Which statement best helps to explain the observation that water boils at 100~^ext{o}C, whereas a 1~ext{molal} aqueous solution of NaCl boils at a higher temperature?

• A. Adding NaCl to water has no effect on the boiling point because NaCl does not interact with water molecules.
• B. The boiling point of water decreases when NaCl is added due to the disruption of hydrogen bonding.
• C. NaCl acts as a catalyst, causing water molecules to evaporate more quickly and thus raising the boiling point.
• D. Dissolving NaCl in water increases the boiling point because the presence of solute particles lowers the vapor pressure of the solution.

Answer: (D)

Verified step by step guidance

1. Understand that the boiling point of a liquid is the temperature at which its vapor pressure equals the external atmospheric pressure.
2. Recognize that adding a non-volatile solute like NaCl to water causes a colligative property known as boiling point elevation.
3. Recall that boiling point elevation occurs because the solute particles lower the vapor pressure of the solvent by disrupting the escape of solvent molecules into the vapor phase.
4. Use the concept that the decrease in vapor pressure means the solution must be heated to a higher temperature to reach the atmospheric pressure and boil.
5. Conclude that the presence of dissolved NaCl increases the boiling point of water due to the lowering of vapor pressure, not because of catalytic effects or disruption of hydrogen bonding alone.