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Solubility chemistry question

Picture an ionic compound that dissolves in water is saturated with a small amount left over
Since the solubility reaction occurs in both directions at equal rates, what's stoping the reverse reaction from forming precipitates that aren't on the surface of the lump of compound left over?
why do the ions almost always precipitate on the already solid compound causing harldy any noticable change?

I think it is somehow easier, thermodynamically,  for the solution to deposit new layers on an existing crystal, than it is to start a new crystal in a volume of solution containing no "seed crystals".

I'm not totally sure why this happens, but I know it happens often enough that there is a name for it: nucleation.
http://en.wikipedia.org/wiki/Nucleation

Also:
http://en.wikipedia.org/wiki/Seed_crystal

It happens with other phase transitions too.  E.g. water vapor condensing into water droplets in clouds, or bubbles of steam forming in a pot of boiling water. 

The other weird thing is the nucleation sites do not have to be made out of the same matter as the changing phase.  For example water droplets in clouds will condense on things besides other water droplets, like dust or subatomic particles.  Steam bubbles in a pot of boiling water usually start on the solid, usually metal or glass,  bottom of the pot.
Yes... Nucleation sites can be a micro particle ( dirt, Dust ), a smooth to
touch container wall or a sonic disturbance in the saturated fluid.
iceng5 years ago
Have you ever grown ionic water soluble crystals where material deposits
an orderly pattern as slow moisture evaporation causes increased saturation and continued deposition of material on a seed crystal.
Potassium ferricyanide grows beautiful red crystals see picture.

Also works with covalent bond materials like sugar ( Rock Candy ).

A
Crystals-Potassium ferricyanide-1-1.jpg