An unknown mixture contains one or two of the following: `CaCO_(3),BaCl_(2),AgNO_(3),Na_(2)SO_(4),ZnSO_(4) and NaOH`. The mixture is completely soluble in water and solution gives pink colour with phenolphthalein. When dilute hydrochloric acid is gradually added to the solution, a precipitate is formed which dissolved with further addition of the acid.
Q. The aqueous solution of mixture gives white precipitate with dil. HCl which dissolves in excess of dil HCl . it confirms.

Options

A. `BaCl_(2)+NaOH`
B. `Na_(2)SO_(4)+NaOH`
C. `ZnSO_(4)+NaOH`
D. `AgNO_(3)+NaOH`

Explanation

To solve the problem, we need to analyze the information given about the unknown mixture and the reactions that occur when dilute hydrochloric acid (HCl) is added. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Components of the Mixture
The unknown mixture contains one or two of the following compounds:
- CaCO₃
- BaCl₂
- AgNO₃
- Na₂SO₄
- ZnSO₄
- NaOH

### Step 2: Analyze the Solubility
The mixture is completely soluble in water, which indicates that it does not contain any insoluble salts. Among the given compounds, all are soluble in water except for CaCO₃, which is insoluble. Therefore, the mixture cannot contain CaCO₃.

### Step 3: Phenolphthalein Test
The solution gives a pink color with phenolphthalein, which indicates that the solution is basic. This suggests the presence of NaOH (sodium hydroxide) since it is the only strong base among the listed compounds.

### Step 4: Reaction with Dilute HCl
When dilute HCl is added to the solution, a white precipitate is formed, which dissolves upon further addition of HCl. This behavior is characteristic of zinc hydroxide (Zn(OH)₂), which is formed from the reaction of NaOH with ZnSO₄.

### Step 5: Write the Reactions
1. When NaOH reacts with ZnSO₄, the following reaction occurs:
\[ 2 \text{NaOH} + \text{ZnSO}_4 \rightarrow \text{Na}_2\text{ZnO}_2 + \text{Na}_2\text{SO}_4 + 2 \text{H}_2\text{O} \]
Here, sodium zincate (Na₂ZnO₂) is formed.

2. When dilute HCl is added to sodium zincate, it produces zinc hydroxide:
\[ \text{Na}_2\text{ZnO}_2 + 2 \text{HCl} \rightarrow 2 \text{NaCl} + \text{Zn(OH)}_2 \]
This results in a white precipitate of zinc hydroxide.

3. On further addition of dilute HCl, zinc hydroxide dissolves:
\[ \text{Zn(OH)}_2 + 2 \text{HCl} \rightarrow \text{ZnCl}_2 + 2 \text{H}_2\text{O} \]
This confirms that the precipitate dissolves in excess HCl.

### Conclusion
The presence of a white precipitate that dissolves with excess dilute HCl confirms that the unknown mixture contains NaOH and ZnSO₄.

Thus, the final answer is:
- The mixture contains **NaOH and ZnSO₄**.

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