The dissolution of calcium carbonate (CaCO3) in water is a bit more complex than a simple dissolving like salt in water. Here’s why and how to represent it:
Why it’s complex:
Low Solubility: Calcium carbonate is only slightly soluble in pure water.
Reaction with Acid: It dissolves much more readily in acidic solutions. This is because the carbonate ion (CO3^2-) reacts with hydrogen ions (H+) to form bicarbonate (HCO3-) and carbonic acid (H2CO3), which then decomposes into carbon dioxide (CO2) and water (H2O).
The Equations:
Dissolution in Pure Water (limited):
CaCO3(s) ⇌ Ca^2+(aq) + CO3^2-(aq) This equation shows the equilibrium between solid calcium carbonate and its dissolved ions in water. The double arrow indicates that the reaction is reversible, and the equilibrium lies heavily to the left, meaning very little CaCO3 dissolves.
Dissolution in Acidic Solution (more significant):
CaCO3(s) + 2H^+(aq) → Ca^2+(aq) + H2CO3(aq)
H2CO3(aq) ⇌ H2O(l) + CO2(g)
The first equation shows calcium carbonate reacting with hydrogen ions (from an acid) to form calcium ions and carbonic acid.
The second equation shows the carbonic acid decomposing into water and carbon dioxide gas.
Overall Reaction in Acidic Solution:
You can combine these two equations to get a more complete picture:
CaCO3(s) + 2H^+(aq) → Ca^2+(aq) + H2O(l) + CO2(g)
Important Points:
The presence of acid (H+ ions) significantly increases the solubility of calcium carbonate. This is why acid rain can dissolve limestone and marble (which are forms of CaCO3).
The carbon dioxide produced in the reaction can escape as a gas, which further drives the reaction to the right, promoting more dissolution of the calcium carbonate.
Therefore, while the first equation represents the very limited dissolution in pure water, the equations involving acid are more representative of how calcium carbonate dissolves in most natural environments.