SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 7

Liquids, Solids, and Phase Changes

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 form called a crystal. The smallest portion of the crystal lattice that is repeated throughout the crystal is called the unit cell. Samples of unit cells are shown in Figure 33.

The crystal structure can also be classified by its internal axis, as shown in Figure 34.

Figure 33. Kinds of Unit Cells

Figure 34. Crystal Structure Classified by Internal Axis

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 · 5H2O; CaSO4 · 2H2O; and Na2CO3 · 10H2O. (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 (without water) powder. The dehydration of hydrated CuSO4 serves as a good example since the hydrated crystals are deep blue because of the copper ions present with water molecules. When this water is removed, the structure crumbles into the anhydrous white powder. Some hydrated crystals, such as magnesium sulfate (Epsom salt), lose the 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.