Triple point - sanju-ten (English spelling) triple point

Japanese: 三重点 - さんじゅうてん（英語表記）triple point

A state in which the gas, liquid, and solid phases of a substance coexist simultaneously and are in thermal equilibrium. Because both the temperature and pressure are constant, it was once used as the defining fixed point of the temperature scale, and the triple point of water on the absolute temperature scale was defined as 273.16 K (→ Absolute temperature). The triple points of hydrogen, oxygen, argon, water, etc. are also designated as defining fixed points on the International Practical Temperature Scale. A one-component system has two degrees of freedom according to the phase rule, so it can be represented on a plane diagram with temperature and pressure as the coordinate axes. Region G represents the gas phase, L represents the liquid phase, and S represents the solid phase. The boundaries of each region are called the evaporation curve between GL, the melting curve between LS, and the sublimation curve between SG. These three curves intersect at one point, and the intersection is the triple point. At the triple point, the degrees of freedom are zero, and the temperature and pressure are fixed values. The evaporation curve has an end point on the high temperature and high pressure side. This point is called the critical point, and the temperature and pressure are fixed values.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

一つの物質の気相，液相，固相が同時に共存し，熱平衡にある状態。温度も圧力も一定であることから，かつて温度目盛りの定義定点に採用され，絶対温度目盛りは水の三重点を 273.16Kとして定義されていた（→絶対温度）。また水素，酸素，アルゴン，水などの三重点は，国際実用温度目盛りの定義定点として，その温度が指定されている。一成分系は相律により自由度 2であるから，温度と圧力を座標軸にもつ平面図で表すことができる。領域 Gは気相，Lは液相，Sは固相を表し，各領域の境界線は GL間が蒸発曲線，LS間が融解曲線，SG間は昇華曲線と呼ばれる。これら 3曲線は 1点で交わり，その交点が三重点である。三重点では自由度がゼロであって，温度と圧力が定まった値をとる。蒸発曲線は高温・高圧側に終点をもつ。この点を臨界点といい，温度と圧力が定まった値をとる。

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