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?

Short answer

In the sealed glass container connected to a vacuum pump, the water appears to boil and then freeze due to changes in pressure and temperature, as illustrated by the phase diagram for water. When the vacuum pump is turned on, it reduces the container's pressure, causing the water to boil at a lower temperature. As the pressure decreases further and the temperature drops, water vapor directly transitions to the solid phase (ice) via deposition. If the vacuum pump is left on indefinitely, the ice will gradually decrease in quantity due to sublimation, and the pressure and temperature inside the container will continue to decrease, potentially affecting the ice's physical properties.

Step-by-step solution

Understand the phase diagram for water

A phase diagram is a graphical representation of the states of matter (solid, liquid, and gas) under different conditions of temperature and pressure. The phase diagram for water displays the boundaries between the solid phase (ice), liquid phase (water), and gaseous phase (water vapor) based on both the pressure and temperature. In this diagram, the boiling point, melting point, and triple point are also represented. The triple point is the unique set of conditions at which all three phases coexist in equilibrium. As the pressure inside the system changes, the phase boundaries will shift, causing different phase transitions to occur at different temperatures.

Analyze the effect of the vacuum pump

A vacuum pump is used to remove gases from a container, therefore reducing the pressure inside the container. As the pump is turned on and starts to reduce the pressure in the sealed glass container, the system's conditions move along the phase diagram. Initially, the water will be in a liquid state under atmospheric pressure and room temperature conditions. As the pressure decreases, the boiling point of water decreases too. This is because atmospheric pressure pushing down on the surface of the liquid is reduced, making it easier for water molecules to escape and turn into vapor. The water appears to boil at a lower temperature than it would under normal pressure conditions.

Explain the freezing of water

Now, as the vacuum pump continues to reduce the pressure, the system moves along the phase diagram to even lower pressures. As a result, the water vapor produced due to boiling will rapidly be removed by the vacuum pump, causing the temperature inside the container to drop as well. If the pressure and temperature conditions reach the boundary between the solid (ice) and gaseous (water vapor) phases, the water in the container will begin to freeze because the vapor will transform directly into the solid phase (a process called deposition) without passing through the liquid phase. The water will appear to freeze as the temperature drops further.

Predict what happens to the ice if the vacuum pump is left on indefinitely

If the vacuum pump is left on indefinitely, it will continue to remove any water vapor that sublimates (changing directly from solid to vapor without going through the liquid state) from the ice. As a result, the total amount of ice inside the container would decrease over time due to sublimation. Additionally, the pressure and temperature would continue to drop inside the container, potentially reaching extreme conditions far from typical atmospheric temperature and pressure, which could affect the physical properties of the ice. However, the vacuum pump's capacity to reduce pressure to near absolute vacuum would be limited, so reaching extremely low pressures might not be achievable indefinitely.

Key concepts

Water Vapor

Water vapor is the gaseous form of water, created when water evaporates due to heat. It plays a crucial role in the water cycle and atmospheric processes. In phase diagrams, water vapor occupies a region which can change with variations in temperature and pressure. Understanding these changes requires a view of the phase diagram for water, where we see how water transitions from liquid to vapor called boiling.

Importantly, as pressure decreases, such as when using a vacuum pump, the boiling point of water lowers. This means water can transition into vapor at much lower temperatures than usual by just reducing the surrounding pressure. Therefore, vacuum pumps can make water boil by reducing the pressure instead of increasing the temperature.

This property is essential in scenarios like space science, where low-pressure environments cause life-support challenges due to the rapid loss of moisture.

Vacuum Pump

A vacuum pump is a device used to remove gases from a container, effectively reducing the pressure inside it. This concept is vital to many industrial and scientific applications where pressure control is essential.

When connected to a system with water, as the vacuum pump begins operating, it decreases the pressure. As the pressure drops, the characteristics of the system change, allowing phenomena like water boiling and freezing to occur at room temperature.

For example, when atmospheric pressure is lowered by a vacuum pump, the boiling point of water is reduced, causing it to boil at a lower than usual temperature. Likewise, by continuing to reduce the pressure, water can transition into solid, i.e., ice, demonstrating how a vacuum pump can facilitate a study of interesting phase changes with just atmospheric manipulation.

Triple Point

The triple point of a substance is a unique condition where solid, liquid, and gaseous phases coexist in thermodynamic equilibrium. For water, this occurs at a specific temperature and pressure.

It's a fundamental point on water's phase diagram and is intrinsic to defining the Kelvin temperature scale. In experiments using vacuum pumps, reaching conditions near the triple point would allow simultaneous observation of ice, water, and vapor.

Understanding the triple point helps clarify why substances behave differently when pressure and temperature vary, offering insights into their physical properties. In water, the triple point underlies phenomena like frost forming at surprising temperatures when pressure conditions are variable, such as with decompression in a vacuum environment.

Boiling Point

The boiling point of a liquid is the temperature at which its vapor pressure equals the surrounding pressure, causing it to change into vapor. In a standard atmosphere, water boils at 100 °C. However, this ensures how environmental conditions influence phase transitions.

When using a vacuum pump to reduce the pressure in a container, water can boil at a temperature far below 100 °C. This is a direct visualization of how phase diagrams function: as pressure drops, the boiling point shifts along the phase diagram, leading to interesting processes like evaporation occurring at lower temperatures.

This concept of a changing boiling point is crucial in understanding everyday and industrial processes, such as cooking at high altitudes or designing efficient cooling mechanisms for machinery.

Sublimation

Sublimation is the phase transition wherein a substance changes directly from a solid to a gas, bypassing the liquid state. Commonly observed in dry ice (solid CO2), sublimation is also possible with water under specific conditions.

In a system with a vacuum pump, water ice can sublimate because the continuous reduction in pressure facilitates the transition of ice to water vapor without melting first. Hence, if the vacuum pump is left running, the amount of ice will diminish as it turns straight into vapor due to sublimation.

Understanding sublimation is significant because it illustrates an important part of water's phase diagram and helps elucidate processes like the preservation of foods and freeze-drying, where products are preserved by removing moisture through sublimation.