- A saturated solution is in equilibrium at the solubility limit.
- A supersaturated solution contains more dissolved solute than the equilibrium limit, making it a metastable state.
- Comparing the options reveals that 'the normal solubility is exceeded' correctly describes this state.
- Therefore, the correct option is \(\boxed{B}\).
Explanation
- Defining Saturated and Supersaturated Solutions
A solution is a homogeneous mixture of two or more substances. Solubility is the maximum amount of solute that can dissolve in a specific amount of solvent at a given temperature and pressure. A saturated solution is one in which the solvent has dissolved the maximum possible amount of solute at that temperature, meaning the dissolved solute is in dynamic equilibrium with any remaining undissolved solute. A supersaturated solution is a metastable state that contains a higher concentration of dissolved solute than the equilibrium solubility limit for that specific temperature and pressure. It is essentially "over-full," holding more solute than it normally would be able to hold under standard stable conditions.
- Evaluating the Options
Now, we evaluate each of the given options:
- Option A: "the solvent has become the solute." This is incorrect. The chemical identities of the solvent (the medium) and the solute (the substance dissolved) do not swap roles.
- Option B: "the normal solubility is exceeded." This aligns perfectly with the definition of a supersaturated solution, where the concentration of dissolved solute is greater than what is normally allowed at equilibrium.
- Option C: "more solute than solvent is present." This is not a requirement. Supersaturation refers to exceeding the equilibrium saturation point, not necessarily having a higher mass of solute than solvent.
- Option D: "the dissolved and undissolved solute are in equilibrium." This describes a saturated solution, not a supersaturated one. In a supersaturated solution, the system is unstable and not in equilibrium with undissolved solid.
- Final Conclusion
Based on the comparison, Option B correctly identifies the defining characteristic of a supersaturated solution. Since a supersaturated solution exists in a state that exceeds the standard solubility limit, it is accurate to say that the normal solubility is exceeded in such a state. Therefore, the correct choice is B.
Examples
Think of a supersaturated solution like a crowded elevator. Normally, the elevator (solvent) can only hold a certain number of people (solute) comfortably, which is its capacity (solubility). If you force more people into the elevator beyond its comfortable limit—making it a 'supersaturated' elevator—it becomes an unstable situation where the slightest movement might cause everyone to spill out, just as a slight disturbance (like adding a 'seed' crystal) causes excess solute to crystallize out of a supersaturated solution.