At the very beginning of this class, when looking at chemistry as the study of matter macroscopically, we briefly thought about the states of matter: solid, liquid, and gas. You then studied gases in Unit 2. In the defining picture of gases (the Kinetic Molecular Theory), gas particles are far apart from each other and moving rapidly. Kinetic energy (energy of motion) of the particles of gas is proportional to temperature.
As the gas is cooled, the individual particles move more slowly. There are attractive forces between individual particles of the gases known as intermolecular forces. If the temperature is high enough, the gas particles are moving so fact these forces have little effect. But with cooling, the particles slow, and as they pass by each other more slowly the forces begin to hold them together. If cooled sufficiently , the intermolecular forces will hold them close together (but not so closely that the particles cannot move) and the gas condenses into a liquid. In the liquid phase, the particles are held together by the intermolecular forces tightly enough that they can’t completely escape each other (so volume is constant), but loosely enough that they can move and slide relative to each other, allowing the liquid to flow and change of shape. With further cooling, the particles have even less energy of motion, and the liquid is frozen into a solid, with all particles locked into place. The individual particles in a solid vibrate slightly unless the temperature is cooled all the way to zero K.
In this chapter we will look at intermolecular forces and more closely examine the properties and behavior of liquids and solids.
