Concept:The boiling point of a liquid depends on the external pressure above its surface.
Explanation:Boiling occurs when the vapour pressure of the liquid equals the surrounding external pressure.
At normal atmospheric pressure, pure water boils at
100∘C.
If the external pressure is reduced, water needs less vapour pressure to boil.
This lower required vapour pressure is reached at a lower temperature.
Therefore, decreasing the external pressure makes pure water boil below
100∘C.
Changing the quantity or distilling water does not alter its boiling point, and increasing pressure raises the boiling point.
Answer:B. decreasing the external pressure