Calcium carbonate gets into water primarily through the dissolution of calcium carbonate-containing rocks, influenced significantly by the presence of carbon dioxide.
Here's a breakdown of the process:
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Dissolution of Rocks: Calcium carbonate is a major component of rocks like limestone and chalk. Rainwater, which is naturally slightly acidic due to dissolved carbon dioxide, can dissolve these rocks.
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Carbon Dioxide's Role: The key factor enhancing the dissolution process is the presence of carbon dioxide (CO2) in the water. Here's how:
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Rainwater absorbs CO2 from the atmosphere and soil: This forms a weak carbonic acid (H2CO3):
H2O (l) + CO2 (g) ⇌ H2CO3 (aq) -
Carbonic acid reacts with calcium carbonate: The carbonic acid then reacts with solid calcium carbonate (CaCO3) to form calcium bicarbonate (Ca(HCO3)2), which is much more soluble in water:
CaCO3 (s) + H2CO3 (aq) ⇌ Ca(HCO3)2 (aq)
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Solubility Considerations: Pure water has a very limited ability to dissolve calcium carbonate (approximately 15 mg/L at 25°C). However, the presence of dissolved carbon dioxide significantly increases the solubility of calcium carbonate because it converts it into the more soluble calcium bicarbonate.
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Temperature Influence: Interestingly, the solubility of calcium carbonate increases as the temperature of the water decreases. This is somewhat unusual, as the solubility of most solids increases with increasing temperature.
Therefore, the primary way calcium carbonate gets into water is through the chemical weathering and dissolution of rocks containing it, a process facilitated by carbon dioxide dissolved in rainwater, forming calcium bicarbonate.
