Which of the following compounds can be used for the detection of `CO_2`

Options

A. `Ca(OH)_2`
B. `Na_2CO_3`
C. `KOH`
D. `H_2O`

Answer

To determine which compound can be used for the detection of carbon dioxide (CO₂), we can analyze the options provided.

### Step-by-Step Solution:

1. **Understand the Detection Method**:
- Carbon dioxide can be detected using a reaction that produces a visible change, such as a color change or precipitate formation. One common method involves using limewater (calcium hydroxide solution).

2. **Identify the Compounds**:
- The compounds mentioned are:
- Beryllium hydroxide (Be(OH)₂)
- Sodium carbonate (Na₂CO₃)
- Calcium hydroxide (Ca(OH)₂)
- S₂O (this seems to be a typographical error; possibly referring to sulfur dioxide or another compound).

3. **Evaluate Each Compound**:
- **Beryllium Hydroxide (Be(OH)₂)**: This compound does not react with CO₂ to produce a detectable change.
- **Sodium Carbonate (Na₂CO₃)**: While sodium carbonate can react with acids to produce CO₂, it is not typically used for the detection of CO₂ itself.
- **Calcium Hydroxide (Ca(OH)₂)**: This is commonly known as limewater. When CO₂ is bubbled through limewater, it reacts to form calcium carbonate (CaCO₃), which is insoluble and causes the solution to turn milky.
- **S₂O**: This compound is not relevant for CO₂ detection.

4. **Conclusion**:
- The compound that can be used for the detection of carbon dioxide is **Calcium Hydroxide (Ca(OH)₂)**.

### Final Answer:
**Calcium Hydroxide (Ca(OH)₂)** is the compound used for the detection of carbon dioxide.