How to identify deviation type from data: (1) Compare observed P_total with Raoult's prediction — higher means positive deviation. (2) delta_H_mix: positive (endothermic) = positive deviation; negative (exothermic) = negative deviation. (3) delta_V_mix: positive (expansion) = positive deviation; negative (contraction) = negative deviation. (4) Similar molecules = ideal (benzene/toluene). Disrupted H-bonds = positive (ethanol/cyclohexane). New H-bonds formed = negative (chloroform/acetone). Special case: two components that can form ion pairs or strong complexes show extreme negative deviation.