Editing Azeotrope (section)

=== Negative azeotropes ===
[[File:Negative Azeotrope.png|thumb|left|Negative azeotrope – mixture of [[formic acid]] and water]]
[[File:negativeAzeotropePhaseDiagram.png|thumb|Phase diagram of a negative azeotrope. Vertical axis is temperature, horizontal axis is composition]]

A solution that shows large negative deviation from [[Raoult's law]] forms a maximum boiling azeotrope at a specific composition. [[Nitric acid]] and water is an example of this class of azeotrope. This azeotrope has an approximate composition of 68% [[nitric acid]] and 32% water [[mass percent|by mass]], with a [[boiling point]] of {{convert|393.5|K|C}}. In general, a negative azeotrope boils at a higher temperature than any other ratio of its constituents. Negative azeotropes are also called ''maximum boiling mixtures'' or ''pressure minimum azeotropes''. An example of a negative azeotrope is [[hydrochloric acid]] at a concentration of 20.2% and 79.8% water (by mass). Hydrogen chloride boils at −85&nbsp;°C and water at 100&nbsp;°C, but the azeotrope boils at 110&nbsp;°C, which is higher than either of its constituents. The maximum boiling point of any hydrochloric acid solution is 110&nbsp;°C. Other examples:

* [[hydrofluoric acid]] (35.6%) / water, boils at 111.35&nbsp;°C
* [[nitric acid]] (68%) / [[water (molecule)|water]], boils at 120.2&nbsp;°C at 1 atm 
* [[perchloric acid]] (71.6%) / water, boils at 203&nbsp;°C 
* [[sulfuric acid]] (98.3%) / water, boils at 338&nbsp;°C

The adjacent diagram shows a negative azeotrope of ideal constituents, X and Y. Again the bottom trace illustrates the boiling temperature at various compositions, and again, below the bottom trace the mixture must be entirely liquid phase. The top trace again illustrates the condensation temperature of various compositions, and again, above the top trace the mixture must be entirely vapor phase. The point, A, shown here is a boiling point with a composition chosen very near to the azeotrope. The vapor is collected at the same temperature at point B. That vapor is cooled, condensed, and collected at point C. Because this example is a negative azeotrope rather than a positive one, the distillate is farther from the azeotrope than the original liquid mixture at point A was. So the distillate is poorer in constituent X and richer in constituent Y than the original mixture. Because this process has removed a greater fraction of Y from the liquid than it had originally, the residue must be poorer in Y and richer in X after distillation than before.

If the point, A had been chosen to the right of the azeotrope rather than to the left, the distillate at point C would be farther to the right than A, which is to say that the distillate would be richer in X and poorer in Y than the original mixture. So in this case too, the distillate moves away from the azeotrope and the residue moves toward it. This is characteristic of negative azeotropes. No amount of distillation, however, can make either the distillate or the residue arrive on the opposite side of the azeotrope from the original mixture. This is characteristic of all azeotropes.