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14
Properties oí Pure Fluids

At relatively large molal volumes, where the distances between molecules are great, the pressure-volume-temperature relations of gases are given ,Úth satisfactoryaccuracy by the ideal-gas law. Since low pressure and high temperature tend to increase molal volume, decrease of pressure and increase of temperature tend to make a gaseous material approach idealbehavior. If the molal volume is low, the assumption of ideal-gas behavior may cause errors as great as 500%. Actual Behavior of Gases. The actual pvT relationships of carbon dioxide are shown by the solidlines in Fig. 132. Each curve represents the relationship between the pressure and the molal volume of carbon dioxide at a fixed indicated temperature. The critical point is indicated by C. Thedouble-crosshatched area represents the region of the liquid state. The plain area is the region of a homogeneous fluid which at low pressures is .recúgnized as a gas but at pressures above the criticalhas continu-ity with the liquid state along the cTitical isotheTrn. The single-crosshatched area represents a region in which both liquid and gaseous carbon dioxide are present in equilibrium with eachother. The line ACB is designated as the satumtion 01' co-existent line. Thus, following along the 21.50 e experimental isotherm an increasing pressure is required to cause a reduction in volume untilthe scttumtion CUTveCDB is reached. At this point D, the attractive forces between the molecules become sufficiently great to start condensation. Upon removal of heat at constant temperature, thevolume ofthe combined phases is diminished without further increase in pressure until the curve CEA is reached at point E. This represents completion of condensation to theliquid state. A furtherincrease of pressure is accompanied by a relatively small decrease in volume. Experimentation has demonstrated the following general"b~havior of pure fluids. l. As the pressure of a gas is reduced at...