There are two types of solids. They are crystalline and amorphous. They have differences that are very important in the study of Physical Pharmacy. They differ in the way they are arranged, their melting points, and the way they should be treated when used in making drugs.
Crystalline solids are arranged in fixed geometric patterns or lattices. Examples of crystalline solids are ice, methanol, and sodium chloride. They have an orderly arranged units and are practically incompressible. Crystalline solids also show a definite melting point and so they pass rather sharply from solid to liquid state. There are various crystalline forms which are divided into six crystal systems or shapes. They are cubic, tetragonal, hexagonal, rhombic, monoclinic, and triclinic. The units that constitute these systems can be atoms, molecules, or ions. Ionic and atomic crystals are hard and brittle with high melting points. Molecular crystals are soft and have low melting points. Metallic crystals are composed of positively charged ions in a field of electron gas or freely moving electrons. Metals are good conductors of electricity because of the free movement of electrons in the lattice.
Amorphous solids are solids with random unoriented molecules. Examples of amorphous solids are glass and plastic. They are considered supercooled liquids in which the molecules are arranged in a random manner some what as in the liquid state. Amorphous solids also unlike crystalline solids do not have definite melting points.
The difference between an amorphous and crystalline solid is very important in drug making. When making a drug in solution, the drug is added to the other chemicals to prolong the shelf life. When the drug is crystallizing, if it forms a crystalline solid, there is space in the crystal for the ice to come out leaving the drug and the components. This process only takes about two or three days. If the drug forms an amorphous solid during the crystallizing phase then it takes about seven days. This is because amorphous solids do not have space for the ice to come out during the freezing therefore the ice must diffuse out. Therefore it is preferable to have crystalline solids in drug making.