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. 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
Pretty much the same thing happens as in unassisted nucleation, except that a
solid surface (like a stone, or brick) 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 adsorb and aggregate on the surface. This is where the
protocrystal forms, and the same process as described above happens.
You can probably see why, from what I wrote above, 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.
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