Problem 143 - Chemistry An Atoms First Approach

Question

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?

Step-by-Step Solution

Answer

When the vacuum pump is turned on, it removes air from the container, causing a decrease in pressure. On the phase diagram, this decrease in pressure lowers the boiling point, causing the water to transition from liquid to gas. As pressure continues to decrease, it reaches the solid-gas equilibrium curve and the water vapor loses kinetic energy, transitioning into solid ice through deposition. If the vacuum pump is left on indefinitely, the ice will remain stable as long as the pressure stays on the solid-gas equilibrium curve. Further pressure decrease may cause the ice to sublimate, but most vacuum pumps cannot achieve such low pressures.

Step 1: Understand the Phase Diagram for Water

The phase diagram for water is a graphical representation of the different states of water (solid, liquid, and gas) under various temperature and pressure conditions. On the diagram, the X-axis represents temperature, and the Y-axis represents pressure. There are three curves on the diagram: the solid-liquid equilibrium curve, the liquid-gas equilibrium curve, and the solid-gas equilibrium curve.

Step 2: Consider the Initial State

Initially, some water is placed in a sealed glass container connected to a vacuum pump. The pressure inside the container is close to atmospheric pressure, and the water is in a liquid state. The temperature of the water is somewhere between 0°C and 100°C (assuming room temperature).

Step 3: Boiling of Water

When the vacuum pump is turned on, it starts removing air from the container, causing a decrease in pressure. As we move towards lower pressures on the phase diagram, the boiling point of water decreases. The decreased pressure causes the water molecules to gain enough kinetic energy to transition from the liquid state to the gas state. This is why the water appears to boil.

Step 4: Freezing of Water

As more and more air is removed from the container, the pressure continues to decrease. Eventually, the pressure becomes low enough to reach the solid-gas equilibrium curve on the phase diagram. At this point, the water molecules lose their kinetic energy and transition from the gas state to the solid state. This process is known as deposition, and it causes the water vapor to freeze into ice.

Step 5: The Ice Left Indefinitely

If the vacuum pump is left on indefinitely, the pressure inside the container will continue to decrease until the pump reaches its limit or the pressure becomes equal to the vapor pressure of ice at that temperature. As long as the pressure remains on the solid-gas equilibrium curve on the phase diagram, the ice will remain stable. If the pressure decreases further, it may lead to a phase change from solid to gas (sublimation) in extreme cases. However, most vacuum pumps cannot achieve such low pressures, so the ice is likely to remain solid and stable.