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The compressibility factor (Z), also known as the compression factor, is the ratio of the molar volume of a gas to the molar volume of an ideal gas at the same temperature and pressure. It is a useful thermodynamic property for modifying the ideal gas law to account for the real gas behavior.〔(Properties of Natural Gases ). Includes a chart of compressibility factors versus reduced pressure and reduced temperature (on last page of the PDF document)〕 In general, deviation from ideal behavior becomes more significant the closer a gas is to a phase change, the lower the temperature or the larger the pressure. Compressibility factor values are usually obtained by calculation from equations of state (EOS), such as the virial equation which take compound specific empirical constants as input. For a gas that is a mixture of two or more pure gases (air or natural gas, for example), the gas composition must be known before compressibility can be calculated. Alternatively, the compressibility factor for specific gases can be read from generalized compressibility charts〔 that plot as a function of pressure at constant temperature. ==Definition and physical significance== The compressibility factor is defined in thermodynamics and engineering frequently as: : where is the density of the gas and is the specific gas constant,〔 page 327〕 being the molar mass, and the temperature is expressed in thermal units (kelvin or rankine). In statistical mechanics the description is: : where p is the pressure, n is the number density, is the temperature in ''energy units'' (i.e. joules, by multiplication for the Boltzmann constant), and is the gas constant. For an ideal gas the compressibility factor is per definition. In many real world applications requirements for accuracy demand that deviations from ideal gas behaviour, i.e., real gas behaviour, is taken into account. The value of generally increases with pressure and decreases with temperature. At high pressures molecules are colliding more often. This allows repulsive forces between molecules to have a noticeable effect, making the molar volume of the real gas (), which causes to exceed one.〔 page 55〕 When pressures are lower, the molecules are free to move. In this case attractive forces dominate, making . The closer the gas is to its critical point or its boiling point, the more deviates from the ideal case. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「compressibility factor」の詳細全文を読む スポンサード リンク
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