Do you have a favorite M&Ms or Skittle candy color? Have you ever wondered what dyes are used to make that particular color? Some candy colors are the result of a single food coloring whereas others may use a combination of dyes to create just the right appearance. In this activity you'll get to do some scientific detective work at home to investigate what different dyes are used to color M&M candies. This is a great activity for some leftover candies!
If you've ever seen a drop of water hit a printout from an inkjet printer or a page with marker drawings, you know that when this happens the ink starts to run. The water is absorbed into the fibers of the paper. As the water travels through and down the paper, it picks up ink particles and carries them along with it. This same process can be put to good use as a technique called paper chromatography.
Chromatography includes a group of techniques that separate the various components in a complex solution. In each chromatography setup there is generally a so-called mobile element (a fluid in which the components are dissolved) and a stationary aspect (the material the fluid moves through). For example, in paper chromatography water is the mobile phase and paper is the stationary phase. The solution's components ideally move at different speeds as they travel through the stationary phase so that they can be separated. In paper chromatography different pigments can be separated based on their solubility, or their ability to dissolve in water. The more soluble a pigment is, the more readily it will dissolve in the mobile phase and the farther it will usually travel as it is carried by the water being absorbed into the paper.
• Paper towels. Thicker ones will work better.
• Large bowl or pot
• Measuring spoon
• Measuring cup
• Large jar with a narrow opening
• Medicine dropper
• Clean plate
• M&Ms, one of each color you want to test
• Timer or clock
• Red, green and blue food coloring liquids
• Drying rack (optional)
• Cut the paper towels into strips that are each about one inch wide. Make each strip at least one inch longer than the height of your large jar. Cut three strips for the food coloring samples and an additional strip for each M&M color you want to test.
• Use a pencil to label which candy color or food coloring will be spotted on each paper strip.
• Draw a pencil line about one inch from the bottom end of each paper strip.
• In a large bowl or pot mix together one eighth teaspoon of salt with four cups of water. (To get approximately one eighth teaspoon you can fill a one quarter teaspoon measuring spoon so that it is half full.) Pour a small amount of the salt solution into a large jar with a narrow opening so that the jar is about one inch full.
• Put a single drop of water on the clean plate. Set an M&Ms candy in the drop of water and leave it there for three minutes.
• After three minutes flip the candy over so its other side is in the drop of water. Leave it there for another three minutes. Then remove the candy.
• Take one of the paper towel strips you prepared and place the pencil line you drew on top of the now-colored drop of water. Do you see the dye soak into the strip, coloring the area around the line? You can get some extra dye on the line by dabbing the sides of the wet candy with it.
• Repeat these steps with the other M&M colors you want to test, using a new strip for each M&M candy.
• Next prepare a strip for each food coloring. Put a small drop of food coloring on the plate and lightly dab the line on a new strip against it. Repeat this so that you have three food coloring strips (one each for the red, green and blue food coloring).
• Carefully put the pigmented end of a strip into the jar you filled with saltwater. Put the strip into the jar until the strip just touches the saltwater. Drape the top of the strip over the jar's opening and secure it with a clothespin. Make sure that each strip is not touching the jar's sides but only contacts the jar where it is secured. Place and secure as many of the other strips as will fit in the jar, being sure that they do not touch the jar's sides or each other.
• Let the water rise up the strips until it is about an inch or two from the top. Remove the strips at this point. (This may take around 10 minutes.) What has happened to the colors on the strips? Keep a close eye on the strips and where the water is—if you let the strips run too long, the water can reach the top of the strips and distort your results.
• If you could not fit your other strips in the large jar, put them in the jar now, as you did with the other strips, and run them until they are similarly done.
• Let the strips completely dry out. You can set them on a drying rack, or on the clothespins laid flat to do this.
• Look at your strips. How many dye components does each M&M color have? Can you identify which dyes that are in the food coloring are also likely in the M&Ms candies? Do your results make sense to you?
• Extra: A more accurate way to identify colored components in a solution using paper chromatography is by determining their retention factor (Rf value). The Rf value is the ratio between how far the component travels and the distance the mobile phase (solvent) travels from a common starting point (the line you drew on the strips). If other conditions are kept the same, the Rf value for a certain component should be consistent. You can do this activity again but this time measure these distances and calculate the Rf value for each component. Based on the Rf values, can you identify which dyes are used in the different colored M&M candies by comparing them with the known food coloring dyes?
• Extra: In this activity you used M&M candies, but you could try it with other colored candies as well, such as Skittles. Does the red in Skittles look the same as the red in M&Ms when you test both using paper chromatography? What about their other colors?
• Extra: You could try this activity again, but this time compare using different kinds of mobile phases (for example, saltwater, water, vegetable oil, isopropyl rubbing alcohol, etcetera). Does a dye travel different distances depending on the mobile phase you use? What do you think this tells you about the solubility of that dye in the different mobile phases?