Question

Water in an open beaker evaporates over time. As the water is evaporating, is the vapor pressure increasing, decreasing, or staying the same? Why?

Short answer

The vapor pressure of water in an open beaker during evaporation remains constant. This is because vapor pressure depends on the equilibrium between the liquid phase and the vapor phase. In an open beaker, water molecules in the liquid phase are just as likely to return to the liquid as water molecules in the vapor phase are to evaporate, maintaining a constant vapor pressure at a given temperature.

Step-by-step solution

Define Vapor Pressure

Vapor pressure is the pressure exerted by the vapor of a substance when it is in equilibrium with its liquid or solid form. In other words, it is the pressure at which the molecules of a substance escape from the liquid phase and become vapor. Vapor pressure depends on temperature; it increases as temperature increases.

Understand the Evaporation Process

Evaporation is the process by which molecules of a liquid escape the surface of the liquid and become vapor. This process occurs when individual liquid molecules possess sufficient energy to overcome the intermolecular forces holding them within the liquid. As molecules escape, the remaining liquid loses some of its most energetic molecules, leading to a decrease in temperature. This is why evaporative cooling occurs.

Analyze the Open Beaker Scenario

In an open beaker, water is allowed to evaporate freely. As water evaporates, the liquid level in the beaker decreases, and the concentration of water molecules in the vapor phase increases. However, since the beaker is open to the environment, water vapor molecules can escape into the surrounding area, thus preventing the accumulation of water vapor above the liquid's surface.

Determine the Effect on Vapor Pressure

Since the water in the open beaker is continually evaporating, the concentration of water molecules in the vapor phase is always changing. However, as the vapor pressure depends on the equilibrium between the vapor and the liquid phase, it remains constant at a given temperature. At any given temperature, water molecules in the liquid phase are just as likely to return to the liquid as water molecules in the vapor phase are to evaporate. Therefore, the vapor pressure of water remains the same throughout the evaporation process, as long as temperature remains constant.

Conclusion

In summary, the vapor pressure of water in an open beaker during evaporation remains the same. This is because vapor pressure is determined by the equilibrium between the liquid phase and the vapor phase, and the free exchange of water molecules between these two phases in an open beaker environment maintains a constant vapor pressure at a given temperature.