Limestone's interaction with water depends significantly on whether the water contains dissolved carbon dioxide. In the absence of carbon dioxide, the interaction is primarily physical rather than chemical dissolution.
Limestone's Interaction with Pure Water
Based on the provided reference, when limestone is mixed with pure water:
- The limestone does not dissolve in water. This is a key characteristic of limestone (calcium carbonate, CaCO₃) in pure water.
- Instead of dissolving, it will form a solution same as that of mud and water. While the reference uses the term "solution," it clarifies the nature of this mixture by stating that Two separate layers can be seen. This observation is characteristic of a suspension, not a true solution. Like mud and water, the solid limestone particles (or fine powder) are suspended in the water but will eventually settle out over time due to gravity, creating distinct layers of solid and liquid.
Key Takeaway: In pure water, limestone does not undergo a significant chemical reaction or dissolve. It forms a suspension where solid particles are dispersed but will settle, similar to how mud behaves in water.
Limestone's Reaction with Water Containing Carbon Dioxide
The interaction changes significantly when the water contains dissolved carbon dioxide (CO₂). Rainwater and natural groundwater often contain dissolved CO₂ from the atmosphere and soil.
- If limestone is mixed with a mixture of carbon dioxide and water it will form calcium hydrogen carbonate. This is a chemical reaction where calcium carbonate (limestone) reacts with water and carbon dioxide to form calcium hydrogen carbonate (also known as calcium bicarbonate, Ca(HCO₃)₂).
- Unlike calcium carbonate, calcium hydrogen carbonate is soluble in water. This chemical reaction is responsible for the formation of karst landscapes, caves, stalactites, and stalagmites over geological time as acidic groundwater dissolves limestone bedrock.
The Chemical Equation (Simplified):
Limestone + Water + Carbon Dioxide → Calcium Hydrogen Carbonate
CaCO₃(s) + H₂O(l) + CO₂(aq) → Ca(HCO₃)₂(aq)
Key Takeaway: When carbon dioxide is present in water, limestone undergoes a chemical reaction, converting the insoluble calcium carbonate into soluble calcium hydrogen carbonate, leading to dissolution.
Summary of Limestone's Interaction with Water
Here's a brief comparison of limestone's behaviour in the two scenarios mentioned in the reference:
| Water Condition | Primary Interaction | Resulting Mixture | Observation |
|---|---|---|---|
| Pure Water | Physical (No dissolution) | Suspension (like mud/water) | Two separate layers |
| Water + Carbon Dioxide | Chemical Reaction (Dissolution) | Solution (Calcium Hydrogen Carbonate) | Dissolves over time |
Understanding these interactions is crucial in geology, environmental science (e.g., water hardness), and construction.
