When calcium carbonate — for instance, in limestone — reacts with carbon dioxide and water, a soluble calcium salt is produced. When boiled, the chemical decomposes back to calcium carbonate, forming limescale. What is the name of the calcium salt?
Calcium carbonate has the formula CaCO₃. Calcium, in group two, forms ions with a two plus charge, while the carbonate anion has a charge of two minus. So these combine together to form a neutral compound, CaCO₃. Calcium carbonate reacts with carbon dioxide, CO₂, and water, H₂O, forming an as-yet-unknown soluble calcium salt. If an aqueous solution of this salt is boiled, like it might be in a kettle, calcium carbonate is regenerated.
What the question is asking for is the name of the calcium salt. When treated with carbon dioxide and water, calcium carbonate will form calcium hydrogen carbonate. HCO₃− is the formula of the hydrogen carbonate anion. It takes two of the hydrogen carbonate anion to counterbalance the two plus charge of the calcium ion. So the name of the salt produced when carbon dioxide and water react with calcium carbonate is calcium hydrogen carbonate.
An interesting demonstration of this reaction occurs with limewater. When carbon dioxide is bubbled through limewater, a solution of calcium hydroxide, a cloudy suspension of calcium carbonate forms, turning the liquid cloudy. This is why limewater is used as a confirmatory test for the presence of carbon dioxide.
However, if the flow of carbon dioxide is continued into the suspension of calcium carbonate, the solution will go clear again because soluble calcium hydrogen carbonate is formed. This reaction is quite slow though. So back to our question, the name of the calcium salt formed when calcium carbonate reacts with carbon dioxide and water is calcium hydrogen carbonate.