Fig. 1: A graph of the total vapour pressure in kilopascals against the mole fraction of liquid A in the liquid, from 0 to 1. A dashed grey straight line, labelled Raoult's law, runs from 7 kilopascals at the left to 18 kilopascals at the right. A solid purple curve, labelled measured, starts at the same two ends but sags below the dashed line in between, with the largest gap in the middle.