supersaturation
/ soo-per-sach-uh-RAY-shun /
Picture honey holding more dissolved sugar than it 'should' — leave it on a shelf and crystals eventually grow. A supersaturated solution is in that same tense, overloaded state: it carries more dissolved material than it can normally hold, just waiting for an excuse to dump the excess as solid.
Supersaturation is the condition where a solution temporarily contains more dissolved solute than its equilibrium solubility allows. It is unstable: the excess will precipitate, but only once something triggers the first solid to appear.
It matters in gravimetry because the degree of supersaturation at the moment of precipitation controls crystal quality. High supersaturation throws down a flood of tiny, impure crystals that are hard to filter; gentle, low supersaturation grows a few large, clean crystals — which is why analysts precipitate slowly and from dilute, warm solutions.
Dropping a single crystal into a clear, supersaturated sodium acetate solution sets off a wave of crystallization that races through the liquid until the excess solute has all become solid.
An overloaded solution unloads its excess the instant it is triggered.
Von Weimarn's rule captures the practical lesson: the finer and more numerous the crystals, the higher the supersaturation was. Keeping supersaturation low is the single most powerful lever for filterable, pure precipitates.