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Raoult's law states that the partial vapour pressure of each volatile component in an ideal solution is proportional to its mole fraction: P_A = x_A * P_A_std and P_B = x_B * P_B_std. Total pressure: P_total = x_AP_A_std + x_BP_B_std = P_B_std + (P_A_std - P_B_std)x_A, which is linear in x_A. The vapour composition: y_A = P_A/P_total = x_AP_A_std/P_total (Dalton's law). The more volatile component (higher P_std) is enriched in the vapour phase — this is the principle behind fractional distillation. Ideal solutions satisfy: delta_H_mix = 0 (no heat absorbed/released), delta_V_mix = 0 (no volume change), and A-B interactions ≈ average of A-A and B-B. Examples: benzene + toluene, n-hexane + n-heptane, chlorobenzene + bromobenzene — all pairs with similar molecular structures. P_total vs x_A is a straight line for ideal solutions. P_total vs y_A gives a curve. On T-x and T-y diagrams, the liquid line (bubble curve) and vapour line (dew curve) enclose the two-phase region. The lever rule gives the ratio of liquid to vapour phases at any point in this region. Understanding ideal solutions provides the baseline for analysing deviations.