Rocks and Minerals > Rock Cycle Activities

Chemical Weathering

by Deborah Hanuscin

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Concept: Chemical weathering

In chemical weathering of rocks, chemical reactions occur that dissolve the rock and create new substances. For example, when rainwater combines with carbon dioxide in the air or carbon dioxide from decaying organic matter in soil, a weak acid called carbonic acid is produced. When this slightly acidic water flows through cracks in limestone it chemically reacts with the rock, forming dissolved calcium and bicarbonate, which is carried away in the water. This action has the effect of dissolving the rock, which can eventually lead to the formation of a cave.

Objective:

Students will be able to describe the effects of chemical weathering on rocks.

Materials:

• clear plastic cups
• vinegar
• litmus paper or other acid/base indicator
• water
• small samples of limestone

Procedure:

1. Fill a cup halfway with vinegar. Place a strip of blue litmus paper in the vinegar, or add another indicator such as red cabbage juice. Observe the color change. The red or pink color indicates that vinegar is an acid. What other acids can you name?
2. Ask students how they think an acid may affect limestone. Why do they think this? Record any predictions.
3. Place a small piece of limestone into the cup of vinegar and allow to soak for several days, while observing. What do you notice happening?
4. Remove the limestone from the vinegar and observe carefully. How has the limestone changed? How might this process occur in nature? Explain.

Extension:

Use the indicator to identify other acids. Then repeat the procedure using the other acids with limestone and with other types of rock.

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