CHAP. 2: STATE BEHAVIOUR [CONTENTS] 46
Obr.2.1:Dependence of pressure on the molar volume at a fixed temperature. The curves denoted
T 1 throughT 3 are isotherms. IsothermT 3 is supercritical, extending over the whole region of gas.
IsothermTcis critical, point C is the critical point,pcis the critical pressure, andVm,c=Vcis the
molar critical volume. The critical point C is an inflection point of the isothermTc. In addition,
the tangent to the isotherm going through point C is parallel to the axisVm. IsothermsT 1 andT 2
are subcritical, each of them consisting of a portion extending in gas and a portion extending in
liquid. The curveL 1 L 2 CG 2 G 1 separates the gaseous and liquid regions. PointsG 1 andG 2 indicate
the saturated vapour at temperaturesT 1 andT 2 and at corresponding saturated pressuresps(T 1 ) and
ps(T 2 ) PointsL 1 andL 2 indicate saturated liquid at the same temperatures and pressures.
Example
Derive the relation for the coefficient of thermal expansion from the equation of state of an ideal
gas (2.15).