Some liquid water is placed in a room-temperature container and sealed. After some time, the level of the liquid drops by a small amount. After that, the water level remains constant.

In the beginning, some liquid water is placed in a room-temperature container. The container has a fixed volume, and the water level is constant initially. The system is in a not fully equilibrium state as there is a transition between the liquid and gas phase.

The liquid water molecules possess a range of energies. Some molecules will have enough energy to overcome the intermolecular forces that hold them together in the liquid state. When this happens, these molecules escape from the liquid surface to enter the gas phase, which is called evaporation. This transition causes a decrease in the liquid water level.

If the container were not sealed, the escaped water molecules would leave the container and disperse into the surrounding environment. Since the container is sealed in this case, the water molecules cannot escape, and they remain in the container's space above the liquid surface.

The escaped water molecules will collide with the liquid surface and can get trapped by the intermolecular forces again, returning to the liquid phase. This process is called condensation. As more and more water molecules evaporate and enter the gas phase, the rate of condensation increases. Eventually, the rate of evaporation will equal the rate of condensation, and the system reaches a dynamic equilibrium. At this point, no net change occurs in the amount of liquid water, so the water level remains constant.

This phenomenon can be explained using the concept of vapor pressure, which is the pressure exerted by vapor in equilibrium with its corresponding liquid at a given temperature. In the sealed container, the water vapor pressure rises until it reaches a value that corresponds to the liquid water's vapor pressure at room temperature. Once this happens, the equilibrium is established, and the water level remains constant. In conclusion, the initial decrease in the liquid water level is due to evaporation, while the eventual constant water level results from the establishment of a dynamic equilibrium between evaporation and condensation in the sealed container. This equilibrium is governed by the vapor pressure of the liquid at room temperature.