Question 1

What are the criteria for an ideal solution?

Accepted Answer

In an ideal solution, the A-A, B-B and A-B intermolecular interactions are essentially equal in strength. Consequently, ΔH_mix = 0 and ΔV_mix = 0, and the solution obeys Raoult's law at all compositions and temperatures.

Question 2

What causes positive deviation from Raoult's law?

Accepted Answer

Positive deviation occurs when A-B interactions are weaker than A-A and B-B interactions. Unlike molecules are held together less strongly, so the mixture has ΔH_mix > 0, ΔV_mix > 0, and each component shows a vapour pressure above its ideal Raoult value.

Question 3

What causes negative deviation from Raoult's law?

Accepted Answer

Negative deviation occurs when A-B interactions are stronger than A-A and B-B interactions. This lowers both components' vapour pressures below their Raoult values and gives ΔH_mix < 0 and ΔV_mix < 0 in the intended comparison.

Question 4

How are minimum- and maximum-boiling azeotropes related to deviation?

Accepted Answer

A minimum-boiling azeotrope is associated with a maximum in the fixed-temperature vapour-pressure curve and positive deviation. A maximum-boiling azeotrope is associated with a minimum in that curve and negative deviation. A deviation alone is not enough; the corresponding extremum must occur.

Question 5

Does negative deviation mean the entropy of mixing is negative?

Accepted Answer

No. Entropy of mixing remains positive in both ideal and non-ideal solutions. A negative value of ΔH_mix for a negatively deviating solution describes its energetic stabilization, not a reversal of mixing entropy.

Ideal vs Non-Ideal: A-B Attraction in 3D

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