Supersaturated solutions are not stable primarily because they hold more dissolved solute than is naturally possible under typical conditions.
The Instability of Supersaturated Solutions
According to the provided reference, supersaturated solutions are highly unstable because the amount of solute that is contained in a supersaturated solution exceeds the natural quantity of solute that should be dissolved. This means they exist in a fragile state, having dissolved an amount of solute beyond their standard solubility limit at that specific temperature and pressure.
Think of it like stacking too many blocks in a tower – the higher you go beyond the stable base, the more likely it is to fall.
Why the Excess Solute Causes Instability
- Beyond Equilibrium: A saturated solution represents an equilibrium point where the rate of dissolving solute equals the rate of solute crystallizing out. A supersaturated solution has pushed past this equilibrium, forcing more solute into solution than the solvent can comfortably hold.
- High Potential Energy: The excess solute molecules are essentially 'uncomfortable' in the dissolved state and have a higher potential energy compared to their stable, crystalline form.
- Tendency to Crystallize: This excess solute has a strong tendency to return to its more stable solid state.
What Happens When a Supersaturated Solution is Disturbed?
Because of this inherent instability and the presence of excess dissolved solute, supersaturated solutions are very sensitive. The reference notes that the excess solute readily crystallizes if the solution is disturbed.
Disturbances can include:
- Adding a seed crystal (a tiny crystal of the solute). This provides a surface for the excess solute to crystallize onto.
- Slight cooling.
- Shaking or stirring.
- Introducing impurities like dust particles.
These disturbances provide the necessary nucleation points or energy fluctuations for the crystallization process to begin, rapidly causing the excess solute to precipitate out of the solution until it reaches a stable, saturated state.
Key Takeaway: The core reason for the instability is the presence of excess dissolved solute that exceeds the equilibrium solubility limit, making the solution poised for crystallization upon disturbance.
