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Chapter 11: Problem 101

A flask of water is connected to a powerful vacuum pump. When the pump is turned on, the water begins to boil. After a few minutes, the same water begins to freeze. Eventually, the ice disappears. Explain what happens at each step.

Short Answer

Expert verified
The water boils at lower temperature due to decreased pressure by the vacuum pump. The heat loss during boiling reduces temperature, causing water to freeze. Eventually, the ice disappears due to sublimation- a phase transition directly from solid (ice) to gas (water vapor) at extremely low pressure.

Step by step solution

01

Understanding the Boiling Point

When the flask of water is connected to the vacuum pump, the pressure inside the chamber decreases. The boiling point of any liquid is the temperature at which its vapor pressure equals the atmospheric pressure. In this case, as the pressure decreases due to the vacuum pump, the boiling point of water also drops. This is why the water begins to boil at room temperature.
02

Freezing Point under Vacuum Condition

As the boiling continues and more heat energy is lost (without any external heat supply), the temperature of the water drops. If the pressure is reduced drastically as in this case with vacuum pump, the water could potentially reach its freezing point and start to form ice crystals. Even without decreasing the room temperature, the water freezes due to the fact that the freezing point of water rises under reduced pressures (In fact, at extremely low pressures, the line between 'liquid' and 'gas' disappears).
03

Sublimation - Solid to Gas State

After a certain time, it is observed that the ice also starts disappearing. This happens because the water molecules are transitioning directly from the solid phase (ice) to the gas phase (water vapor), bypassing the liquid phase. This is also a characteristic effect of reduced pressure and is known as sublimation.

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