PvT Surface

The equilibrium states of a simple, compressible substance can be specified in terms of its pressure, volume and temperature. If any two of these state variables is specified, the third is determined. This implies that the states of the substance can be represented as a surface in a three dimensional PvT space. The PvT surface above represents a substance which contracts upon freezing. Most substances do so, the notable exception being water which expands upon freezing. A considerable amount of information about the phases of matter can be illustrated with the PvT surface.

简单可压缩物质的平衡状态可以用其压力、体积和温度来指定。如果其中任意两个状态变量被指定，第三个变量就被确定。这表明物质的状态可以表示为三维PvT空间中的一个表面。上述PvT表面表示一种在冻结时收缩的物质。大多数物质都是如此，例外的是水，它在冻结时会膨胀。用PvT表面可以展示大量关于物质相的信息。

The solid, liquid and gas(vapor) phases can be represented by regions on the surface. Note that there are regions on the surface which represent a single phase, and regions which are combinations of two phases. A point lying on a line between a single-phase and a two-phase region represents a "saturation state". The line between the liquid and the liquid-vapor regions is called the liquid-saturation line and any point on that line represents a saturated-liquid state. A point on the boundary between the vapor and the liquid-vapor regions is called a saturated-vapor state.

固体、液体和气体（蒸汽）相可以在表面的区域上表示。需要注意的是，表面的某些区域代表单一相，而其他区域则是两种相的组合。位于单一相区域和两种相区域之间的线上的点表示“饱和状态”。液体与液体-蒸汽区域之间的线称为液体饱和线，该线上任何一点都表示饱和液体状态。位于蒸汽与液体-蒸汽区域边界上的点称为饱和蒸汽状态。

Note the critical state where the saturated-liquid and saturated-vapor lines meet. The state variables of this unique point are denoted by P_c, v_c and T_c. If a substance is above the critical temperature T_c, it cannot condense into a liquid, no matter how high the pressure. This merging of the liquid and vapor states above the critical temperature is a characteristic of all known substances. While a pure vapor state can exist at a pressure lower than P_c, at pressures above P_c it is constrained to be a vapor. States with pressures above P_c are described as "supercritical states".

注意临界态，这是饱和液体和饱和蒸气线相交的地方。该唯一点的状态变量用 P_c、v_c 和 T_c 表示。如果物质的温度高于临界温度 T_c，无论压力多高都无法凝结成液体。在临界温度以上，液体和蒸气状态的合并是所有已知物质的特征。虽然纯蒸气状态可以在压力低于 P_c 时存在，但在压力高于 P_c 时则被限制为蒸气。压力高于 P_c 的状态被称为‘超临界状态’。

The remarkable "triple state" of matter where solid, liquid and vapor are in equilibrium may be characterized by a temperature called the triple point. The triple state is represented by a line parallel to the Pv plane with a characteristic pressure for the substance but variable volume. The triple point temperature of water is assigned the value 273.16 K and the triple state of water is used as the reference for establishing the Kelvin temperature scale.

物质的奇妙‘三相平衡’状态，其中固态、液态和气态处于平衡，可以由一个称为三相点的温度来表征。三相平衡状态由一条与Pv平面平行的线表示，该线具有物质的特征压力但体积可变。水的三相点温度被定义为273.16 K，水的三相平衡状态被用作建立开尔文温标的基础。

PvT Surface for a Substance which Contracts Upon Freezing