Kids, in this activity you can pretend to be a geologist and test some rocks and other natural materials using a common chemical method.

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 a small sampling of rocks, making sure that some of them are limestone or marble, chalk, a few seashells, vinegar and/or lemon juice, and some clear plastic cups. Put the seashells in one cup, each rock in it's own cup, and a piece of chalk in a cup. Label each of them if necessary, especially to keep track of the rocks. Pour the lemon juice or vinegar over each material and note what happens. You should see bubbles form on some of the materials, although not from all of the rocks.

What's happening here? Vinegar and lemon juice are both weak, dilute acids (acetic acid and citric acid, respectively). Calcium carbonate is a chemical compound found in many natural materials, such as limestone and seashells. When calcium carbonate is exposed to acid, it chemically changes into new materials. One of these is carbon dioxide gas, which form the bubbles that you observe in the cup. When limestone is exposed to great heat and pressure under the earth's surface for many years, it turns into marble. It is still calcium carbonate and that is why marble will form bubbles with acid also. You get similar results when using chalk because it, too, is made of limestone. 

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
May 1997

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Reference: "Earth Science for Every Kid" by Janice VanCleave, NY: Wiley Publ., 1991, p. 32 and "Simple Science Experiments with Everyday Materials" by Muriel Mandell, NY: Sterling, 1990, p. 54.