Physical Origin
Ideality of solutions is analogous to ideality for gases, with the important difference that intermolecular interactions in liquids are strong and can not simply be neglected as they can for ideal gases. Instead we assume that the mean strength of the interactions are the same between all the molecules of the solution.
More formally, for a mix of molecules of A and B, the interactions between unlike neighbors (UAB) and like neighbors UAA and UBB must be of the same average strength i.e. 2 UAB = UAA + UBB and the longer-range interactions must be nil (or at least indistinguishable). If the molecular forces are the same between AA, AB and BB, i.e. UAB = UAA = UBB, then the solution is automatically ideal.
If the molecules are almost identical chemically, e.g. 1-butanol and 2-butanol, then the solution will be almost ideal. Since the interaction energies between A and B are almost equal, it follows that there is a very small overall energy (enthalpy) change when the substances are mixed. The more dissimilar the nature of A and B, the more strongly the solution is expected to deviate from ideality.
Read more about this topic: Ideal Solution
Famous quotes containing the words physical and/or origin:
“When one walks, one is brought into touch first of all with the essential relations between one’s physical powers and the character of the country; one is compelled to see it as its natives do. Then every man one meets is an individual. One is no longer regarded by the whole population as an unapproachable and uninteresting animal to be cheated and robbed.”
—Aleister Crowley (1875–1947)
“We have got rid of the fetish of the divine right of kings, and that slavery is of divine origin and authority. But the divine right of property has taken its place. The tendency plainly is towards ... “a government of the rich, by the rich, and for the rich.””
—Rutherford Birchard Hayes (1822–1893)