Concept:Boiling occurs when the saturated vapour pressure of a liquid equals the external pressure, and both dissolved solutes and external pressure changes affect the boiling point.
Explanation:Statement I is correct: a liquid boils when its saturated vapour pressure is equal to the external pressure.
Statement II is correct: dissolved substances in pure water reduce the vapour pressure, which raises the boiling point.
Statement III is correct: an increase in external pressure requires a higher boiling point before vapour pressure can match it.
Statement IV is incorrect: dissolved substances increase the boiling point, not decrease it.
Therefore, the correct statements are I, II and III only.
Answer:A. I, II and III only