What are non-ideal solutions? Distinguish between positive and negative deviations from Raoult's law, with one example and a graph for each. What type of azeotrope does each give?

1. 1 · Define. A non-ideal solution does not obey Raoult's law over the whole range of composition; its vapour pressure is higher or lower than Raoult's law predicts.
2. 2 · Explain positive deviation. A-B attraction is weaker, so vapour pressure is higher; ΔmixH > 0 and ΔmixV > 0. Example: ethanol + acetone (acetone breaks some hydrogen bonds of ethanol).
3. 3 · Explain negative deviation. A-B attraction is stronger, so vapour pressure is lower; ΔmixH < 0 and ΔmixV < 0. Example: chloroform + acetone (a hydrogen bond forms between them).
4. 4 · Draw both graphs. Dotted straight Raoult's law lines in each; solid curves above them for positive deviation and below them for negative deviation. Label p₁°, p₂°, p_total and the axes.
5. 5 · Name the azeotropes. Large positive deviation gives a minimum boiling azeotrope (ethanol and water, about 95% ethanol). Large negative deviation gives a maximum boiling azeotrope (68% nitric acid and water).
6. 6 · Link to real life. Ethanol for petrol blending cannot be dried beyond the azeotrope by distillation alone.
7. 7 · Close in one line. So the sign of the deviation follows the strength of the A-B attraction compared with A-A and B-B.

Scoring tip: A neat two-column table plus two labelled graphs is the fastest way to collect all five marks.