Which of the following represents a metastable system? 

A supersaturated solution

The correct answer is option 4. A supersaturated solution.

A metastable system is one that is in a temporary state of equilibrium, appearing stable for a period but capable of transitioning to a more stable state upon disturbance. Here’s a detailed explanation of why a supersaturated solution represents a metastable system:

Metastable State: In this state, the system is not in its lowest energy configuration but remains in its current state because it requires an energy input or disturbance to transition to a more stable state.

Analysis of Options:

1. Unsaturated Solution: Contains less solute than the solvent can dissolve.

Stable: More solute can be added without any issues. There’s no tendency for the system to change spontaneously.

2. Dilute Solution:Contains a small amount of solute relative to the solvent.

Stable: Like an unsaturated solution, it’s in equilibrium and doesn’t change unless more solute is added or solvent is evaporated.

3. Saturated Solution: Contains the maximum amount of solute that can dissolve at a given temperature and pressure.

Stable: It is in equilibrium with any undissolved solute, and no more solute can dissolve unless conditions change (e.g., temperature).

4. Supersaturated Solution: Contains more solute than the solvent can theoretically dissolve at a given temperature and pressure.

Metastable: This state is achieved by dissolving solute at a higher temperature and then cooling the solution without disturbing it. The solution appears stable, but it’s not in true equilibrium. A small disturbance, such as adding a seed crystal or shaking the container, can cause rapid crystallization of the excess solute, transitioning the solution to a saturated state.

Why Supersaturated Solutions are Metastable:

Formation: Supersaturated solutions are typically formed by dissolving solute at a high temperature and then carefully cooling it. As the solution cools, it holds more solute than it normally would at that lower temperature.

Apparent Stability: The solution appears stable because the excess solute remains dissolved due to the lack of nucleation sites for crystallization to begin.

Trigger for Transition: Any disturbance, such as adding a small crystal of the solute, shaking, or even scratching the container, provides a nucleation site for the excess solute to crystallize out. This rapid crystallization demonstrates the system’s transition from the metastable supersaturated state to a more stable saturated state.

Conclusion: The supersaturated solution is metastable because it is not in true equilibrium and can readily transition to a more stable state when disturbed.