The water in a beaker of water left in a room will slowly evaporate until the beaker is dry. However, place that same beaker in a sealed box and the water level in the beaker will drop a bit but then remain constant. Is the latter case an example of equilibrium? Explain your answer.

Evaporation is a surface phenomenon in which molecules of a liquid gain enough energy to transition into the gaseous state. This process occurs when the individual molecules of a substance, like water, absorb energy from their surroundings, typically in the form of heat. Once they reach a certain energy threshold, these molecules are able to overcome the attractive forces holding them in the liquid and escape into the air as vapor.

It's important to understand that evaporation is a cooling process, as the highest energy molecules are the ones most likely to evaporate, leaving the remaining liquid with a lower average kinetic energy and thus a lower temperature. An everyday example of evaporation is the drying of wet clothes in the air. The rate at which evaporation happens depends on several factors, including temperature, surface area, humidity, and air movement.

Condensation is the opposite process of evaporation, where water vapor in the air loses energy and changes back into liquid form. This occurs when the temperature of the vapor falls below its dew point, or when it comes into contact with a surface that is cooler than the air around it. In a sealed system, condensation plays a critical role in achieving equilibrium.

One familiar example of condensation is the formation of dew on grass in the early morning. As the grass cools overnight, it cools the air around it to below the dew point, causing water vapor in the air to condense into tiny droplets on the surface of the grass. Inside a closed system, the condensed vapor returns to the liquid state, thereby preventing the loss of molecules that would otherwise result in a dry beaker.

A closed system is a concept critical to understanding chemical equilibrium. In such a system, matter is not allowed to enter or leave, although energy in the form of heat or work may be exchanged with the surroundings. This containment of matter is what enables the establishment of equilibrium, as seen in the example of a beaker of water in a sealed box.

The concept of a closed system is key in many scientific and engineering disciplines. In thermodynamics, it helps us understand how energy is conserved and transferred. In our beaker example, the contained water vapor can't escape, and so the rates of evaporation and condensation eventually balance each other out—no net loss of water occurs, and the water level stabilizes, illustrating the dynamic balance of a system in equilibrium.