Question

Q18.26: A beaker of water at room temperature is placed in an enclosure, and the air pressure in the enclosure is slowly reduced. When the air pressure is reduced sufficiently, the water begins to boil. The temperature of the water does not rise when it boils; in fact, the temperature drops slightly. Explain these phenomena.

Short answer

Only if the atmospheric pressure becomes equal to the vapor pressure, will the liquid start boiling. Thus, on reducing the pressure, at a constant temperature, the liquid starts boiling at low temperatures.

Step-by-step solution

Definition of boiling 

The liquid boils when its boiling point and the atmospheric pressure match. This means once the pressure at the surface of the liquid becomes the same as the vapor pressure, the liquid starts boiling.

Normally, water boils at atmospheric pressure, when it reaches a temperature of $100°\mathrm{C}$. But, when the pressure is lowered, the boiling will occur at a lower temperature, with less heat involved.

Temperature drops slightly when boiling

During boiling, the molecules on the surface with greater kinetic energy escapes away. As the temperature is quantified as the average kinetic energy, the decrease in average kinetic energy results in a decrease in temperature.

Hence, when the air pressure is reduced sufficiently, the water boils at a lower temperature.

The decrease in average kinetic energy due to the escape of high-speed molecules results in a decrease in temperature.