Flame Test
Teaching team presenter information

Concepts

• Atomic structure - electrons, orbitals, ions
• Periodicity - metals

Overview

Because the valence electrons in metals are weakly held, the energy in a flame is sufficient to excite them to higher energy levels. When they return to the ground state, energy is released, some of it in the form of visible light. Because of differences in electronic structure (which orbitals the valence electrons occupy), the color of the light emitted is distinctive for each metal. Students will determine which colors are emitted by various metals, then identify three unknown metal salt solutions by the colors they emit. (Some students may notice that metal ions have no valence electrons and wonder how the metal can emit colored light. This is because there is an equilibrium in the solution between M and M+, so some of the metal is in its atomic, not ionic, form and has its valence electrons.)

Safety

Students must wear goggles during this activity. Loose hair must be tied back. Warn the students that hydrochloric acid is very caustic. Acid spills should be reported to the presenter immediately so he or she can neutralize them with NaHCO3. If students come in contact with the acid they should flush the affected area with water immediately. Instruct the students to wash their hands if they touch any of the other solutions.

Procedure

Turn on the portable Bunsen burner by turning the red knob on the top of the burner counterclockwise 2/3 of the way around (listen for a hissing sound), then pressing the red knob on the side of the burner. Demonstrate cleaning the nichrome loop by dipping it into the acid and holding it in the outer edge of the flame. Wait briefly for the loop to cool. Dip the loop into one of the metal ion solutions, then hold it in the flame. Clean the loop using the acid and flame as before.

Students can take turns seeing the color produced by each solution (cleaning the loop in between different solutions) and noting it on their handouts. Once they have tested all of the known solutions, they can determine the identities of the unknowns by the flame colors they produce.

Materials

• goggles
• portable Bunsen burners, nichrome wire loops
• NaHCO3 (baking soda) to neutralize acid spills
• vials containing concentrated HCl or 0.5 M solutions of salts of the following metals: K+ (violet), Li+ (crimson), Ca2+ (deep orange), Ba2+ (lime green; 0.2 M), Cu2+ (jade green)
• vials containing "unknown" metal salt solutions: A) Ba2+, B) K+