Compared with pure water, the boiling point of water containing a dissolved…
2023
Compared with pure water, the boiling point of water containing a dissolved non-volatile impurity:
Answer: B. increases — ConceptA liquid boils when its vapour pressure becomes equal to the surrounding atmospheric pressure. A dissolved non-volatile solute lowers the solvent's…
- A.
is same
- B.
increases
- C.
decreases
- D.
More than one of the above
- E.
None of the above
Attempted by 4 students.
Show answer & explanation
Correct answer: B
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
Apply the standard colligative-property convention: the impurity is dissolved and non-volatile.
At the boiling point of pure water, the impure water has a lower vapour pressure than pure water.
Heating must continue until the impure water's vapour pressure reaches atmospheric pressure.
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.