Phases of Matter

Matter ordinarily exists in four phases: solid, liquid, gas, and plasma. Plasma is a high-temperature phase not encountered in the typical organic chemistry laboratory and thus not considered in this book. Most pure chemical compounds exist in each of these phases. The phase of a compound at a particular moment depends on its temperature and pressure. For example, water at 1 atmosphere pressure is a solid below 0oC, a liquid from 0oC to 100oC, and a gas above 100oC. The temperatures at which a compound makes the transitions between phases are unique for each compound. These temperatures are a measure of some of the unique physical properties of that compound.

Because you have studied solids, liquids, and gases from your earliest years of school, you already know many facts about them. The goal for this section is to look at solids, liquids, and gases from the

organic chemistry point of view—to cover their characteristics and to define any terms that are important in organic chemistry.

Consider a hypothetical ideal compound as it makes its transition through each phase. For simplicity's sake, assume that each molecule of this compound is spherical. A sphere is a simple shape that is easy to visualize with minimal intermolecular interactions (interactions between molecules)

Starting at some low temperature, the compound is a solid. There are two extremes of solid forms: the crystalline form and the amorphous form. Because nearly all solid organic compounds are in the crystalline form, this book does not discuss the amorphous form. Spheres pack easily into an ordered matrix, thus, the idealized solid is a crystal. Envision this crystalline form by considering a box full of identically sized marbles. The box’s size is such that one layer of marbles fits exactly in the bottom. The consecutive layers of marbles then fit on top of each other in an ordered set of layers. Figure 4.1 illustrates such a regular ordered matrix of molecules in a crystalline solid.

The intermolecular attractive forces hold the molecules in place in the crystal lattice. The stronger these forces are, the greater the en-ergy that is required to break down the crystal lattice. When you apply sufficient heat to a solid, the crystalline structure of that solid breaks down.

As you heat a solid, it undergoes a transition from the solid phase to the liquid phase. Chemists call this transition fusion and the characteristic temperature at which a compound undergoes fusion its melting point. In the liquid phase the molecules are no longer packed in an ordered matrix. There is more disorder, but still a definite boundary between the liquid and its surroundings. Figure 4.2 illustrates the liquid phase using the idealized compound.

As you continue to increase the temperature, the compound undergoes another transformation. It moves from its liquid phase to its gas phase. Chemists call the process of going from the liquid phase to the gas phase vaporization. The characteristic temperature at which a compound undergoes vaporization is its boiling point. In its gas phase a compound has even more disorder than it has in its liquid phase. There is little intermolecular interaction because the molecules now have enough energy to counteract these forces. Since there is so little intermolecular interaction, the molecules move freely, filling all the available space. There is no obvious boundary between the molecules of the gas and its surroundings. Figure 4.3 illustrates the gas phase using the idealized compound.