Comparison of Gases, Liquids, and Solids

These are some of the questions we will examine in this chapter. They concern certain physical properties that, as you will see, can often be related to the bonding and structure of the liquid and solid states. 

Dry ice is soiid carbon dioxide, CO2, whose temperature is about —78°C. it passes directiy from the solid to the gaseous state. The white plumes are composed of ice crystals (H20,not CO2) formed by condensation from the cold air. 

The different states of matter were defined in Section 1.4, and the gas laws and the kinetic theory of gases were defined in Chapter 5. Here we want to recall the salient features of those discussions and then compare gases, liquids, and solids. We especially want to compare how these states are viewed in terms of kinetic-molecular theory. 

Gases are compressible fluids. According to kinetic-molecular theory, gases are composed of molecules or single atoms that are in constant random motion throughout mostly empty space (unless the gas is highly compressed). A gas is easily compressed because the molecules can be pushed into a smaller space. A gas is fluid because individual molecules can move easily relative to one another. 

Liquids are relatively incompressible fluids. According to kinetic-molecular theory, the molecules of a liquid are in constant random motion (as in a gas) but are more tightly packed, so there is much less free space. Because the molecules can move relative to one another as in a gas, a liquid can flow (it is flnid). Bnt the lack of empty space explains why a liquid, unlike a gas, is nearly incompressible. 

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FIGURE 10.1 A molecular comparison of gases, liquids, and solids, (a) In gases, the particles feel little attraction for one another and are free to move about randomly, (b) In liquids, the particles are held close together by attractive forces but are free to move over one another, (c) In solids, the particles are rigidly held in an ordered arrangement. 

SECTION 11.1 A Molecular Comparison of Gases, Liquids, and Solids 427... 

MOLECULAR COMPARISONS OF GASES, LIQUIDS, AND SOLIDS (section 11.1)... 

A MOLECULAR COMPARISON OF GASES, LIQUIDS, AND SOLIDS We begin with a comparison of solids, liquids, and gases from a molecular perspective. This comparison reveals the important roles that temperature and intermolecular forces play in determining the physical state of a substance. 

As we learned in Chapter 10, the molecules in a gas are widely separated and in a state of constant, chaotic motion. One of the key tenets of kinetic-molecular theory is the assumption that we can neglect the interactions between molecules. (Section 10.7) The properties of liquids and solids are quite different from gases largely because the intermoiecuiar forces in liquids and solids are much stronger. A comparison of the properties of gases, liquids, and solids is given in TABLE 11.1. 

For a simple calculation based on the Lindemann criterion and comparison with experiment see for example Tabor, D. Gases, Liquids and Solids, 1969, Penguin Books, Baltimore, p. 207. The calculation assumes harmonic vibration of the particles, against Young s modulus. Melting temperatures are calculated with reasonable accuracy for metals and even for quartz, but not for organic molecules. 

The spectra of liquids and solids are known to have strong induced components. Liquids and solids are, however, so dense that many-body terms dominate the spectra the binary and ternary spectral components which are the main topic of this work (and which are usually measurable in compressed gases at densities much lower than liquid state) will often resemble the spectra of liquids and solids, but a critical comparison will reveal important qualitative and quantitative differences. Nevertheless, a study of binary spectra will help to illuminate important aspects of the theoretical descriptions of liquid spectra and may be considered a basic input into the theory of liquid interactions with radiation. 

The capillary condensation phenomenon was discovered by Zsigismody [139], who investigated the uptake of water vapour by silica materials. Zsigismody proved that the condensation of physicosorbed vapours can occur in narrow pores below the standard saturated vapour pressure. The main condition for the capillary condensation existence is the presence of liquid meniscus in the adsorbent capillaries. As it is known, the decrease of saturated vapour pressure takes place over the concave meniscus. For cylindrical pores, with the pore width in the range 2-50 nm, i.e., for the mesopores, this phenomenon is relatively well described by the Kelvin equation [14]. This equation is still widely applied for the pore size analysis, but its main limitations remain unresolved. Capillary condensation is always preceded by mono- and/or multilayer adsorption on the pore walls. It means that this phenomenon plays an important, but secondary role in comparison with the physical adsorption of gases by porous solids. Consequently, the true pore width can be assessed if the adsorbed layer thickness is known. 

One characteristic property of a gas is its compressibility—its ability to be squeezed into a smaller volume by the application of pressure. By comparison, liquids and solids are relatively incompressible. The compressibility of gases was first studied... 

A pyrolysis technique was investigated as a method for the chemical recycling of glass fibre-reinforced unsaturated polyester SMC composites. The proeess yielded liquid products and gases and also a solid residue formed in the pyrolysis of glass fibres and fillers. The solid residue was used as a reinforeement/filler in unsaturated polyester BMC composites, and the influenee on mechanical properties was studied in comparison with BMC prepared entirely from virgin materials. 

One of the consequences of close packing in solids and liquids is much higher densities in comparison with gases for instance, ice and water have densities that are a thousand times higher than water vapor at room pressure. Another consequence is that solids and liquids have much lower compressibility, so that the density is not sensitive to the pressure. The bulk modulus B is defined as B = —AP/ AV/V), which has the units of pressure. This parameter measures the fractional volumetric response of a material, when pressure is applied to all faces of the material at the same time. 

The values for pure solids and liquids are usually omitted from the K expression because their concentrations vary only slightly. Additionally, the concentrations of pure solids and liquids are extremely large in comparison to the concentrations of gases and dissolved materials (normally, as measured in mol/L). This includes situations when the solvent is one of the products or reactants, as in the following, the hydrolysis of urea in aqueous solution ... 

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