Chalk chromatography is a fun and easy science project that uses chalk, alcohol, and water to separate pigments in food coloring or ink.
Chalk Chromatography Materials
You need a few basic, inexpensive materials for chalk chromatography:
- 50-70% alcohol (isopropyl or rubbing alcohol works best)
- Food coloring, ink, or dye
- Small cup
- Plastic wrap (optional)
Use regular, cheap chalk (not dustless chalk). You can dilute 95% to 99% alcohol with water, if you can’t find 50% to 70% alcohol. Alternatively, you can use vodka.
What You Do
- Apply the ink, dye, or food coloring about 1 cm (1/2 inch) from the bottom of a piece of chalk. You can place a single dot or draw a band all the way around the chalk. Tip: Some inks and food colorings only contain one pigment, so you may wish to combine several to get a good color display.
- Pour enough alcohol into the cup so that the liquid level will be below the dot or line on the chalk.
- Place the chalk in the cup so the liquid is about 1/2 cm below the marking on the chalk.
- Optional: Cover the cup with plastic wrap to slow evaporation of the alcohol.
- Observe the color rising up the chalk. Remove the chalk when you’re satisfied with the separation of the colors.
- Let the chalk dry before using it for writing.
Here’s a video of chalk chromatography, so you know what to expect.
How Chalk Chromatography Works
Chromatography separates components of a mixture based on the different rates the components travel as a fluid (the mobile phase) through a fixed material (the stationary phase). In this project, the fluid phase consists of the alcohol, water, and dye mixture. The stationary phase is chalk, which is a porous form of calcium carbonate.
The reason for using a mixture of alcohol and water is that some dyes and inks are water-soluble, but others are not. For young children, you can perform this project using only water and food coloring.
Capillary action draws the fluid up the chalk. Different materials adsorb on the stationary phase (stick to the the chalk) longer or shorter amounts of time. Larger particles move more slowly through the pores in the chalk more slowly than smaller particles, which can follow a more direct path through the solid. The net effect is that components of a mixture separate out over time.
In this project, the pigments separate running up the stick of chalk, but the dye only permeates the outer surface of the material. If you break the chalk, its exterior is colored, but the entire piece of chalk is not. Chalk chromatography is similar to paper chromatography, but because it involves only the outer layer of the material, it’s really a type of thin-layer chromatography.
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- Geiss, F. (1987). Fundamentals of Thin Layer Chromatography Planar Chromatography. Heidelberg. Hüthig. ISBN 3-7785-0854-7.
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- 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.