Which of the following method uses Ca(OH)₂ (slaked lime) for water softening?

Explanation:

Clark's method is one of the earliest and simplest methods of removing hardness (i.e., Ca²⁺ and Mg²⁺ ions) from water. It is a lime-soda process and was first proposed by Dr. Clark in the year 1841. Here's how the process works:

Step 1 - Adding Slaked Lime: In this method, slaked lime [Ca(OH)₂] is added to hard water, which precipitates calcium ions as calcium carbonate and magnesium ions as magnesium hydroxide. The chemical reactions are as follows:

For Calcium: Ca²⁺ (aq) + CO₃²⁻ (aq) ⇌ CaCO₃ (s)

For Magnesium: Mg²⁺ (aq) + 2 OH⁻ (aq) ⇌ Mg(OH)₂ (s)

This process can reduce the hardness from water up to a considerable extent but not completely. This means the water is still hard after this treatment.

Step 2 - Adding Soda Ash: The remaining calcium and magnesium ions are then treated with 'soda ash' (sodium carbonate, Na₂CO₃), which precipitates the remainder of the calcium ions as calcium carbonate and the magnesium ions as magnesium hydroxide:

For Remaining Calcium: Ca²⁺ (aq) + CO₃²⁻ (aq) ⇌ CaCO₃ (s)

For Remaining Magnesium: Mg²⁺ (aq) + CO₃²⁻ (aq) ⇌ MgCO₃ (s)

After these precipitation reactions occur, the precipitates are allowed to settle and are then filtered out, resulting in softened water.