Crystallization

Many substances form a repeated pattern structure as they come out of solution. The structure is bounded by plane surfaces that make definite angles with each other to produce a geometric from called a crystal. The smallest portion of the crystal lattice that is repeated throughout the crystal is called the unit cell.

A substance that holds a definite proportion of water in its crystal structure is called a hydrate. The formulas of hydrates show this water in the following manner: CuSO4 * 5 H20; CaSO4 * 2 H2O; and Na2CO3 * 10 H2O (the * is read as "with") When these crystals are heated gently, the water of hydration can be forced out of the crystal and the structure collapses into an anhydrous white powder. Some hydrated crystals, such as magnesium sulfate lose water of hydration on exposure to air at ordinary temperatures. They are said to be efflorescent. Other hydrates, such as magnesium chloride and calcium chloride, absorb water from the air and become wet. They are said to be deliquescent, or hydroscopic. This property explains why calcium chloride is often used as a drying agent in laboratory experiments.