专业英语第三章概要1.ppt

* * Unit 3 Structure and Properties 3.1 Introduction Except for air, most of the substances that you encountered are in a liquid or solid state. The mathematical treatment of solids and liquids, unlike that of gases, is complicated by the fact that their molecules (or ions or atoms) are close together and thus the forces of attraction or repulsion among them cannot be ignored. Because of the order in solids, it is possible to evaluate the forces operating among their constituent particles. The ions, atoms, or molecules of solids are present in a regular, unchanging arrangement. The study of liquids is complicated by the fact that they do not have the order present in solids, and although their molecules are in constant motion, the motion is not random as it is in gas. Thus theories to describe the behavior of liquids are particularly difficult to develop. 3.2 Vapor Pressure The term Vapor is used instead of gas to refer to the gaseous state of substances which are not gases under ordinary conditions. Liquids produce vapor when energetic molecules at the liquid surface escape into the gas phase. This is evaporation (or vaporization). The vapor pressure of a liquid is the pressure exerted by the gas molecules above a liquid, molecules that are produced from evaporation of the liquid. Liquids that evaporate easily have relatively high vapor pressures and are said to be volatile. It is also possible for solids to exert vapor pressures when solid molecules become energetic enough to escape from the solid phase to the gas phase. This process is called sublimation. The vapor pressure of a solid can be used predict whether of not it could have a detectable odor. For solids to have odors they must be volatile. 3.3 Distillation Distillation is the process of first vaporizing and then condensing (liquefying) the liquid components of a mixture to separate them from each other or from the solid components which may be present.