What are crystals?
Crystals are organized groupings of atoms, or molecules. Each crystal has different shapes and properties. Sugar crystals, for example, are oblong and slanted at the ends. On the other hand, salt crystals are cubic. Some elements can make more that one crystalline form. Carbon, in the form of graphite, acts as a lubricant between moving parts, is used as a writing tool (a pencil), is an electrical conductor, and strengthens steel. As a diamond, carbon is used as an industrial cutting tool and as gemstones in all kinds of jewelry.
How do crystals form?
Crystals start growing through process called “nucleation”. Nucleation can either start with the molecules themselves (unassisted nucleation), or with the help of some solid matter already in the solution (assisted nucleation).
In unassisted nucleation, molecules of the “solute” (the stuff of which you want to grow crystals) are in solution, and most of the time they see only solvent molecules around them. Solvent molecules are the molecules in which a solute is being dissolved. Occasionally, some solute molecules 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 will pull them apart. Sometimes, the two molecules stay together long enough to meet up with a third, a fourth, and a fifth solute molecule. This process can keep going for a long time, which is why some crystal formations are very large. 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 is very similar to 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.
I hope you liked this background on crystals. Now go on and try some crystal-making experiments!