# Clausius–Clapeyron relation

The **Clausius–Clapeyron relation**, named after Rudolf Clausius[1] and Benoît Paul Émile Clapeyron,[2] is a way of characterizing a discontinuous phase transition between two phases of matter of a single constituent.

## Definition

On a pressure–temperature (P–T) diagram, the line separating the two phases is known as the coexistence curve. The Clausius–Clapeyron relation gives the slope of the tangents to this curve. Mathematically,

where is the slope of the tangent to the coexistence curve at any point, is the specific latent heat, is the temperature, is the specific volume change of the phase transition, and is the specific entropy change of the phase transition.

## Derivations

### Derivation from state postulate

Using the state postulate, take the specific entropy for a homogeneous substance to be a function of specific volume and temperature .[3]^{:508}

The Clausius–Clapeyron relation characterizes behavior of a closed system during a phase change, during which temperature and pressure are constant by definition. Therefore,[3]^{:508}

Using the appropriate Maxwell relation gives[3]^{:508}

where is the pressure. Since pressure and temperature are constant, by definition the derivative of pressure with respect to temperature does not change.[5]^{:57, 62 & 671} Therefore, the partial derivative of specific entropy may be changed into a total derivative

and the total derivative of pressure with respect to temperature may be factored out when integrating from an initial phase to a final phase ,[3]^{:508} to obtain

where and are respectively the change in specific entropy and specific volume. Given that a phase change is an internally reversible process, and that our system is closed, the first law of thermodynamics holds

where is the internal energy of the system. Given constant pressure and temperature (during a phase change) and the definition of specific enthalpy , we obtain

Given constant pressure and temperature (during a phase change), we obtain[3]^{:508}

Substituting the definition of specific latent heat gives

Substituting this result into the pressure derivative given above (), we obtain[3]^{:508}[6]

This result (also known as the **Clapeyron equation**) equates the slope of the tangent to the coexistence curve , at any given point on the curve, to the function of the specific latent heat , the temperature , and the change in specific volume .

### Derivation from Gibbs–Duhem relation

Suppose two phases, and , are in contact and at equilibrium with each other. Their chemical potentials are related by

Furthermore, along the coexistence curve,

One may therefore use the Gibbs–Duhem relation

(where is the specific entropy, is the specific volume, and is the molar mass) to obtain

Rearrangement gives

from which the derivation of the Clapeyron equation continues as in the previous section.

### Ideal gas approximation at low temperatures

When the phase transition of a substance is between a gas phase and a condensed phase (liquid or solid), and occurs at temperatures much lower than the critical temperature of that substance, the specific volume of the gas phase greatly exceeds that of the condensed phase . Therefore, one may approximate

at low temperatures. If pressure is also low, the gas may be approximated by the ideal gas law, so that

where is the pressure, is the specific gas constant, and is the temperature. Substituting into the Clapeyron equation

we can obtain the **Clausius–Clapeyron equation**[3]^{:509}

for low temperatures and pressures,[3]^{:509} where is the specific latent heat of the substance.

Let and be any two points along the coexistence curve between two phases and . In general, varies between any two such points, as a function of temperature. But if is constant,

or[5]^{:672}

These last equations are useful because they relate equilibrium or saturation vapor pressure and temperature to the latent heat of the phase change, *without* requiring specific volume data.

## Applications

### Chemistry and chemical engineering

For transitions between a gas and a condensed phase with the approximations described above, the expression may be rewritten as

where is a constant. For a liquid-gas transition, is the specific latent heat (or specific enthalpy) of vaporization; for a solid-gas transition, is the specific latent heat of sublimation. If the latent heat is known, then knowledge of one point on the coexistence curve determines the rest of the curve. Conversely, the relationship between and is linear, and so linear regression is used to estimate the latent heat.

### Meteorology and climatology

Atmospheric water vapor drives many important meteorologic phenomena (notably precipitation), motivating interest in its dynamics. The Clausius–Clapeyron equation for water vapor under typical atmospheric conditions (near standard temperature and pressure) is

where:

- is saturation vapor pressure
- is temperature
- is the specific latent heat of evaporation of water
- is the gas constant of water vapor

The temperature dependence of the latent heat , and therefore of the saturation vapor pressure , cannot be neglected in this application. Fortunately, the **August–Roche–Magnus formula** provides a very good approximation, using pressure in hPa and temperature in Celsius:

(This is also sometimes called the *Magnus* or *Magnus–Tetens* approximation, though this attribution is historically inaccurate.)[9] But see also this discussion of the accuracy of different approximating formulae for saturation vapour pressure of water.

