Editing Raoult's law (section)

==Real solutions==
In many pairs of liquids, there is no uniformity of attractive forces, i.e., the [[Adhesion|adhesive]] (between dissimilar molecules) and [[Cohesion (chemistry)|cohesive]]  forces (between similar molecules)   are not uniform between the two liquids. Therefore, they deviate from Raoult's law, which applies only to ideal solutions.

Notably, when the concentration of A is small, its vapor pressure instead follows [[Henry's law]], and likewise for substance B when its concentration is small.

===Negative deviation=== 
[[File:Raul minus.jpg|thumb|upright|Negative deviation from Raoult's law]]

When the adhesion is stronger than the cohesion, fewer liquid particles turn into vapor thereby lowering the vapor pressure and leading to negative deviation in the graph.

For example, the system of [[chloroform]] (CHCl<sub>3</sub>) and [[acetone]] (CH<sub>3</sub>COCH<sub>3</sub>) has a negative deviation<ref>P. Atkins and J. de Paula, ''Physical Chemistry'' (8th ed., W. H. Freeman 2006) p. 146.</ref> from Raoult's law, indicating an attractive interaction between the two components that have been described as a [[hydrogen bond]].<ref>{{cite journal |last1=Kwak |first1=Kyungwon |last2=Rosenfeld |first2=Daniel E. |last3=Chung |first3=Jean K. |last4=Fayer |first4=Michael D. |date=6 November 2008 |title=Solute-Solvent Complex Switching Dynamics of Chloroform between Acetone and Dimethylsulfoxide Two-Dimensional IR Chemical Exchange Spectroscopy |journal=J. Phys. Chem. B |volume=112 |issue=44 |pages=13906–13915 |doi=10.1021/jp806035w |pmc=2646412 |pmid=18855462 |citeseerx=10.1.1.560.3553 }}</ref> The system HCl–water has a large enough negative deviation to form a minimum in the vapor pressure curve known as a (negative) [[azeotrope]], corresponding to a mixture that evaporates without change of composition.<ref>Atkins and de Paula, p. 184.</ref> When these two components are mixed, the reaction is exothermic as ion-dipole intermolecular forces of attraction are formed between the resulting ions (H<sub>3</sub>O<sup>+</sup> and Cl<sup>–</sup>) and the polar water molecules so that Δ''H''<sub>mix</sub> is negative.

===Positive deviation===
[[File:Raoult's and Henry's laws en 5000x5000 256 cols.png|thumb|Positive deviation from Raoult's law]]
When the adhesion is weaker than cohesion, which is quite common, the liquid particles escape the solution more easily that increases the vapor pressure and leads to a positive deviation.

If the deviation is large, then the vapor pressure curve shows a maximum at a particular composition and forms a positive azeotrope (low-boiling mixture). Some mixtures in which this happens are (1) [[ethanol]] and [[water]], (2) [[benzene]] and [[methanol]], (3) [[carbon disulfide]] and [[acetone]], (4) [[chloroform]] and ethanol, and (5) [[glycine]] and water. When these pairs of components are mixed, the process is endothermic as weaker intermolecular interactions are formed so that Δ<sub>mix</sub>''H'' is positive.

===Mixed deviation===

It is possible to have mixed deviations, which are positive for one component and negative for the other, and which switch between positive and negative while moving from <math>x=0</math> to <math>x=1</math>. These are not merely theoretically possible, as actual examples of mixed deviation exist.<ref name=McGlashan>{{cite journal|doi=10.1021/ed040p516 |title=Deviations from Raoult's law |date=1963 |last1=McGlashan |first1=M. L. |journal=Journal of Chemical Education |volume=40 |issue=10 |page=516 |bibcode=1963JChEd..40..516M }}</ref> The possible physical deviations are not entirely arbitrary however, as they are constrained by the [[Duhem–Margules equation]]: for example, if one component has positive deviation over the entire range then the other component cannot have negative deviation over the entire range.<ref name=McGlashan/>