The triple point is the unique physical property of a substance at which it co-exists in all three states of matter i.e. solid, liquid, and gas simultaneously at a particular temperature and pressure. For example, the triple point of water happens to be at 0.01°C and 0.00604 atm (4.59 mm of Hg).

According to the Gibbs phase rule, a three-phase situation in a one-component system has no degrees of freedom. i.e. it is invariant. As the triple point occurs at a specific temperature and pressure, any change in either variable will cause the disappearance of any of the three forms of state.

For most substances, the triple point has a pressure requirement of less than 1 atm, such substances then have a liquid-vapor transition at 1 atm (normal boiling point). However, if this triple point has a pressure above 1 atm, the substance surpasses directly from solid to vapor at 1 atm.

Every pure substance has its specific properties. Their physical form, identity, and properties can easily be understood by their particular phase diagrams.

Outline

## Significance

Triple points are easily reproducible and very specific. This makes them suitable standards for temperature scales because the already used standards/references are not so easily reproducible and specific.

## Examples

### Triple Point of Water

The triple point of water occurs at the temperature at which all three states of water coexist i.e. ice, liquid water, and vapors. The triple point of water is important because it is sometimes taken as the standard reference point for defining the Kelvin temperature scale.

The triple point of water occurs at 273.16 °K which is, therefore, read 0.01 on a centigrade scale.

Pressure is also a sensitive factor in triple points. For the above reading to be exact and reproducible, a pressure of 0.00604 atm or 0.61 kPa is required.

#### Phase Diagram of Water

The phase diagram of water is a pressure-temperature diagram that represents all physical states of water, including water, steam, and ice, under different conditions. It represents the phase changes along the solid-liquid-gas curves. The complete diagram is given below;

### Triple Point of Carbon Dioxide

The triple point of carbon dioxide occurs at -56.4 °C and 5.1 atm. Just like other triple points, it is easily reproducible and sophisticated. However, just like others, it only exists under these specific conditions.

A common assumption is that CO2 will sublime rather than melt. If solid CO2 kept at 1 atm pressure is warmed slowly, its vapor pressure rises along the solid-gas curve, until it reaches atmospheric pressure at -78.5 °C. Above this temperature, solid CO2 will no longer be stable at 1 atm pressure and will turn into gas, thus sublimation occurs.

Above 5.1 atm pressure, CO2 exhibits melting and vaporization behaviors like water and other substances. By going horizontal as shown in the phase diagram below, at around 6 atm pressure, the intersection at the solid-liquid line is the melting point of the solid CO2, and the intersection at the liquid-gas line is the boiling point of liquid CO2.

#### Phase Diagram of CO2

The phase change diagram shown is the extended form of the phase diagram of CO2, which shows all the phase-changing points along with pressure-temperature effects. The colored areas are regions of single phases (solid, liquid, and gas) and the boundary lines are conditions of two-phase equilibrium. The three phases coexist at the triple point (-56.4 °C and 5.1 atm).

The conditions for the sublimation of CO2 are -78.5 °C at 1 atm according to the diagram. is the sublimation temperature at 1 atm pressure.

The straight line up from the triple point marks the condition of equilibrium between solid and liquid, showing how the melting point changes with pressure. This can be justified by Le Chatelier’s principle. An increase in pressure favors the dense phase for this type of compound.

The increase in pressure will result in an increase in temperatures of sublimation Ts, melting Tm, and boiling Tb, as more energy is required to bring molecules into a less dense phase.

Similar phase diagrams can be made for all the substances. The triple points of several substances are shown in the table below:

## Concepts Berg

What is a Triple Point?

The triple point is the unique physical property of a substance at which it co-exists in all three states of matter i.e. solid, liquid, and gas simultaneously at a particular temperature and pressure.

For example, the triple point of water is at 0.01°C and 0.00604 atm (4.59 mmHg).

How do you achieve the triple point?

A triple point can be achieved at a single point in the P-T phase diagram where the three phases of a substance coexist. To get the triple point, pressure and temperature should be set to specific values. To confirm the triple point, triple point cells are used.

Is triple point a state of matter?

Every pure substance has a particular state of matter at a specific temperature and pressure and the states vary with temperature and pressure. The triple point is the unique physical property at a particular temperature and pressure, where there is an equilibrium in three states of matter, therefore it cannot be called as a separate state of matter.

Why is the triple point of water important?

• The triple point of water is important because it is taken as the standard reference point for defining the Kelvin temperature scale.
• The triple point of water is also used in sealed cells as pressure transfer standards.

What is meant by triple point and eutectic point?

• Triple Point is the specific temperature and pressure at which three states are in equilibrium.
• Eutectic Point is the condition at which two solids and a liquid phase are in equilibrium; (SolidA+SolidB+Liquid).

Why doesn’t helium have a triple point?

Helium does not solidify except under extreme pressure even at extremely low temperatures. At atmospheric pressure, helium gas does not even condense until the temperature drops to 4.2 °K, only four degrees above Absolute Zero. (That discovery was made by Kamerlingh Onnes in 1908.)

The liquid could be further cooled using vacuum pumps to lower the vapor pressure. At lower pressures, it goes to the superfluid state, but solid is not possible. That is the reason, helium doesn’t have a triple point.

References Books

• R. M. Rosenberg, Principles of Physical Chemistry, Oxford University Press.
• F. W. Sears, Thermodynamics, Addison-Wesley, 2nd edition