Rock Pop

The Geological Society of America

Activity Source: 

Geological Society of America. Adapted with permission.

How can a cave form from solid rock? Most caves are found in limestone, a rock made of materials of calcium carbonate.  This rock is unusual because the solid minerals it is made of easily dissolve in weak acids.  The most common weak acid in the environment is actually water!

This acid forms when carbon dioxide in the atmosphere dissolves in the rain water to form carbonic acid.  The acid is so weak, we do not notice it when we drink water, but over very long periods of time, it is capable of eating away mountains of rock.  When the rock is eaten away by acid, it releases carbon dioxide into the air.

Materials

  • 1.0-1.5 liter clear plastic drink bottle
  • Limestone rock (or uncoated chalk) fragments small enough to fit into bottle
  • White vinegar
  • A balloon
  • Paper and pen or pencil

Procedure

  1. You will be using the plastic drink bottle, some vinegar (also a weak acid but much stronger than carbonic acid), and the limestone fragments to test the suggestion that a weak acid can eat away solid rock. Make a prediction about the outcome and write it down.
  1. Place in the bottle between 3 and 5 wet limestone fragments that are just small enough to fit. Place vinegar in the bottle until it is about 25 percent full. Stretch the balloon over the mouth of the bottle. Leave the bottle undisturbed for about 10 minutes.
  1. Observe the contents of the bottle. Is there any evidence of a gas being released? Observe the balloon. Is there any evidence of a gas being released? Record your results in a table like the one shown here.
  BOTTLE OBSERVATIONS BALLOON OBSERVATIONS
Beginning    
Trial 1    
Trial 2    
Trial 3    
  1. Conduct the test three times and watch what happens each time. Make a note about the outcome, whether it agreed with your prediction and whether the outcome was the same each time.
  1. Discuss: How did your experiment show that a weak acid can form caves in limestone?