Summary

In this lab, students will learn that mixtures are created by the combination of two or more substances. Many colors are actually mixtures of several different pigments. Mixtures can be separated, and in this lab students will perform chromatography to separate mixtures found in candy and in leaves.

Grade Level

Middle school

Objectives

By the end of this lesson, students should be able to

• explain what a mixture is.
• separate a mixture into its individual components using chromatography.

Chemistry Topics

• Separating Mixtures

Time

Teacher Preparation: 30 minutes each day

Lesson: 2 - 50 minute classes

Materials

• Water
• Plate (paper or plastic)
• M&Ms and/or Skittles
• Coffee filters
• Tall glass/beaker
• Salt
• Pencil
• Toothpicks
• Empty 2 liter bottle
• Measuring cups/spoons
• Leaves (variety)
• Small jars with lids
• Rubbing alcohol
• Hot water
• Shallow pan
• Kitchen utensils

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Food in the lab should be considered a chemical not for consumption.
• Keep rubbing alcohol away from open flames and sources of heat as it is highly flammable.
• Parts of this experiment should be done under a fume hood.

Teacher Notes

• Chromatography is defined as the separation of mixtures into their constituents by preferential adsorption by a solid, as a column of silica, a strip of filter paper, or a gel (dictionary.com). Many colorful things are actually a mixture of different colors, including the dyes used to color candies and the pigments that produce color in leaves. Many students do not understand that the colors are already in the leaves, but the green chlorophyll that is used to produce food for the plant overpowers these other colors. Once daylight hours decrease, the plant stops making food, so the chlorophyll disappears, allowing the other colors to show.
• I suggest completing the Skittles chromatography lab on the first day, followed by the Leaf Chromatography.
• Some helpful resources:
  • A video that explains why leaves change color.
  • Article that explains why leaves change color.
  • You can also have a discussion about art and the mixing of pigments to form different colors.
    
• Day 1: Candy

1. Put students into small groups (2-3 students per group).
2. Give each group one Skittle of each color (in the original package this would mean 5 skittles total per group), one coffee filter (cut into a square), a glass or beaker containing water, a ruler, a dropper, toothpicks, a salt solution (make by dissolving ~1 gram of salt in 250ml of water).
3. Have students use the ruler to draw a line ½ inch from one edge of the square filter paper.
4. Put five pencil dots along the line, evenly spaced out. Label these for the colors of the Skittles that will go there. Make sure a pen is not used for the dots or the labels.
5. Use the dropper to put five individual drops of water spaced out on a plate.
6. Put one candy on each water drop and wait approximately one minute until some of the color can be seen in the water drop. The more color that is removed from the candy shell, the better. When finished, remove the candy from the water.
7. Dip a different toothpick into each color and dab the color onto the pencil dot for that color on the filter paper. Keep the dots small. Let dry. Repeat this two more times, adding more pigment to the dots so they will become concentrated.
8. When dry, place the edge of the filter paper that has the dots on it into the salt water (do not submerge the dots). Have students hold this end of the filter paper steadily in the solution for several minutes. Capillary action will draw the salt solution upward, separating the dots into different colors if the color is a mixture of pigments.
9. When the salt water has moved to almost the top, remove it and put it on a flat, clean surface to dry and then analyze.
10. Compare the dyes of different colors with other groups.

• Day 2: Leaves

1. Give each group a variety of leaves (more than one of each type), a small jar with a lid (for each type of leaf), rubbing alcohol, a shallow pan, and some strips of coffee filters.
2. Have students break the leaves into very small pieces and put them in the jars, placing only one type of leaf in each jar.
3. Have students add a small amount (drop by drop is suggested) of rubbing alcohol, just enough to cover the leaves.
4. When the alcohol has removed the color from the leaves, it will be visible in solution and this process is complete.
5. Place one strip of filter paper in each jar so that one end is in the alcohol and the other is hanging out of the end of the jar. Leave the cap off the jar.
6. Make sure this is conducted in a well ventilated area and students are not inhaling the vapors of rubbing alcohol. Using a fume hood is suggested.
7. As the alcohol evaporates, it will pull the pigment up the paper. After 30-90 minutes, remove the strips and allow to them dry. Compare strips. The comparison may have to occur during a different class time, as the process is lengthy.