What is relative volatility and why is it important in distillation?

Relative volatility (alpha) is the ratio of the vapor-liquid equilibrium ratios (K-values) of two components: alpha = K1/K2 = (y1/x1)/(y2/x2). It indicates the ease of separation - higher relative volatility means easier separation requiring fewer stages. When alpha approaches 1, separation becomes difficult or impossible by ordinary distillation, and alternative methods may be needed.