When a liquid mixtures is heated in a closed container, the mixture boils and starts forming vapor. The vapor will have different composition compared to the liquid. The vapors are obviously richer in more volatile component. If such vapors are collected and condensed, the condensed liquid will have different composition form original mixture and is richer in high volatile component. Read more
In order to separate binary mixtures using distillation process, there must be differences in volatilities of the components. The greater the difference, the easier it is to do so. A measure for this is termed relative volatility. Volatility is a way of expressing relationship between components of vapor Ya and that of liquid Xa in equilibrium. It is defined as ratio of partial pressure of component to mole fraction of component in liquid.