Animated Solution for Chemistry - s and p-Block Elements: The one that is not suitable for the removal of permanent hardness of water is

Have you ever tried washing your hands with soap, only to find that instead of a rich lather, you get a sticky, useless scum? Or perhaps you've noticed a chalky white crust building up inside your electric kettle? Welcome to the world of hard water.

In chemistry, water is considered "hard" when it contains high concentrations of dissolved multivalent metallic ions, predominantly calcium ($C{a}^{2+}$) and magnesium ($M{g}^{2+}$). While perfectly safe to drink, these ions wreak havoc on plumbing, boilers, and cleaning processes. To tackle this problem, chemists classify hardness into two distinct categories based on the specific anions accompanying these metals: Temporary Hardness and Permanent Hardness.

The Fragile Bicarbonates

Temporary hardness is caused by the presence of soluble bicarbonates, specifically calcium bicarbonate, $Ca(HC{O}_3{)}_2$, and magnesium bicarbonate, $Mg(HC{O}_3{)}_2$. The beauty of temporary hardness is right there in the name—it is temporary.

Bicarbonates are thermally unstable. When you simply boil the water, the heat drives off carbon dioxide gas, forcing the soluble bicarbonates to decompose into insoluble carbonates. These solid precipitates can then be easily filtered out, leaving you with soft water. But boiling large municipal water supplies isn't economically feasible. Enter Clark's Method.

Precision Chemistry

Instead of heat, Clark's method uses a chemical trick. By adding a precisely calculated amount of slaked lime, $Ca(OH{)}_2$, to the water, we can force the bicarbonates to precipitate at room temperature.

The reaction for calcium bicarbonate is elegant:

For magnesium bicarbonate, it requires a bit more lime to fully precipitate the magnesium as a hydroxide:

Notice a crucial detail here: Clark's method relies entirely on the chemical properties of the bicarbonate ion ($HC{O}_3^{-}$). It is a targeted strike against temporary hardness.

The Stubborn Salts

What happens when the calcium and magnesium ions are paired with chlorides ($C{l}^{-}$) or sulphates ($S{O}_4^{2-}$)? You get Permanent Hardness.

Unlike bicarbonates, these salts are highly stable. You can boil the water all day, and the chlorides and sulphates will happily stay dissolved. Clark's method is completely useless here because slaked lime will not precipitate these stable salts. To defeat permanent hardness, we need heavy-duty chemical artillery.

We use methods like: 1. Treatment with Washing Soda ($N{a}_{2}C{O}_3$): This directly provides carbonate ions to force precipitation. 2. Calgon's Method: Using sodium hexametaphosphate to "sequester" or trap the $C{a}^{2+}$ and $M{g}^{2+}$ ions in complex soluble molecules so they can't react with soap. 3. Ion-Exchange Method: Passing the water through synthetic resins that physically swap the offending $C{a}^{2+}$ and $M{g}^{2+}$ ions for harmless $N{a}^{+}$ ions.

Returning to our original problem, we were asked to identify the method not suitable for removing permanent hardness.

As we've explored, Ion-exchange, Calgon's method, and Sodium carbonate treatment are the exact tools designed to combat the stubborn chlorides and sulphates of permanent hardness. Clark's method, however, is a specialized tool that only works on bicarbonates. Therefore, it is entirely unsuitable for permanent hardness, making it the correct answer to our question.