Laboratory Activity 2: Teacher Notes

Continued

Teacher-Student Interaction

Circulate in the laboratory. Check spots on prepared chromatograms to make sure they are properly placed and not too large.

Anticipated Student Results

The brown color is a mixture of red and blue dyes.

Answers to Data Analysis and Concept Development

1. No, blue and red.
2. Yellow and blue.
3. Carry out a chromatographic analysis using the M & M® candies and the Reese's Pieces® along with the unknown. Check the chromatogram for a match.

Answers to Implications and Applications

1. Use paper chromatography to separate and compare the dyes used in the candies. Make sure that students are aware that there are only five or six food colors approved by the Food and Drug Administration (FDA), so some look-alike candies may indeed have chromatograms that look alike as well.
2. Obtain food dye samples of known composition to use as standards. A convenient source is the food colors sold in supermarkets since each color is labeled with the dye or dyes that it contains. Chromatographing these dyes side by side with candy samples will probably help identifying the candy dyes.

Post-Laboratory Discussion

Discuss this laboratory activity in terms of separation (see Separations module). Ask students why they think separations are so important to chemists. Ask students to suggest ways in which separations can be used in the food industry. Ask them to imagine that they are trying to break into a candy market that has been dominated by a certain manufacturer. What chemical information might they need? How can they obtain this information? How can the concept of chromatographic separtions help? Explain that there are numerous commercial analytical laboratories that perform some of these functions for businesses. Also pertinent to this laboratory activity is the fact that Red (Dye) No. 3, used in fruit cocktail cherries, is being phased out by the FDA because extensive tests showed that this dye could cause thyroid cancer in rats. Ask students to discuss a way for testing for Red No. 3 in various food products. What products might they choose? Do these products necessarily have to be red? (See Extensions.)

Extensions

1. Students may wish to examine products at home or in the grocery store to find out which ones contain Red No. 3. Boxes of food colors state that they contain Yellow No. 5, Red No. 40, Red No. 3, and Blue No. 1. Students can use paper chromatography to determine the ingredients of green, red, and yellow colors. There are many other foods that chan be chromatographed, such as numerous varieties of candy, diet Jello®, or Kool-Aid®. Washable colored pens are advertised as using only FDA approved colors, but do they? Design an experiment to verify or disprove these claims.
2. Do Activity 1 in the Forensic Chemistry module on identifying over-the-counter drugs by chromatography.

References

Red No. 3 and other colorful controversies. (1990, May). FDA Consumer; 24(4).

Additives for eye appeal. (1973, July-August). FDA Consumer.

1. Draw representative chromatograms. Show how an unknown can be identified from a chromatogram.
2. Give students two chromatograms and have them identify the substance(s) common to both.

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