SAT Subject Test Chemistry




Liquids, Solids, and Phase Changes


Whereas particles in gases have the highest degree of disorder, the solid state has the most ordered system. Particles are fixed in rather definite positions and maintain definite shapes. Because of their variation in packing, solids can be divided into three categories: Crystalline solids have a three-dimensional representation much like a brick wall. They have a regular structure, in which the particles pack in a repeating pattern from one edge of the solid to the other. Amorphous solids (literally, “solids without form”) have a random structure, with little if any long-range order.Polycrystalline solids are an aggregate of a large number of small crystals or grains in which the structure is regular, but the crystals or grains are arranged in a random fashion.

Particles in solids do vibrate in position, however, and may even diffuse through the solid. (Example: Gold clamped to lead shows diffusion of some gold atoms into the lead over long periods of time.) Other solids do not show diffusion because of strong ionic or covalent bonds in network solids. (Examples: NaCl and diamond, respectively.)

When heated at certain pressures, some solids vaporize directly without passing through the liquid phase. This is called sublimation. Solids like solid carbon dioxide and solid iodine exhibit this property because of unusually high vapor pressure.

The temperature at which atomic or molecular vibrations of a solid become so great that the particles break free from fixed positions and begin to slide freely over each other in a liquid state is called the melting point. The amount of energy required at the melting point temperature to cause the change of phase to occur is called the heat of fusion. The amount of this energy depends on the nature of the solid and the type of bonds present.