When a standard solution of NaOH is left in air for a few hours:

Options

A. a precipitate will form
B. strength of solution will decrease
C. the strength of solution will increase
D. the concentration of `Na^(+)` ion in solution will remains same

Answer

To solve the question regarding what happens when a standard solution of NaOH is left in air for a few hours, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding NaOH in Solution**:
- Sodium hydroxide (NaOH) is a strong base that dissociates completely in water to give Na⁺ and OH⁻ ions.

2. **Exposure to Air**:
- When a standard solution of NaOH is exposed to air, it can absorb carbon dioxide (CO₂) from the atmosphere.

3. **Formation of Sodium Carbonate**:
- The absorbed CO₂ reacts with the hydroxide ions (OH⁻) in the solution to form sodium carbonate (Na₂CO₃) and water:
\[ 2 \text{NaOH} + \text{CO}_2 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} \]

4. **Effect on Concentration**:
- As NaOH converts to Na₂CO₃, the concentration of NaOH decreases, leading to a decrease in the strength of the solution.

5. **Precipitate Formation**:
- NaOH itself does not form a precipitate unless it reacts with certain metal ions (like Al³⁺, Ca²⁺, or Mg²⁺) in solution. Since we are only considering NaOH in the absence of these ions, no precipitate will form.

6. **Conclusion**:
- Therefore, the correct conclusion is that the strength of the NaOH solution will decrease when left in air for a few hours.

### Final Answer:
The strength of the solution will decrease.

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