Limestone makes water alkaline primarily because it contains calcium carbonate (CaCO3), which dissolves and reacts with water, increasing the water's pH.
The Process Explained
When water flows over or through limestone formations, a chemical reaction occurs. This reaction is what leads to increased alkalinity. Here's a breakdown:
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Dissolution of Calcium Carbonate (CaCO3): Limestone is predominantly composed of calcium carbonate. When water comes into contact with limestone, the calcium carbonate begins to dissolve. The degree of dissolution depends on factors like water temperature and acidity. Cooler and more acidic water dissolves limestone more readily.
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Reaction with Water: The dissolved calcium carbonate reacts with water (H2O) and carbon dioxide (CO2) present in the water to form calcium bicarbonate (Ca(HCO3)2).
CaCO3 (s) + H2O (l) + CO2 (aq) ⇌ Ca2+ (aq) + 2HCO3- (aq)
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Increase in Bicarbonate Ions (HCO3-): The reaction results in an increase in bicarbonate ions (HCO3-) in the water. Bicarbonate is a base, and its presence raises the pH of the water, making it more alkaline.
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Buffering Capacity: The dissolved calcium and bicarbonate also contribute to the water's buffering capacity. This means the water becomes more resistant to changes in pH when acids or bases are added.
Factors Affecting Alkalinity Increase
Several factors influence how much limestone increases water alkalinity:
- Contact Time: The longer the water is in contact with the limestone, the more calcium carbonate dissolves and the greater the increase in alkalinity.
- Water Chemistry: The initial pH and CO2 content of the water play a significant role. More acidic water with higher CO2 content dissolves limestone more readily.
- Surface Area: A larger surface area of limestone exposed to the water promotes greater dissolution. Fractured or heavily weathered limestone will increase alkalinity more significantly than a solid, smooth surface.
- Temperature: Lower water temperatures generally increase the solubility of CO2 which then drives limestone dissolution and enhances alkalinity.
Effects of Alkaline Water
Increased alkalinity in water can have various effects:
- Neutralizing Acid Rain: Alkaline water can help neutralize the effects of acid rain in ecosystems.
- Aquatic Life: While some aquatic organisms thrive in alkaline conditions, extreme alkalinity can be harmful to others.
- Water Treatment: Alkalinity is an important parameter in water treatment processes, affecting coagulation and disinfection.
- Taste: Highly alkaline water may have a slightly bitter taste.
In summary, limestone increases water alkalinity by dissolving and releasing calcium carbonate, which reacts with water and carbon dioxide to form bicarbonate ions, thereby raising the pH of the water.
