Britannica Sources used in this answer

How does carbon dioxide in water dissolve calcium carbonate deposits?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A labeled diagram of the periodic table entry for calcium, showing its atomic number 20, symbol Ca, atomic weight 40.078, electron configuration, and classification as an alkaline-earth metal. Icons indicate calcium is a solid with a face-centered cubic crystal structure and strongly basic oxides at room temperature. — calcium The chemical element calcium is the most abundant metallic element in the human body.

Carbon dioxide dissolves calcium carbonate deposits through the formation of carbonic acid when carbon dioxide reacts with water.12

When rainwater infiltrates soil, it absorbs carbon dioxide to create a dilute solution of this acid. As this acidic water comes into contact with limestone (which is primarily composed of calcium carbonate), a chemical reaction occurs that converts the insoluble calcium carbonate into calcium hydrogen carbonate, $C a (H C O_{3})_{2}$ , which is water-soluble.21

This process is expressed by the following reaction:
$C a C O_{3} + H_{2} O + C O_{2} \to C a (H C O_{3})_{2}$

This dissolution process is responsible for the formation of caves in limestone bedrock. Conversely, if the water loses carbon dioxide—such as when it emerges into a cave atmosphere—the reaction reverses, causing the calcium carbonate to precipitate and form features like stalactites and stalagmites.12