Under typical atmospheric conditions, the denominator of the exponent depends weakly on (for which the unit is Celsius). Therefore, the August–Roche–Magnus equation implies that saturation water vapor pressure changes approximately exponentially with temperature under typical atmospheric conditions, and hence the water-holding capacity of the atmosphere increases by about 7% for every 1 °C rise in temperature.[10]

## Example

One of the uses of this equation is to determine if a phase transition will occur in a given situation. Consider the question of how much pressure is needed to melt ice at a temperature below 0 °C. Note that water is unusual in that its change in volume upon melting is negative. We can assume

and substituting in

- (latent heat of fusion for water),
- K (absolute temperature), and
- (change in specific volume from solid to liquid),

we obtain

To provide a rough example of how much pressure this is, to melt ice at −7 °C (the temperature many ice skating rinks are set at) would require balancing a small car (mass = 1000 kg[11]) on a thimble (area = 1 cm²).

## Second derivative

While the Clausius–Clapeyron relation gives the slope of the coexistence curve, it does not provide any information about its curvature or second derivative. The second derivative of the coexistence curve of phases 1 and 2 is given by [12]

where subscripts 1 and 2 denote the different phases, is the specific heat capacity at constant pressure, is the thermal expansion coefficient, and is the isothermal compressibility.

## References

- Clausius, R. (1850). "Ueber die bewegende Kraft der Wärme und die Gesetze, welche sich daraus für die Wärmelehre selbst ableiten lassen" [On the motive power of heat and the laws which can be deduced therefrom regarding the theory of heat].
*Annalen der Physik*(in German).**155**(4): 500–524. Bibcode:1850AnP...155..500C. doi:10.1002/andp.18501550403. - Clapeyron, M. C. (1834). "Mémoire sur la puissance motrice de la chaleur".
*Journal de l'École polytechnique*(in French).**23**: 153–190. ark:/12148/bpt6k4336791/f157. - Wark, Kenneth (1988) [1966]. "Generalized Thermodynamic Relationships".
*Thermodynamics*(5th ed.). New York, NY: McGraw-Hill, Inc. ISBN 978-0-07-068286-3. - Çengel, Yunus A.; Boles, Michael A. (1998) [1989].
*Thermodynamics – An Engineering Approach*. McGraw-Hill Series in Mechanical Engineering (3rd ed.). Boston, MA.: McGraw-Hill. ISBN 978-0-07-011927-7. - Salzman, William R. (2001-08-21). "Clapeyron and Clausius–Clapeyron Equations".
*Chemical Thermodynamics*. University of Arizona. Archived from the original on 2007-06-07. Retrieved 2007-10-11. - Alduchov, Oleg; Eskridge, Robert (1997-11-01),
*Improved Magnus' Form Approximation of Saturation Vapor Pressure*, NOAA, doi:10.2172/548871 — Equation 25 provides these coefficients. - Alduchov, Oleg A.; Eskridge, Robert E. (1996). "Improved Magnus Form Approximation of Saturation Vapor Pressure".
*Journal of Applied Meteorology*.**35**(4): 601–9. Bibcode:1996JApMe..35..601A. doi:10.1175/1520-0450(1996)035<0601:IMFAOS>2.0.CO;2. Equation 21 provides these coefficients. - Lawrence, M. G. (2005). "The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air: A Simple Conversion and Applications" (PDF).
*Bulletin of the American Meteorological Society*.**86**(2): 225–233. Bibcode:2005BAMS...86..225L. doi:10.1175/BAMS-86-2-225. - IPCC, Climate Change 2007: Working Group I: The Physical Science Basis, "FAQ 3.2 How is Precipitation Changing ?", URL http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-3-2.html Archived 2018-11-02 at the Wayback Machine
- Zorina, Yana (2000). "Mass of a Car".
*The Physics Factbook*. - Krafcik, Matthew; Sánchez Velasco, Eduardo (2014). "Beyond Clausius–Clapeyron: Determining the second derivative of a first-order phase transition line".
*American Journal of Physics*.**82**(4): 301–305. Bibcode:2014AmJPh..82..301K. doi:10.1119/1.4858403.

## Bibliography

- Yau, M.K.; Rogers, R.R. (1989).
*Short Course in Cloud Physics*(3rd ed.). Butterworth–Heinemann. ISBN 978-0-7506-3215-7. - Iribarne, J.V.; Godson, W.L. (2013). "4. Water-Air systems § 4.8 Clausius–Clapeyron Equation".
*Atmospheric Thermodynamics*. Springer. pp. 60–. ISBN 978-94-010-2642-0. - Callen, H.B. (1985).
*Thermodynamics and an Introduction to Thermostatistics*. Wiley. ISBN 978-0-471-86256-7.