Question

Explain what is meant by a regular solution; what additional features distinguish a real solution from a regular solution?

Answer 1

A regular solution is a solution whose entropy of mixing is equal to that of an ideal solution with the same composition, but is non-ideal due to a nonzero enthalpy of mixing. Such a solution is formed by random mixing of components without strong specific interactions,and its behavior diverges from that of an ideal solution only moderately.Its entropy of mixing is equal to that of an ideal solution with the same composition, due to random mixing without strong specific interactions.

If atoms A and B have different sizes, the statistical or quasi-chemical model will underestimate ΔHmix. The energy of the elastic strain fields due to the mismatch in atomic sizes should betaken into account. If the size difference is large, the contribution of the strain energy term could as important as thechemical (bonding) term.
If the size difference between the atoms is very large, theninterstitial solid solutions are energetically favorable.In systems with strong chemical bonding between the atoms there is a tendency for formation of intermediate phases. The intermediate phases can have a different crystal structure and may be highly ordered.