What could cause crystals to form in a hot saturated sodium acetate solution?

[Camila_W]

I'm trying to follow a procedure that calls for a saturated solution of sodium acetate, but I'm getting crystals forming even while it's still hot on the stir plate. I thought a saturated solution should hold all the solute at a given temperature. Is my starting material possibly less pure than I assumed, or am I misunderstanding the definition of saturation here?

[Charles61]

I once watched a saturated solution of sodium acetate crystallize while the bath was still hot. There were a few microscopic seed crystals on the glass and some dust in the air acting as nucleation points, and the stirrer kept shoving new surface in contact with the solution. That combination can make crystals pop up even when you think you’re above the saturation point. So saturation isn’t a fixed line you cross; seeds and impurities matter, and you can have a supersaturated edge that crystallizes on disturbance.

[William_M]

Also, the thermostat on the stir plate isn’t a perfect read of the liquid temperature. Convection and surface cooling can leave portions of the solution cooler than the plate surface, but hot pockets can still exist and nucleation can happen where the liquid meets air or the glass.

[JerryFH]

Purity of the starting material can matter a lot. If the salt isn’t completely dry or has absorbed moisture or other ions, it can alter solubility and provide tiny seeds. In my experiments, drying or using fresh material changed when crystals showed up, sometimes delaying it and sometimes not.

[Camila.H]

Maybe the real issue isn’t the definition of saturation but whether you actually have seed material or dust acting as nucleation sites. If you see crystals despite heat, do you see seed material or dust in the solution?