Chalky Chromatography

    Kids, do you think you can unmix your favorite marker color? Some of the bright colors in your watercolor marker set are not made from a single pigment. Rather, just the right amount of different pigments is often mixed together. You can unmix, or separate, all of these pigments using a process called chromatography. This is one way that chemists use to separate mixtures into their individual parts. We have talked about chromatography previously in this column (see columns on the web from 10/92 and 2/95), but we have never asked chalk to do the work for us before.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    You'll need watercolor markers (several different colors and brands, be sure to try black), water, long fat pieces of white sidewalk chalk, and clear cups. Fill a cup with 1/4 inch of water. Using a marker, draw a line of color around a piece of chalk, about 1/2 inch from the bottom. Stand the chalk in the water with the color line just above the water line. Don't let the water line touch the color line otherwise the color will just wash into to the water. Instead, what you want is for the water to percolate up the chalk. It will then dissolve the ink pigments and carry them up the stalk of chalk. Different colors of pigment will travel up the chalk at different rates, depending on the size, shape, weight, and composition of the pigment molecules.

    Eventually the chalk will be decorated with a colorful, smeared-out band. Let it dry out and then use this designer chalk to decorate your sidewalks and driveways. Try it again with new pieces of chalk and other colors. 


    Kathleen Carrado Gregar, PhD, Argonne National Labs
    June 2002


    Reference: Joan Silberlicht Epstein, Scientific American Explorations magazine, Spring 2001, page 45.