Chalk Chromatography Science Project

Separate Pigments Using Chalk Chromatography

These chalk chromatography examples were made using chalk with ink and food coloring.
These chalk chromatography examples were made using chalk with ink and food coloring. Anne Helmenstine

Chromatography is a technique used to separate components of a mixture. There are many different types of chromatography. While some forms of chromatography require expensive lab equipment, others can be performed using common household materials. For example, you can use chalk and alcohol to perform chromatography to separate the pigments in food coloring or inks. It's a safe project and also a very quick project, since you can see bands of color forming within minutes. After you've finished making your chromatogram, you'll have colored chalk. Unless you use a lot of ink or dye, the chalk won't be colored all the way through, but it will still have an interesting appearance.

Key Takeaways: Chalk Chromatography

  • Chalk chromatography is a simple separation method used to distinguish between different pigments in a dye or ink.
  • The pigment molecules separate based on their size, which affects how quickly they can be drawn up porous chalk by a solvent.
  • The pigments tend to travel only up the outer surface of a piece of chalk, making chalk chromatography a type of thin-layer chromatography.

Chalk Chromatography Materials

You only need a few basic, inexpensive materials for the chalk chromatography project:

  • Chalk
  • Alcohol (isopropyl alcohol or rubbing alcohol seems to work best)
  • Ink, dye, or food coloring
  • Small jar or cup
  • Plastic wrap

What You Do

  1. Apply your ink, dye or food coloring to a piece of chalk about 1 cm from the end of the chalk. You can place a dot of color or stripe a band of color all the way around the chalk. If you are mainly interested in getting bands of pretty colors rather than separating individual pigments in the dye, then feel free to dot multiple colors, all in the same place.
  2. Pour enough rubbing alcohol into the bottom of a jar or cup so that the liquid level is about half a centimeter. You want the liquid level to be below the dot or line on your piece of chalk.
  3. Place the chalk in the cup so that the dot or line is about half a centimeter higher than the liquid line.
  4. Seal the jar or put a piece of plastic wrap over the cup to prevent evaporation. You can probably get away with not covering the container.
  5. You should be able to observe the color rising up the chalk within a few minutes. You can remove the chalk whenever you are satisfied with your chromatogram.
  6. Let the chalk dry before using it for writing.

Here's a video of the project, so you can see what to expect.

How It Works

Example of thin-layer chromatography
Example of thin-layer chromatography.  tonaquatic / Getty Images

Chalk chromatography is similar to paper chromatography, where pigments travel through a sheet of paper based on particle size. Larger particles have a harder time navigating "holes" in the paper, so they don't travel as far as smaller particles. The pigment molecules are drawn through the paper via capillary action as the solvent moves. However, since pigments really only travel along the outer surface of a piece of chalk, it's more an example of a type of thin-layer chromatography. The chalk serves as the adsorbent or stationary phase of chromatography. Alcohol is the solvent. The solvent dissolves the non-volatile sample to form the liquid phase of chromatography. Separation is achieved as the analytes (pigments) travel at different rates. To best assess the properties of the pigments, the progress of the solvent should be marked, as well as the progress of each pigment or color. Some dyes and inks only consist of a single pigment, so they will only leave one band of color. Others contain multiple pigments, which are separated using chromatography. For a student demonstration, the most interesting results will be obtained if the sample consists of a mixture of different colors.

Sources

  • Block, Richard J.; Durrum, Emmett L.; Zweig, Gunter (1955). A Manual of Paper Chromatography and Paper Electrophoresis. Elsevier. ISBN 978-1-4832-7680-9.
  • Geiss, F. (1987). Fundamentals of Thin Layer Chromatography Planar Chromatography. Heidelberg. Hüthig. ISBN 3-7785-0854-7.
  • Reich, E.; Schibli A. (2007). High-Performance Thin-Layer Chromatography for the Analysis of Medicinal Plants (Illustrated ed.). New York: Thieme. ISBN 978-3-13-141601-8.
  • Sherma, Joseph; Fried, Bernard (1991). Handbook of Thin-Layer Chromatography. Marcel Dekker. New York NY. ISBN 0-8247-8335-2.
  • Vogel, A.I.; Tatchell, A.R.; Furnis, B.S.; Hannaford, A.J.; Smith, P.W.G. (1989). Vogel's Textbook of Practical Organic Chemistry (5th ed.). ISBN 978-0-582-46236-6.