper. To help convince them otherwise, ensure that they use a roaring Bunsen flame and show them that a beaker (containing water) which is heated with the same flame does not get coated in black powder. Inside the envelope, the copper remains as it was at the start.

Copper, like many transition metals, only reacts slowly with oxygen in the air. When heated it forms a layer of black copper oxide on its surface:

Copper + Oxygen → Copper oxide

2Cu(s) + O2(g) → 2CuO(s)

This experiment could be used as an illustration of the likely reactions of other transition metals with oxygen, as they all have similar properties. It could also provide a contrast to the reactions of Group 1 and 2 metals with oxygen.