Of state. Contents and Concepts 1. Comparison of Gases, Liquids, and Solids Changes of State

FIGURE 11.3

Sublimation of iodine

Left: The beaker contains iodine crystals, I2; a dish of ice rests on top of the beaker. Right: Iodine has an appreciable vapor pressure even below its melting point (114_C); thus, when heated carefully, the solid sublimes without melting. The vapor deposits as crystals on the cool underside of the dish.
11 States of Matter; Liquids and Solids

When a substance that is normally a gas, such as carbon dioxide, changes to the

liquid state, the phase transition is often referred to as liquefaction. The following

diagram summarizes these phase transitions.

Vapor Pressure

Liquids, and even some solids, are continuously vaporizing. If a liquid is in a closed vessel with space above it, a partial pressure of the vapor state builds up in this space. The vapor pressure of a liquid is the partial pressure of the vapor over the liquid, measured at equilibrium at a given temperature. To understand what we mean by equilibrium, let us look at a simple method for measuring vapor pressure. You introduce a few drops of water from a medicine dropper into the mercury column of a barometer (Figure 11.4A). Being less dense than mercury, the water rises in the tube to the top of the mercury, where it evaporates, or vaporizes. To understand the process of vaporization, it is necessary to realize that the molecules in a liquid have a distribution of kinetic energies (Figure 11.5). Those molecules moving away from the surface and toward the vapor phase will escape only if their kinetic energies are greater than a certain minimum value equal to the potential energy from the attraction of molecules in the body of the liquid.