The significance of crystals in chemistry is often overlooked. Working in an organic chemistry lab, scientists often use the process of crystallization to purify a specific compound. Crystals are often formed in nature, under similar circumstances.
Crystallization occurs when a starting material, usually a powder or some other solid state unit, is melted into a liquid. As the liquid cools, either quickly using a freezer or at room temperature, the solid most often solidifies as a crystal. The reason this phenomenon occurs is because the individual molecules that originally held together the solid compounds are rearranged to form a more stable formation, the crystal.
The melting process involves applying heat to the solid compound. Heat is energy which causes the hydrogen bonds and intermolecular forces of the molecules to become unstable. As more energy is applied to the compounds, the excitement of the molecules causes the bonds to break and the intermolecular forces of the molecules are unable to hold the compound in a solid state. As the molecules slowly cool, energy in the form of heat is lost and the molecules begin forming intermolecular attractions with each other. The intermolecular attractions are due to polarity within the individual atoms of the molecules. A partially negative atom will attract a partially positive atom in what is known as the intermolecular attraction. These very attractions are responsible for the states of matter. So as the liquid compound slowly cools, weak polar bonds are made between atoms and the compound begins to solidify. An important principle in chemistry is Gibbs free energy. Basically, all chemical activity tends to create stable products with the lowest energy. Using the idea of Gibbs free energy, we can understand why the liquid compound will reform in the most stable formation.
A crystal is the most stable formation because all the bonds are perfectly arranged in a network that strengthens the overall integrity of the molecule. A diamond, for instance, is the most stable form of pure carbon, as its molecules are tightly packed in an organized crystal.
When a mixture of different compounds is crystallized, a phenomenon known as Co-crystallization often occurs. A co-crystal is a crystal that involved the organization of 2 different molecules bonding in a 1:1 or 1:2 ratio. Co-crystallization occurs because the co-crystal formed is more stable than the 2 pure crystals that would form otherwise. This once again supports the idea that chemical reactions favor the most stable product.