Which substance would give a solution with a boiling point below that of pure wate rrather than above?

Options

A. Sodium chloride (solid)
B. Ethyl alcohol (liquid, b.p. `61^(@)C`)
C. sulphuric acid (liquid, b.p.gt`300^(@)C`)
D. sucrose sugar (solid)

Answer

To solve the question of which substance would give a solution with a boiling point below that of pure water, we need to analyze the properties of the given substances in terms of their dissociation in solution and the effect on boiling point.

### Step-by-Step Solution:

1. **Understand Boiling Point Elevation**:
- The boiling point of a solution is elevated compared to that of the pure solvent (water in this case) when a solute is added. This phenomenon is known as boiling point elevation, which can be calculated using the formula:
\[ \Delta T_b = i \cdot K_b \cdot m \]
where:
- \(\Delta T_b\) = boiling point elevation
- \(i\) = van 't Hoff factor (number of particles the solute dissociates into)
- \(K_b\) = ebullioscopic constant of the solvent
- \(m\) = molality of the solution

2. **Identify the Substances**:
- The substances given are:
1. Sodium chloride (NaCl)
2. Ethyl alcohol (C₂H₅OH)
3. Sulfuric acid (H₂SO₄)
4. Sucrose (C₁₂H₂₂O₁₁)

3. **Determine the van 't Hoff Factor (i)**:
- **NaCl**: Dissociates into Na⁺ and Cl⁻, so \(i = 2\).
- **Ethyl alcohol (C₂H₅OH)**: Does not dissociate into ions, so \(i = 1\).
- **Sulfuric acid (H₂SO₄)**: Dissociates into 2 H⁺ and SO₄²⁻, so \(i = 3\).
- **Sucrose**: A non-electrolyte that does not dissociate, so \(i = 1\).

4. **Analyze the Effect on Boiling Point**:
- The higher the value of \(i\), the greater the boiling point elevation. Conversely, substances with a lower \(i\) will result in a lower boiling point compared to pure water.
- Comparing the values:
- NaCl: \(i = 2\)
- Ethyl alcohol: \(i = 1\)
- Sulfuric acid: \(i = 3\)
- Sucrose: \(i = 1\)

5. **Conclusion**:
- Both ethyl alcohol and sucrose have the same van 't Hoff factor of 1, indicating they do not contribute to boiling point elevation. However, since sucrose is a non-electrolyte and does not dissociate, it will not elevate the boiling point at all.
- Therefore, the substance that would give a solution with a boiling point below that of pure water is **sucrose**.

### Final Answer:
The substance that would give a solution with a boiling point below that of pure water is **sucrose**.