The blue litmus is turned red by an aqueous solution of

Options

A. `KNO_3`
B. `NaCl`
C. ` CuSO_4`
D. ` CH_3COONa.`

Answer

To determine which aqueous solution turns blue litmus red, we need to analyze the given options based on their acidic or basic nature.

### Step-by-Step Solution:

1. **Understanding Litmus Test**:
- Blue litmus paper turns red in acidic solutions. Therefore, we need to identify which of the given compounds produces an acidic solution when dissolved in water.

2. **Analyzing Each Compound**:
- **Option 1: KNO3 (Potassium Nitrate)**:
- When KNO3 dissolves in water, it dissociates into K+ and NO3- ions.
- KNO3 is formed from a strong base (KOH) and a strong acid (HNO3), making it a neutral salt.
- **Conclusion**: KNO3 does not turn blue litmus red.

- **Option 2: NaCl (Sodium Chloride)**:
- NaCl dissociates into Na+ and Cl- ions in water.
- It is formed from a strong base (NaOH) and a strong acid (HCl), thus it is also a neutral salt.
- **Conclusion**: NaCl does not turn blue litmus red.

- **Option 3: CuSO4 (Copper(II) Sulfate)**:
- CuSO4 dissociates into Cu2+ and SO4^2- ions in water.
- It is formed from a strong acid (H2SO4) and a weak base (Cu(OH)2).
- Since it is derived from a strong acid, the solution is acidic.
- **Conclusion**: CuSO4 will turn blue litmus red.

- **Option 4: CH3COONa (Sodium Acetate)**:
- CH3COONa dissociates into CH3COO- and Na+ ions in water.
- It is formed from a weak acid (CH3COOH) and a strong base (NaOH), making it a basic salt.
- **Conclusion**: CH3COONa does not turn blue litmus red.

3. **Final Answer**:
- Among the options, only CuSO4 produces an acidic solution that will turn blue litmus red. Therefore, the correct answer is **Option 3: CuSO4**.