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Chapter 10: Problem 153

Some water is placed in a sealed glass container connected to a vacuum pump (a device used to pump gases from a container), and the pump is turned on. The water appears to boil and then freezes. Explain these changes using the phase diagram for water. What would happen to the ice if the vacuum pump was left on indefinitely?

Short Answer

Expert verified
Initially, water is in a liquid state in a sealed container connected to a vacuum pump. As the vacuum pump removes gas molecules and reduces pressure inside the container, the boiling point of water decreases, causing it to boil at room temperature. Further decrease in pressure causes the remaining water to reach the solid-liquid coexistence line in the phase diagram, resulting in freezing. If the vacuum pump is left on indefinitely, the ice will enter the solid-gas coexistence region, causing it to sublimate directly into water vapor, leaving only water vapor in the container.

Step by step solution

01

Understanding the phase diagram of water

A phase diagram is a graphical representation of the physical states of a substance (solid, liquid, and gas) as a function of temperature and pressure. For water, the diagram shows the regions where solid (ice), liquid (water), and gas (water vapor) coexist and where phase transitions occur.
02

Initial state of water in the container

Initially, water is placed in a sealed glass container connected to a vacuum pump. At this point, the water is at room temperature and atmospheric pressure, which puts it in the liquid phase according to the phase diagram.
03

Turning on the vacuum pump

When the vacuum pump is turned on, it starts removing gas molecules from the container, which reduces the pressure inside. As the pressure decreases, the boiling point of water also decreases, as indicated by the leftward movement along the liquid-gas coexistence curve in the phase diagram. When the pressure gets low enough, the water begins to boil at room temperature.
04

Water appears to boil

The water appears to boil not because it is being heated but because the pressure inside the container is being reduced, causing the water to undergo a phase transition from liquid to gas (evaporation). If the vacuum pump keeps reducing the pressure, the water will continue to boil and produce water vapor.
05

Water freezes

As the pressure continues to decrease and more water evaporated, the remaining water will eventually reach the solid-liquid coexistence line in the phase diagram (triple point). At this point, any further decrease in pressure (while keeping the temperature constant) will cause the water to freeze, forming ice.
06

What happens when the vacuum pump is left on indefinitely?

If the vacuum pump continues to run and lower the pressure even further, the ice will enter the solid-gas coexistence region in the phase diagram. Thus, the ice will sublimate or directly turn from a solid to a gas without going through the liquid phase. Over time, all the ice will eventually turn into water vapor, and the container will be left with only water vapor in it.

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