 The center trace is a straight line, which is what Raoult's law predicts for an ideal mixture.In general solely mixtures of chemically similar solvents, such as "n"-hexane with "n"-heptane, form nearly ideal mixtures that come close to obeying Raoult's law.The top trace illustrates a nonideal mixture that has a positive deviation from Raoult's law, where the total combined vapor pressure of constituents, X and Y, is greater than what is predicted by Raoult's law.