Problem 9 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following represents a metastable system? (a) an unsaturated solution (b) a dilute solution (c) a saturated solution (d) a supersaturated solution

Step-by-Step Solution

Answer

A supersaturated solution (d) is metastable, as it can precipitate with slight disturbance.

Step 1: Understanding the Concepts

A metastable system is not in the thermodynamically most stable state but can persist for a significant time. It's often in a temporary state that will change upon slight disturbance.

Step 2: Analyze Each Option

Consider each option based on the definition of a metastable system: - (a) An unsaturated solution is stable because it can dissolve more solute. - (b) A dilute solution is stable for the same reason as the unsaturated solution. - (c) A saturated solution is stable as it is in equilibrium with undissolved solute. - (d) A supersaturated solution contains more dissolved solute than a saturated solution and is thus metastable, as any disturbance can cause precipitation.

Step 3: Identify the Metastable System

Based on the analysis, a supersaturated solution represents a metastable system because it is not at its most stable state and a slight disturbance can cause it to change to a more stable state (i.e., precipitate to a saturated solution).

Key Concepts

Supersaturated Solution

A supersaturated solution occurs when a solution contains more solute than it can ideally hold at a given temperature. Imagine dissolving sugar in water; once it reaches the point where no more sugar can dissolve, it is saturated. If you heat this solution, allowing more sugar to dissolve, and then let it cool without disturbance, you have a supersaturated solution.
These solutions are fascinating because they hold more dissolved particles than typical circumstances would allow. This is why they are considered metastable. Despite being in a sensitive state, they appear stable until disturbed, making them intriguing studies in solution chemistry.

• Supersaturation is achieved typically through changing temperatures or evaporating the solvent.
• Even a minor disturbance, like shaking or adding a tiny particle, can force the excess solute to crystallize.
• This process is often used in industries for crystallization processes to obtain purified substances.

Chemical Equilibrium

Chemical equilibrium in solutions is the state where the concentration of dissolved and undissolved solutes remains constant over time. At this point, processes such as dissolving and crystallization occur at equal rates.
In the context of saturated solutions, equilibrium is reached where no more solute can be dissolved in the solvent at a specific temperature and pressure. For supersaturated solutions, they have surpassed this equilibrium, making them delicate or unstable.
Equilibrium in saturated solutions can be disturbed by:

• Changing the temperature, which often affects the solubility of the solute.
• Altering pressure, especially in the case of gases in liquid solutions.
• Adding more solute or solvent, which shifts the equilibrium state.

Understanding these principles is crucial as they underpin many chemical processes and reactions.

Solution Stability

Solution stability refers to a solution's ability to remain homogeneous and unchanged over time without external influences. It's about maintaining a balance where dissolved particles remain consistently suspended within the solvent.

• Saturated solutions are stable because they are at equilibrium, with no net movement of solute between the dissolved state and any undissolved particles.
• Supersaturated solutions, however, are on the edge of instability; they remain seemingly at rest until triggered to form precipitate, indicating a shift to a stable state.
• Factors such as temperature, impurity presence, and mechanical disturbances can influence this stability.

In practice, maintaining solution stability is critical in industries like pharmaceuticals, where predictable solution behavior ensures product consistency and efficacy.
Understanding solution stability helps in anticipating changes that may occur when solutions are exposed to environmental changes.