Flame Tests, Atomic Spectra and Applications Activity

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(See teacher background information in Flame Tests, Atomic Spectra and Applications Activity)

Introduction:

Have you ever seen a fireworks display? Where do all of the colors come from?

Below are some links to the chemistry of fireworks:

http://chemistry.about.com/od/fireworkspyrotechnics/Fireworks_Pyrotechnics.htm

http://alchemy.chem.uwm.edu/amalgamator/NCW/ncw2001/fireworks.html

In this activity, you will investigate the colors of flame produced by solutions of metal salts.

A flame test is a procedure used to test qualitatively for the presence of certain metals in chemical compounds.  When the compound to be studied is excited by heating it in a flame, the metal ions will begin to emit light.   Based on the emission spectrum of the element, the compound will turn the flame a characteristic color.  This technique of using certain chemical compounds to color flames is widely used in pyrotechnics to produce the range of colors seen in a firework display.

Certain metal ions will turn the flame very distinctive colors; these colors in turn can help identify the presence of a particular metal in a compound.  However, some colors are produced by several different metals, making it hard to determine the exact ion or concentration of the ion in the compound.  Some colors are very weak and are easily overpowered by stronger colors.  For instance, the presence of a potassium ion in a compound will color a flame violet. But on the other hand, even trace amounts of sodium ions in a compound produce a very strong yellow flame, often times making the potassium ion very difficult to detect.  To counteract the effects of any sodium impurities, one can view the flame through a piece of cobalt blue glass.  The cobalt glass absorbs the yellow light given off by sodium while letting most other wavelengths of light pass through. 

In this activity, wooden splints dipped in solutions of metal salts are heated using a Bunsen burner, producing different colored flames. By comparing the color given off by an unknown with the known metal salts, the identity of the metal salt can be determined.

Flame Tests Activity (As an option, this could be a demo rather than a student activity)

Materials:

▪ Bunsen burner
▪ Wooden splints (9 per group)
▪ Solutions (1.0 mol/L) of the following metal salts
➢ lithium chloride
➢ barium chloride
➢ strontium chloride
➢ calcium chloride
➢ copper(II)chloride
➢ sodium chloride
➢ potassium chloride
➢ sodium chloride/potassium chloride mixture
➢ an unknown metal chloride solution
▪ cobalt blue glass (if available)

Procedure:

1. Obtain a cobalt blue glass and 9 wooden splints that have been soaking in the metal salt solutions. (Why is soaking the splints important?). Be sure to label each wooden splint with the names of the salts so they are not mixed up.

2. Light the Bunsen burner and open the air vent to obtain a non-luminous flame with two blue cones. Be sure to avoid a yellow flame. (Why?)

3. Carefully place the end of the wooden splint that was soaked in the metal salt solution at the top of the inner blue cone. Record the color and intensity (bright/faint) of the flame in the data table. The color given off by the salt is the initial color observed, not the yellow-orange color produced by the burning wood. (To avoid burning the wood, wave the splint through the flame rather than holding it right in the flame).

4. Repeat with the other 8 salts. Be sure to record the colors as precisely as possible.

5. For the sodium potassium mixture, observe the colors as before and then again by looking through the cobalt glass. The cobalt glass cuts out any yellow-orange color.

6. If more observations are needed, dip the clean end of the wooden splint in the solutions for a few minutes and repeat. Otherwise, discard the wooden splints at the end...
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