What is the triple point of a phase diagram?

The phase diagram is a diagram that represents the variation between the different states of matter depending on certain variables given. The simplest phase diagram uses two variables, which is why it is also known as a 2D (two-dimensional) phase diagram.

The two most commonly used variables are Pressure and the temperature. In a phase diagram of this type, corresponding to a pure substance, three zones can be distinguished in which the substance exists in a single state: solid, liquid or gas. There is a point where the intersection of the three zonesand it is this point that is known as triple point:

Between the separation of two zones a line is formed that represents pairs of pressure and temperature in which the substance passes from one state to another. For example, the points defined by pressure and temperature values ​​at which a liquid changes to a solid state form the freezing line, represented in the previous image with the continuous green line. The green dotted line is characteristic of water, which exhibits a different behavior than most liquids due to hydrogen bonding.

Thus, if the pressure to which a liquid is subjected is known, the temperature at which it will solidify can be known.

In a similar way, the vaporization line is formed, between the liquid and gaseous states, and the sublimation line, which represents the direct passage from the solid state to the gaseous state without passing through the liquid phase.

The triple point is a point that is formed by the intersection of the three lines, the freezing line, the vaporization line and the sublimation line. This means that At the triple point, matter coexists in all three states..

The triple point has many applications, one of the most common is the calibration of thermometers, especially precision and industrial thermometers.

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