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   Boiling Points

The variation of boiling point with pressure

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Trouton's rule ( F. Trouton. Nature 1883, 27, 292.) states that the entropy of vaporisation has almost the same value for many different liquids. Starting from this rule, and a liquid's boiling point at a particular pressure, the boiling point at any other pressure can be estimated. The yellow line on the applet shows the result of this calculation.

Some common substances deviate from Trouton's rule. Water and methane have unusually high and unusually low entropies of vaporisation. The results of calculations using these values are also shown on the applet (in blue and grey, respectively), to illustrate the uncertainty in the calculations.

The best results come from the Trouton-Hildebrand-Everett rule, and these are shown in green.

  • Blue - Trouton's rule modified for water
  • Yellow - Trouton's rule
  • Grey - Trouton's rule modified for methane

  • Green - if the initial pressure is atmospheric pressure, a green line corresponding to the Trouton-Hildebrand-Everett rule is also drawn. In the absence of other information, this line is the best guess.

Enter a known initial pressure and associated boiling point, and click the button just below. It is best to enter the boiling point at atmospheric pressure, if this is known. If it is not known, the program will estimate this value using the T-H-E rule. Click on the graph to trace along the resulting vapour-pressurecurve, or enter either the new temperature or pressure in the fields beneaththe graph and click calculate - if both fields have numbers in them the new pressure is calculated by default. The units of pressure can be changed between kPa and mmHg by clicking the kPa/mmHg button.

J. M. Goodman, P. D. Kirby, and L. O. Haustedt Tetrahedron Lett. 2000, 41, 9879-9882.

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department of chemistry University of Cambridge