Question

Match the following

The correct answer is

• A. A -- III, B -- IV, C -- I, D -- II
• B. A -- III, B -- IV, C -- II, D -- I
• C. A -- IV, B -- II, C -- I, D -- III
• D. A -- II, B -- IV, C -- I, D -- III

Hint:

Remember the unique precipitation product for Magnesium temporary hardness on boiling: it forms \(\text{Mg(OH)}_2\), not \(\text{MgCO}_3\), due to solubility product differences (\(K_{sp}\) of \(\text{Mg(OH)}_2<\text{MgCO}_3\)).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The question asks to match reactions involved in removing hardness of water with the specific methods used.

Step 2: Analyzing Each Reaction:

1. Reaction A:
   \( \text{Mg(HCO}_3)_2 \xrightarrow{\Delta} \text{Mg(OH)}_2 \downarrow + 2\text{CO}_2 \uparrow \)
   This reaction represents the removal of temporary hardness caused by magnesium bicarbonate by boiling. Note that unlike calcium bicarbonate which forms carbonate, magnesium bicarbonate precipitates as hydroxide because \( \text{Mg(OH)}_2 \) is less soluble than \( \text{MgCO}_3 \). (Matches with III).
2. Reaction B:
   \( \text{M}^{2+} + \text{Na}_6\text{P}_6\text{O}_{18}^{2-} \rightarrow [\text{Na}_2\text{MP}_6\text{O}_{18}]^{2-} + 2\text{Na}^+ \)
   The compound \( \text{Na}_6\text{P}_6\text{O}_{18} \) is commercially known as Calgon (Sodium hexametaphosphate). It complexes with metal ions like \( \text{Ca}^{2+} \) and \( \text{Mg}^{2+} \) to keep them in solution but inactive. This is Calgon's method. (Matches with IV).
3. Reaction C:
   \( \text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \rightarrow 2\text{CaCO}_3 \downarrow + 2\text{H}_2\text{O} \)
   This involves adding calculated amounts of lime \( (\text{Ca(OH)}_2) \) to precipitate out calcium carbonate. This is known as Clark's method. (Matches with I).
4. Reaction D:
   \( 2\text{NaZ} + \text{Ca}^{2+} \rightarrow \text{CaZ}_2 + 2\text{Na}^+ \)
   Here, Z represents Zeolite (hydrated sodium aluminium silicate). The exchange of sodium ions for calcium/magnesium ions on the zeolite surface is characteristic of the ion exchange method (Permutit/Zeolite process). (Matches with II).

Step 3: Matching:
A → III, B → IV, C → I, D → II.

Final Answer:
Option (A).