What is the product of a reaction of calcium carbonate, water and carbon dioxide?

Calcium hydrogen carbonate

The correct answer is option 4. Calcium hydrogen carbonate.

The reaction of calcium carbonate (CaCO₃) with water (H₂O) and carbon dioxide (CO₂) leads to the formation of calcium hydrogen carbonate (Ca(HCO₃)₂). The overall chemical equation for this reaction is:

\[ \text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{Ca(HCO}_3\text{)}_2 \]

Here's a step-by-step explanation of the reaction:

1. Dissolution of Calcium Carbonate:

\[ \text{CaCO}_3 + \text{H}_2\text{O} \rightarrow \text{Ca}^{2+} + \text{CO}_3^{2-} + \text{H}_2\text{O} \]

Calcium carbonate dissolves in water to form calcium ions (\(\text{Ca}^{2+}\)), carbonate ions (\(\text{CO}_3^{2-}\)), and water.

2. Reaction with Carbon Dioxide:

\[ \text{CO}_3^{2-} + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3\text{)}_2 \]

Carbon dioxide reacts with carbonate ions in water to form calcium hydrogen carbonate. The resulting product, calcium hydrogen carbonate (Ca(HCO₃)₂), is soluble in water, contributing to the "hardness" of water. This reaction is also important in the natural weathering of rocks containing calcium carbonate, as carbon dioxide in the atmosphere combines with rainwater to form a weak carbonic acid, which can react with calcium carbonate in rocks.