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

FIGURE 11.2

Representation of the states of matter In a solid, the basic units (atoms, ions, or molecules) are closely packed and vibrate about fixed sites. In a liquid, molecules are rather closely packed, similar to those in a molecular solid, but can slip by one another and are in constant random motion. In a gas, molecules are widely spaced and, similar to those in a liquid, are in constant random motion.

atoms, molecules, or ions) exist in close contact and (unlike those in a gas or liquid) do not move about but oscillate or vibrate about fixed sites. This explains the rigidity of a solid. And, of course, the compact structure explains its incompressibility. Figure 11.2 compares the gaseous, liquid, and solid states in kineticmolecular terms. Recall that gases normally follow closely the ideal gas law, PV = nRT. The simplicity of this equation is a result of the nearly negligible forces of interaction between molecules and the nearly negligible molecular size compared with the total volume of gas. Still, to be accurate we should account for both of these quantities, and the van der Waals equation, which we discussed in Section 5.8, is one attempt to do this.

(P +n²a/V²)(V - nb) _ nRT

In this equation, the constants a and b depend on the nature of the gas. The constant a depends on the magnitude of the intermolecular forces; the constant b depends onthe relative size of the molecules. No such simple equations exist for the liquid and solid states. The size of the particles making up the liquid and solid states cannot be neglected relative to the total volume or accounted for in a simple way. Similarly, the forces of interaction between particles in the liquid and the solid states cannot be neglected; indeed, these forces of interaction are crucial when you describe the properties of the liquid and solid states. Some of the most important properties of liquids and solids are concerned with changes of physical state. Therefore, we begin with this topic. In later sections, we will look specifically at the liquid and solid states and see how their properties depend on the forces of attraction between particles.