|Material & Resources:||Water|
Coffee filters cut into strips about 1 in. wide and 7-9 in. long
Colored food products, such as Skittles, Kool-Aid powder, M & M’s--anything with color that can easily be removed
Large disposable cups
Toothpicks, coffee stirrers, or capillary pipettes, if available
|In this activity, students will be using chromatography to develop patterns of "known" compounds (food coloring). These then will be compared to colored food products to determine what colorings are used in coloring everyday foods.|
Demonstrate the procedure for completing a proper chromatography:
- Draw in pencil a horizontal line about 1 in. up from the bottom of a filter strip (holding the paper so it’s longer vertically)
- Label in pencil, and under the line, the compound that will be used on this strip
- Use a toothpick, coffee stirrer (holding a finger over the top of it like a straw), or a capillary tube to make a very small dot of liquid on the line
- After the liquid dries, roll the top of the paper over a pencil and staple together.
- Add about 1-2 cm of water to a cup and carefully lower the strip in, supported on the rim by the pencil
- Watch the level of the water as it moves up the paper--when it’s almost to the top of the paper, remove it from the water and let it dry
- Use one strip of paper for each compound
After the demonstration, instruct students to do the procedure outlined above for each color of food coloring that you have. Once they have chromatographic data for each color, they should create a table that illustrates the different color components (i.e., some food coloring colors will only be composed of one color, while others may be composed of more).
So far, the purpose of this activity has been to set up the the preliminary knowledge about chromatography and the "known" compounds. Now inform students that in the past, a food coloring was found to be carcinogenic in large doses. Because of this, it was banned from the market. Tell the students that they will be pretending that this has happened again and choose a color to be considered dangerous. You would like to figure out which common foods contain this compound. Present the foods and candies that you have brought to be tested. Allow students to develop their own methods for analyzing these compounds (e.g., Kool-Aid powder may need to be mixed into a thin paste, whereas candies may need to have color "blotted" off of them).
If you have the time and desire, you may actually require students to measure how far a color band traveled in reference to the water level (the Rf, a fraction expressed as how far the top of the color band traveled divided by how far the water traveled). Or you may simply require them to roughly compare bands. In any case, the information acquired by their comparisons should be expressed on their tables, where they should also note possible food coloring ingredients of these foods. Which foods may contain the "carcinogenic" substance?