Forces, Solids and Liquids

You should read this chapter if you need to review or learn about  

Types of intermolecular forces Properties of liquids Surface tension Viscosity Capillary action Structures of solids Phase changes and diagrams 

Copyright 2007 by John Moore and Richard Langley. Click here for terms of use 

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The most important manifestation of intermolecular forces is the very existence of liquids and solids. Without intermolecular forces, solids and liquids would not exist and all matter would be gaseous. In liquids, we can observe several other manifestations of intermolecular forces including surface tension and viscosity. 

Intermolecular forces are present everywhere. If it were not for the attractive forces to hold the molecules together, there would be no solids and liquids the kinetic energy of the molecules would spread them all over the available space. Everything, including ourselves, would be in the gaseous state. And if it were not for the repulsive forces, solids and liquids would not resist compression. 

The existence of intennolecular interactions is apparent from elementary experimental observations. There must be attractive forces because otherwise condensed phases would not fomi, gases would not liquefy, and liquids would not solidify. There must be short-range repulsive interactions because otherwise solids and liquids could be compressed to much smaller volumes with ease. The kernel of these notions was fomuilated in the late eighteenth century, and Clausius made a clear statement along the lines of this paragraph as early as 1857 [1]. 

In a filtering centrifuge, separating sohds from liquid does not require a density difference between the two phases. Should a density difference exist between the two phases, sedimentation is usually at a much more rapid rate compared to filtration. In both cases, the solid and liquid phases move toward the bowl under centrifugal force. The sohds are retained by the filter medium, while the liqmd flows through the cake solids and the filter. This is illustrated in Fig. 18-138/ . 

Solids and liquids are called condensed phases. The attractive forces in a condensed phase, either a solid or a liquid, tend to hold the molecules close together. In liquids, molecules are irregularly spaced and randomly oriented. In a crystalline solid, the molecules occupy regular positions, resulting in additional stability (relative to the liquid). 

The Maxwell theory of X-ray scattering by stable systems, both solids and liquids, is described in many textbooks. A simple and compact presentation is given in Chapter 15 of Electrodynamics of Continuous Media [20]. The incident electric and magnetic X-ray helds are plane waves Ex(r, f) = Exo exp[i(q r — fixO] H(r, t) = H o exp[/(q r — fixO] with a spatially and temporally constant amplitude. The electric field Ex(r, t) induces a forced oscillation of the electrons in the body. They then act as elementary antennas emitting the scattered X-ray radiation. For many purposes, the electrons may be considered to be free. One then finds that the intensity /x(q) of the X-ray radiation scattered along the wavevector q is... 

The resulting model would therefore consist of component balance equations for the soluble component written over each of the many solid and liquid subsystems of the packed bed, combined with the component balance equation for the coffee reservoir. The magnitude of the recirculating liquid flow will depend on the relative values of the pressure driving force generated by the boiling liquid and the fluid flow characteristics of the system. 

The need to separate solid and liquid phases is probably the most common phase separation requirement in the process industries, and a variety of techniques is used (Figure 10.9). Separation is effected by either the difference in density between the liquid and solids, using either gravity or centrifugal force, or, for filtration, depends on the particle size and shape. The most suitable technique to use will depend on the solids concentration and feed rate, as well as the size and nature of the solid particles. The range of application of various techniques and equipment, as a function of slurry concentration and particle size, is shown in Figure 10.10. 

Intermolecular forces exist between covalent molecules in the solid and liquid states. There are two categories ... 

In the solid and liquid phases, molecules are very close to each other. This is because forces hold the molecules together in the solid and liquid states. We have already studied intramolecular bonds within molecules in the previous chapter. In this chapter, we will examine the forces of attraction between the particles in liquids and solids. 

The triple point of a substance is reached when the vapor pressure of the solid phase is equal to that of the liquid phase. If both solid and liquid are subjected to external pressure (which may be caused by capillary forces), their curves of vapor pressure versus temperature lie above those for uncompressed phases and intersect at a temperature different from the triple point. The melting point Tm observed at atmospheric pressure, as a rule, is very near to the triple point. Thus the freezing temperature Tmr of a drop of radius r should be different from Tm. 

Several techniques (SFA, TFB, AFM, MCT, etc.) have allowed very important advances in the field of surface forces between solid and liquid surfaces. Force... 

Intermolecular forces are responsible for the condensed states of matter. The particles making up solids and liquids are held together by intermolecular forces, and these forces affect a number of the physical properties of matter in these two states. Intermolecular forces are quite a bit weaker than the covalent and ionic bonds discussed in Chapter 7. The latter requires several hundred to several thousand kilojoules per mole to break. The strength of intermolecular forces are a few to tens of kilojoules per... 

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