What is your favorite color of Skittles® candy? Do you want to know what dyes were used to make that color? Check out this science project to find out how you can do some scientific detective work to find out for yourself.
Using a simple scientific technique called chromatography, you can separate and identify the various compounds in a complex mixture or solution.
In this activity, water is used as the mobile phase (or solvent), a fluid the solution is dissolved in. The stationary phase, the material the fluid moves through, is filter paper (a coffee filter cut into strips will work well). The water is absorbed into the fibers of the paper by capillary action. As the water travels through the paper, it picks up ink particles (the solute) and carries them along. This same process that spoils a perfect print-out can also be put to good use.
The components in the dye mixture move at different speeds as they travel through the stationary phase due to the different properties of the solution’s components, such as their molecular size, electrical charge, or other chemical properties. In paper chromatography, different pigments can be separated out from a solution based on the solubility of the pigments. A pigment that is more soluble (or more hydrophilic) than another pigment will generally travel farther because it will be easier for it to dissolve in the mobile phase (water) and be carried along the stationary phase (filter paper). A pigment that is less soluble (or more hydrophobic), or interacts more with the filter paper than the water, will generally travel a shorter distance.
How are the dyes used in hard-shelled candies similar? How are they different?
- Candies with a colored coating ~ I recommend testing four different colors and five identical candies of each color
- Filter paper ~ I used a white coffee filter cut into rectangles of approx. 1″ x 3″
- Petri dish (or small plate)
- Pipet or eyedropper
- Cup of water
- Skewer or chopstick
- Prepare the test strips by making a faint line on each strip with a pencil about 1″ from the bottom of the strip.
- Tape the other end to the skewer so that the strip hangs freely.
- Place the candies in a shallow dish and add a few drops of water atop each candy. In a few moments a small colored puddle should form.
- With the pipet, place a drop of the colored candy solution onto the line you drew on the test strip.
- Carefully place the strip into the cup containing the water. The pencil mark should NOT be in the solution but rather be about 1/2 inch above the water. Make sure, though, that the end of the paper is in the water. Watch the water as it moves up the strip of paper (due to capillary action), and see what happens as it comes in contact with the candy solution. Leave the strip in the solution until the dye no longer travels up the strip with the water.
- When this is complete, remove the strip of paper and place it somewhere on your desk so that it can dry thoroughly. Continue to test the remaining strips.
Why do different compounds travel different distances on the piece of paper? In paper chromatography, you can see the components separate out on the filter paper and identify the components based on how far they travel. To do this, we calculate the retention factor (Rf value) of each component. The Rf value is the ratio between how far a component travels (the dye) and the distance the solvent (the water) travels from a common starting point (the origin). To calculate the Rf value, divide the distance traveled by the sample component by the distance traveled by the solvent. For example, 2.5cm ÷ 5.0cm = 0.5
You can use Rf values to identify different components as long as the solvent, temperature, pH, and type of paper remain the same. This ratio will be different for each component due to its unique properties, primarily based on its adhesive and cohesive factors.
Take it Further
- Try this project with a variety of candies— for example, does the red in Skittles® look the same as the red in M&Ms® when processed with chromatography? Is the average Rf value nearly the same? Look in the ingredients on each package – were the same dyes used?
- Try this experiment again but this time use different kinds of solvents (e.g., salt water, vegetable oil, isopropyl rubbing alcohol, etc.). Does a dye travel different distances depending on the solvent you use? What do you think this tells you about the solubility of that dye in the different solvents?
- Do the dyes you tested travel differently on different kinds of filter paper (lightweight paper towels, heavyweight paper towels, white coffee filter papers, etc.)?
You can probably now imagine how chromatography can be used to separate specific components from a complex mixture and identify chemicals, for example crime scene samples like blood, drugs, or explosive residue. Highly accurate chromatographic methods are used for process monitoring, for example to ensure that a pharmaceutical manufacturing process is producing the desired drug compound in pure form.