Safety-First Flame Tests

Introduction

Coloring flames with metallic salts is a great way to demonstrate the emission spectra of metal salts. Put safety first with this unique flame test demonstration by eliminating the need for methanol, which is a common cause of flashback fires.

Concepts

• Flame tests
• Absorption/emission

Materials

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remove all flammable materials from the demonstration area. The demonstration must be done on a heat-resistant surface. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulation that may apply, before proceeding. Each of the metallic salts may be disposed of in the trash according to Flinn Suggested Disposal Method #26a.

Procedure

1. Set up six watch glasses.
2. Sprinkle some of each metallic salt on five watch glasses in rainbow order: strontium chloride (red), strontium chloride and sodium chloride (orange), sodium chloride (yellow), boric acid (green), potassium chloride (violet). Leave one watch glass with no salts for blue (natural flame color).
3. Add a pump of hand sanitizer to each salt.
4. Dim the lights.
5. Using a lighter, ignite the salts.
6. For a more dramatic demonstration, sprinkle a small amount of salt onto the flame.
7. (Optional) To restart the demonstration after the flames have gone out, apply more gel and light it.

Teacher Tips

• When mixing strontium chloride and sodium chloride to create an orange flame, use very little sodium chloride or the sodium salt will over-power the strontium salt and the color will be yellow.
• More metallic salt on the watch glass will result in a more vibrant, noticeable color.
• The flames will start out blue in color but will change to the color characteristic of the metallic salt as they dissolve in the hand sanitizer.
• The flame color is due to the metal in each metallic salt, not the chloride ion. Other metallic salts, such as nitrates, can be used to obtain the same results.
• Other salts that can be used for different colors:
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• Never add more fuel to the flames as the hand sanitizer burns.

Discussion

Your students will notice that when each salt is heated, the chemicals emit different colors. The colors of the chemicals come from the particles of light, or photons, being emitted by each chemical. While the chemicals are heated, they are absorbing energy, which allows the electrons to be promoted from a ground energy state to an excited energy state. From these excited states, the electrons want to make a transition back down to the ground state because the ground state has a lower energy level and is more stable. When the electron makes this transition from a high energy state to a low energy state, photons are emitted. In diagrams, the photon is typically represented by a squiggly line (see Figure 1). The amount of energy emitted with each photon is equal to the difference in energy between the excited state and the ground state. The amount of energy emitted by the photon is observed through the color in the flame.

Every element emits a certain wavelength of light because each element has a different electronic configuration. Since the electronic configurations vary, the electronic transitions will vary. Therefore, the amount of energy emitted with photons and the color observed is unique to each chemical. As a result, the color seen in a flame test can be used to identify a chemical.