Make a Cave

Objective:

Students will demonstrate how carbonic acid can slowly dissolve limestone and form caves.

Materials:

• Powdered lemonade or Kool-Aid
• A piece of limestone, preferably with an imbedded fossil
• Water
• Soda pop, preferably clear

Background Information:
Discuss with the students that after the limestone was made some of it was dissolved away to make the rooms and passageways of the cave. Make a pitcher of lemonade for the class to demonstrate how materials can dissolve in water.

Like the powdered lemonade or Kool-Aid, the limestone of the cave also is dissolved away when it reacts to carbonic acid. Carbonic acid is created when water travels through the soil as it seeps into the earth. The water picks up carbon dioxide from decaying plants or soil. When you mix carbon dioxide with water, it becomes carbonic acid. That acid can dissolve limestone. Carbonic acid is what is in soda pop that makes it fizz. Explain that over time the limestone of the cave was eroded away forming the passageways and rooms we have today.

Procedure:

1. Show the class the example of limestone. Pass it around the room and allow each student to closely examine it. Review how limestone forms.

2. Ask the students how caves are formed in limestone. (Water dissolves the rock.) Pour water over the limestone. What happens? Do we have a cave? No, we have a wet rock! By itself, water cannot dissolve limestone. What else do we need?

3. Explain how carbon dioxide in the soil can change the water into carbonic acid. What will happen when carbonic acid seeps through the soil and reaches the underlying bedrock? If this bedrock is limestone (or another carbonate rock), it will slowly dissolve it.

4. Ask the class where they might find carbonic acid in their every-day lives. Soda pop is acidic because it contains carbonated water, or carbonic acid.

5. Pour the soda pop over the limestone. Make sure that every student has a chance to see the limestone fizz and slowly dissolve. The soda will fizz on any material, but the acid will dissolve the rock. You could use hydrochloric acid in a demonstration, but if you do make sure the students follow all the safety procedures.

Give the students different types of rocks, one of them limestone, and a dropper of white vinegar. Have students test the rocks by dropping vinegar on them. The limestone rock will bubble as it is dissolved away, releasing carbon dioxide.

Discuss how carbonic acid can travel through the limestone (via cracks) and create cave passages. To continue the premis of this activity move on to Make a Cave Part 2.