Compared with pure water, the boiling point of water containing a dissolved…

Concept

A liquid boils when its vapour pressure becomes equal to the surrounding atmospheric pressure.

A dissolved non-volatile solute lowers the solvent's vapour pressure. Therefore, a higher temperature is needed to reach the boiling condition; this is boiling-point elevation.

Application

1. Apply the standard colligative-property convention: the impurity is dissolved and non-volatile.

2. At the boiling point of pure water, the impure water has a lower vapour pressure than pure water.

3. Heating must continue until the impure water's vapour pressure reaches atmospheric pressure.

4. Hence, the boiling-point change is positive.

Cross-check and contrast

• “Is same” would require no vapour-pressure lowering.

• “Increases” matches the need for additional heating after vapour-pressure lowering.

• “Decreases” would require the mixture to reach atmospheric pressure at a lower temperature.

• “More than one of the above” would require more than one direction for the same fixed condition.

• “None of the above” is excluded because positive, zero, and negative changes exhaust the possibilities.

Therefore, compared with pure water, the boiling point increases.