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Chemiluminescence (Blue Light!)

In this demo, students witness the creation of blue light from a chemical reaction, an example of luminescence. This is similar to the reactions that fireflies uses to emit light, and to those used in "glow-sticks" and some roadside emergency lights.

In the demonstration vial is a mix of luminol, perborate and copper sulphate. When water is added the copper sulphate dissolves and reacts with the luminol. This forms a molecule that has an excited electronic state. The molecule then sheds this extra energy in the form of light. This is a "cool" light (that is, there is no heat created along with it). It will continue to be emitted until one of the reactants is used up.

Luminol is used by forensic scientists to detect blood. Crime lab investigators can spray a luminol solution and in the dark the bloodstains will glow with blue light. Haemoglobin (found in our red blood cells) contains iron which reacts with the luminol like the copper in our light.

Objectives

  • Explain that a chemical reaction can produce luminescence via the release of energy.

Materials

  • Per Demo:
    water
    sodium Carbonate (NaCO3)
    sodium Bicarbonate (NaHCO3)
    ammonium Carbonate (NH4CO3)
    copper II Sulphate Pentahydrate (CuSO4·5H2O)
    Luminol (C8H7N3O2)
    3% Hydrogen Peroxide (H2O2)

    Teacher Tip: Chemicals for this activity are available from science suppliers such as Boreal.

Key Questions

  • Why is light released in this chemical reaction?
  • Why does the light fade after a short time?

What To Do

Note: Luminol demonstration kits may be purchased from science suppliers such as Boreal or Teacher Source. If you’re using a kit, follow the associated instructions.

Preparation
Prepare Solution A and Solution B. The amounts given above will allow you to do the demonstration 5 times.

Solution A (Luminol Solution) [Blue]
250 mL Water
2.0 g Sodium Carbonate (Na2CO3)
12.0 g Sodium Bicarbonate (NaHCO3)
0.25 g Ammonium Carbonate ((NH4)2CO3)
0.2 g Copper II Sulphate Pentahydrate (CuSO4·5H2O)
0.1 g Luminol (C8H7N3O2)

Solution B (Oxidizing Solution) [Colourless]
25 mL 3% Hydrogen Peroxide (H2O2)
250 mL Water

Instructions

  1. Make the room as dark as possible.
  2. Pour 50 mL of Solution A and 50 mL Solution B together into a clear, colourless container.
  3. Admire the beautiful blue glow — it will last about 30 seconds.

Extensions

  • This reaction is catalyzed by a metal ion. Iron is similar and can be found in human blood. Can you think of a practical use for the luminol reaction in solving crimes?
  • Experiment with glow sticks — they use the same sort of reaction. Does temperature affect how long they glow?