Relative volatility is a measure used in the field of chemical engineering, particularly in distillation and separation processes, to quantify the ease with which one component in a mixture can be separated from another component. It is defined as the ratio of the vapor pressures of two components in a liquid mixture.
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For a two-component mixture, let be component A's mole fraction in the liquid and its mole fraction in the coexisting vapor. At interior compositions, define the relative volatility of A with respect to B by the ratio of composition odds:Thus means A is enriched in the vapor, while means . Solving this definition givesHere a constant relative volatility is a stipulated model of vapor-liquid equilibrium, not a measured property of a named chemical solution. Real relative volatility can vary with composition, temperature, and pressure. The relation specifies no absolute temperature scale: even fixing does not determine a boiling-temperature curve or a heat requirement.