Q.You are provided with a solid 'A' and three solutions of A dissolved in water - one saturated, one unsaturated, and one super saturated. How would you determine which solution is which?

Step 1. Definitions. A saturated solution holds the maximum amount of solute that can dissolve at that temperature; an unsaturated solution holds LESS than that maximum (it can still dissolve more solute); a supersaturated solution holds MORE solute than the normal saturation limit -- an unstable, non-equilibrium state.

Step 2. The test. Add a small amount (a seed crystal) of solid A to each of the three solutions and observe:

• In the unsaturated solution, the added crystal dissolves completely -- the solution can still take up more solute.
• In the saturated solution, the added crystal neither dissolves further nor grows -- it simply sits at equilibrium with the already-dissolved solute (a very close look may show it stays essentially unchanged in size).
• In the supersaturated solution, the added crystal acts as a seed and triggers RAPID, visible crystallisation -- excess dissolved solute suddenly comes out of solution as solid, until the solution drops back to the true saturation concentration. …

Recall that a supersaturated solution is metastable and will crystallise dramatically when seeded, while an unsaturated solution simply keeps dissolving more solute, and a saturat …

• Trying to distinguish the three purely by appearance (all three look like clear solutions) instead of using the seed-crystal (nucleation) test, which is the standard practical method. …