A liquid is in equilibrium with its vapour in a sealed container at a fixed temperature. The volume of the container is suddenly increased. ?

When a liquid is in equilibrium with its vapor in a sealed container, it means that the rate of evaporation of the liquid equals the rate of condensation of the vapor. This dynamic balance is influenced by temperature and pressure. Now, if the volume of the container is suddenly increased, several interesting changes occur in the system.

The Impact of Volume Increase on Equilibrium

To understand what happens, let’s break it down step by step:

1. Initial Conditions

In the initial state, the liquid and vapor are in equilibrium. The vapor pressure is established based on the temperature and the amount of liquid present. The molecules of the liquid are constantly evaporating into the vapor phase, while vapor molecules are condensing back into the liquid.

2. Sudden Volume Expansion

When the volume of the container is suddenly increased, the immediate effect is a decrease in the pressure of the vapor phase. This is because the same number of vapor molecules now occupies a larger space, leading to a lower concentration of vapor molecules per unit volume.

3. Shift in Equilibrium

According to Le Chatelier's principle, when a system at equilibrium is subjected to a change in conditions, the system will adjust to counteract that change. In this case, the decrease in vapor pressure due to the increased volume will disturb the equilibrium. To restore balance, the system will respond by favoring the evaporation of more liquid into vapor. This means that more liquid will evaporate until a new equilibrium is established.

4. Temperature Considerations

It’s important to note that this process occurs at a fixed temperature. The temperature remains constant, so the vapor pressure of the liquid at that temperature is also constant. However, the system will adjust the amount of liquid and vapor present to reach a new equilibrium state.

5. Final Outcome

Ultimately, after the volume increase, you will find that there is more vapor present than before, as the system has compensated for the drop in pressure by increasing the amount of vapor through additional evaporation. The liquid level may decrease slightly as a result of this increased evaporation, but the temperature and the vapor pressure at that temperature will remain unchanged.

Real-World Analogy

Think of this scenario like a balloon filled with air. If you suddenly stretch the balloon (increase the volume), the air inside becomes less concentrated, and to balance things out, more air will try to fill the space until a new equilibrium is reached. Similarly, in our liquid-vapor system, the liquid will evaporate more to fill the increased volume with vapor.

This dynamic interplay between liquid and vapor phases illustrates fundamental principles of thermodynamics and equilibrium, showcasing how systems respond to changes in their environment. Understanding these concepts is crucial in fields such as chemistry, physics, and engineering, where phase changes and equilibrium play significant roles in various processes.