In virginia and some other parts of the world, water has carved vast caverns out of underground limestone deposits. water, however, must contain dissolved carbon dioxide in order to break down the limestone. what role does the carbon dioxide play?

The role of the carbon dioxide in water is that it has the ability to form an acid in which it has the capacity to react  with underground limestones deposits. If it is able to react with the limestone, then it will most likely produce a substance that can be water-soluble.

Exothermic,  physical change after it is already baked