It's great that you're diving into Le Chatelier's Principle and its application to phase equilibria! Let's clarify your question about the equilibrium between solid water (ice) and liquid water, represented as H2O (s) ⇌ H2O (l). You mentioned that the correct answer is (b), which states that more ice melts if the pressure on the system is increased. Let's break this down step by step.
Understanding the Equilibrium
In the equilibrium reaction H2O (s) ⇌ H2O (l), we have two phases of water: solid (ice) and liquid. At equilibrium, the rate of melting ice equals the rate of freezing liquid water. This means that the system is balanced, but it can still respond to changes in conditions, such as pressure.
Le Chatelier's Principle Explained
Le Chatelier's Principle states that if an external change is applied to a system at equilibrium, the system will adjust itself to counteract that change and restore a new equilibrium. This principle is often used to predict how a system will respond to changes in concentration, temperature, and pressure.
Effect of Pressure on the System
Now, let's focus on pressure. In the case of H2O (s) and H2O (l), both phases are present in equal amounts in terms of moles. However, the key factor here is that the solid phase (ice) occupies more volume than the liquid phase (water). When pressure is increased, the system will respond by favoring the phase that occupies less volume to minimize the effect of that pressure increase.
Why More Ice Melts
When you increase the pressure, the equilibrium shifts to the right, favoring the formation of liquid water. This means that more ice will melt into liquid water, which is why option (b) is correct. The system is trying to reduce the volume by converting solid into liquid, even though the number of moles remains the same. This is a crucial point: while the number of moles is the same on both sides, the volume occupied by the solid is greater than that of the liquid.
Addressing Your Doubt
You mentioned that since the number of moles is the same, option (a) should be correct. However, it's important to remember that Le Chatelier's Principle does not only consider the number of moles but also the volume that each phase occupies. In this case, the solid phase has a larger volume than the liquid phase, which is why the system responds to increased pressure by melting more ice.
Conclusion
In summary, when pressure is increased in the equilibrium H2O (s) ⇌ H2O (l), the system shifts to favor the liquid phase, resulting in more ice melting. This is a fascinating example of how physical changes can influence chemical equilibria, and it highlights the importance of considering both moles and volume when applying Le Chatelier's Principle. If you have any more questions or need further clarification, feel free to ask!
