Triple point

Triple point is the unique physical property of a substance at which it co-exist in all three states of matter i.e. Solid,Liquid, Gas simultaneously at a particular temperature and pressure

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

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 triple point occurs at specific temperature and pressure, any change in either variable will occur in disappearance of any of three forms of state.

For most substances, triple point has a pressure 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 passes 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 the phase diagrams.

Phase diagram of water

The water phase diagram is a pressure-temperature diagram that represents various water phases, including water, steam, and ice, under different conditions.Also the different phase changes along the solid-liquid-gas curves. Below is the phase diagram of water.

For details of the phase diagram of water click the link below.

Triple point of water

The triple point of water occurs is the temperature at which it co-exist in all three states of matter i.e. Ice, liquid and vapors. Triple point of water occurs at 0.01°C and 0.00604 atm (4.59 mmHg).

The triple point of water is important because it is taken as the standard reference point for defining the Kelvin temperature scale.

For details of the triple point of water do visit the link below.

Phase diagram of CO2

A plot of pressure vs temperature indicating the conditions at which Solid, Liquid and Gas are thermodynamic stable. Also shows at which points two and even three phases are in equilibrium.

Let’s demonstrate this graph.

CO2 we have is gas at 1 bar and 0oC.Now by lowering the temperature slowly keeping pressure 1bar, it will still be a gas i.e. – 70oC & 01bar(shown in blue). Here by increasing slowly its pressure, it’ll start becoming a solid i.e. -70oC & 05 bar (shown in pink)

 

Similarly, a solid CO2 can easily be converted into a gaseous phase by increasing temperature. At some temperature it will be converted into gas which is called sublimation.

This case is the same for Gas to solid and Liquid to Gas. By manipulation of temperature and pressure, it can easily be understood whether it’ll be solid, liquid or gas.

Liquid and Gas are in equilibrium along this line. Below the line CO2 is stable in gaseous state and above that line it is stable in liquid form.

Triple Point of CO2

The diagram shown is the extended form of phase diagram of CO2,which indicates all the phase changing points along with pressure – temperature effects. The areas are regions of single phases and the boundary lines are conditions of two-phase equilibrium. Three phases co-exist at the triple point. Pc and Tc are coordinates of the critical point and P3 and T3 are coordinates of the triple point. Ts 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 with Le Chatelier’s principle. An increase in pressure favours the most dense phase. The increase in pressure will result in increase of temperatures of sublimation Ts, melting Tm and boiling Tb. Because more energy is required to bring molecules into less dense phase.

At only one set of conditions, triple point of molecule can exist. The triple point for CO2 occurs at 5.1 atm pressure and – 56.6oC. We generally think CO2 will sublime rather than melting. If solid CO2 under 1 atm pressure is warmed slowly, its vapor pressure rises along the solid-gas curve until it reaches atmospheric pressure at – 78oC. Above this temperature, solid CO2 will no longer be stable at 1 atm pressure.

Above 5.1 atm pressure, CO2 exhibits the melting and vaporization behaviors that are more likely to water and other substances. By going horizontal as shown in figure at 06 atm pressure, the intersection with the solid-liquid line is the melting point of the solid, and that with liquid-gas line is the boiling point of liquid.

Similar phase diagrams can be made for all the substances that exist as solid, liquid and gas. The triple points of several substances are shown in the table below:

Substances Pressure (atm) Temperature (Celsius)
Oxygen0.0026-218
Iodine0.12113.50
Hydrogen0.0695-259

Concepts Berg

What is a Triple Point?

Triple point is the unique physical property of a substance at which it co-exist in all three states of matter i.e. Solid,Liquid, Gas simultaneously at a particular temperature and pressure.

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

How do you achieve the triple point?

Triple point can be achieved at a single point in the P-T phase diagram where the three phases of water 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 specific temperature and pressure and this state varies with temperature and pressure. Triple point is the unique physical property at some temperature and pressure, where there is an equilibrium in three states of matter i.e. Solid, Liquid and Gas.

Why does water boil at triple point?

As boiling point is the temperature at which external pressure is equal to the vapor pressure exerted by the liquid. By lowering the temperature and pressure, its boiling point also decreases. At triple point, all three states co-exist at specific temperature and pressure. So, this could be inferred that boiling point becomes equals to freezing point. That’s why water boils at triple point.

But it is impossible to drop the boiling point below the triple point.

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: Solid, liquid and gas.

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 , that is, only four degrees above Absolute Zero. (That discovery was made by Kamerlingh Onnes in 1908.) The liquid could be further cooled by using vacuum pumps to lower the vapor pressure. At lower pressures, it goes to the superfluid state, but solid is not possible. That’s why Helium doesn’t have a triple point.

References

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

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