How do crystals form and how do they grow?
Crystals start growing by a process called "nucleation". Nucleation can either start with the molecules
themselves (we'll call this unassisted nucleation), or with the help of some solid matter already in the solution
(we'll call this assisted nucleation).
Unassisted nucleation
When molecules of the "solute" (the stuff of which you want to grow crystals) are in solution, most of the time
they see only solvent molecules around them. However, occasionally they see other solute molecules. If the
compound is a solid when it is pure, there will be some attractive force between these solute molecules.
Positive ion and negative ions will attract to each other. Most of the time when these solute molecules meet
they will stay together for a little while, but then other forces eventually pull them apart. Sometimes though, the
two molecules stay together long enough to meet up with a third, and then a fourth (and fifth, etc.) solute
molecule.
Most of the time when there are just a few molecules joined together, they break apart. However, once there
becomes a certain number of solute molecules, a so-called "critical size" where the combined attractive forces
between the solute molecules become stronger than the other forces in the solution which tend to disrupt the
formation of these "aggregates". This when this "protocrystal" (a sort of pre-crystal) becomes a nucleation site.
As this protocrystal floats around in solution, it encounters other solute molecules. These solute molecules feel
the attractive force of the protocrystal and join in. That's how the crystal begins to grow.
It continues growing until eventually, it can no longer remain "dissolved" in the solution and it falls out (as
chemists like to say) of solution. Now other solute molecules begin growing on the surface of the crystal and it
keeps on getting bigger until there is an equilibrium reached between the solute molecules in the crystal and
those still dissolved in the solvent.
Assisted Nucleation
When molecules of the "solute" (the stuff of which you want to grow crystals) are in solution, most of the time
they see only solvent molecules around them. A solid surface (like a stone, or brick, or glass) acts as a place
for solute molecules to meet. A solute molecule encounters the surface of a stone, it adsorbs to this surface,
and stays on it for a certain time before other randomizing forces of the solution knock it off. Solute molecules
will tend to aggregate on the surface. This where this "protocrystal" (a sort of pre-crystal) becomes a nucleation
site. As this protocrystal floats around in solution, it encounters other solute molecules, and the same process
as described above happens.
You can probably see why, crystals grow fastest in a solution in which the concentration is near saturation. If
there are more solute molecules in a given volume, then there is more of a chance they will meet one another.
You also don't want to heat up the solution because that acts as the major randomizing force in solution which
causes the aggregates of molecules to break up.
Brief Overview of our crystal procedures:
Grow a Seed Crystal:
1.)
2.)
3.)
4.)
5.)
Measure out solid solute.
Add deionized water, stir to create a saturated solution with extra solid at the bottom.
Dissolve all the solid by heating to create a hot unsaturated solution.
Slowly cool to make that solution a supersaturated solution.
Add a few (3-4) pretty tiny seed crystals from the stock bottle to encourage “assisted nucleation” and discourage
“unassisted nucleation”.
6.) Nucleation occurs overnight, seed crystals form.
Re-supersaturate the solution and re-seed it
1.)
2.)
3.)
4.)
5.)
7.)
Pour off saturated solution into another beaker.
Observe & chip the seed crystals out of the beaker. Keep a few of the nicest ones.
Measure out a little solute (you can use the not-nice seed crystals you just decided not to keep).
Add the solute to the saturated solution in the beaker. Dissolve all the solid by heating to create a hot unsaturated solution.
Slowly cool to make that solution a supersaturated solution.
Hang the nicest seed crystal on a string in the center of the solution to encourage “assisted nucleation” and discourage
“unassisted nucleation”.
6.) Nucleation occurs overnight, seed crystals form.