Solute Stabilization in Water Passing through Limestone

When water passes through limestone, a form of solute stabilization occurs known as liming.

Explanation:
Limestone is a sedimentary rock primarily composed of calcium carbonate (CaCO3). When water comes into contact with limestone, it undergoes a chemical reaction known as dissolution. This reaction can be represented by the following equation:

CaCO3 + H2O → Ca2+ + HCO3- + OH-

In this reaction, calcium carbonate dissolves in water, producing calcium ions (Ca2+), bicarbonate ions (HCO3-), and hydroxide ions (OH-). These ions contribute to the solute concentration in the water.

Process of Liming:
Liming is the process of adding limestone or lime to water in order to stabilize the solute concentration. When water passes through a limestone bed or when lime is added directly to water, the following reactions occur:

1. Neutralization: Lime (CaO) reacts with the bicarbonate ions in water to form calcium carbonate and water. The reaction can be represented as follows:

CaO + HCO3- → CaCO3 + H2O

In this reaction, lime reacts with bicarbonate ions to form calcium carbonate, which is insoluble and precipitates out of the water. This reduces the concentration of bicarbonate ions and stabilizes the solute concentration.

2. Re-carbonation: The calcium carbonate that precipitates out of the water can dissolve back into the water through a process called re-carbonation. This occurs when the water is exposed to carbon dioxide (CO2) in the atmosphere or through the addition of carbon dioxide gas. The reaction can be represented as follows:

CaCO3 + CO2 + H2O → Ca2+ + 2HCO3-

In this reaction, carbon dioxide reacts with calcium carbonate to form calcium ions and bicarbonate ions. This re-introduces the solute back into the water, but at a lower concentration compared to the initial dissolution of limestone.

3. Precipitation: The calcium ions present in the water can also precipitate as calcium carbonate if the concentration of calcium exceeds its solubility limit. This can occur when the water is heated or the pH of the water increases.

Conclusion:
In summary, when water passes through limestone, the process of liming occurs. Liming involves the neutralization of bicarbonate ions with lime, the re-carbonation of precipitated calcium carbonate, and the potential precipitation of calcium ions as calcium carbonate. These processes help stabilize the solute concentration in the water.