why do crystal form?

Answer 1

A crystal lattice forms when ionic compounds form together. The different reactions of the elements are what form the crystal lattice.

Answer 2

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.