Distillation 

Distillation is a commonly used method for purifying liquids and separating mixtures of liquids into their individual components.  As the liquid being distilled is heated, the vapors that form will be richest in the component of the mixture that boils at the lowest temperature. Purified compounds will boil, and thus turn into vapors, over a relatively small temperature range ; by carefully watching the temperature in the distillation flask, it is possible to affect a reasonably good separation. As distillation progresses, the concentration of the lowest boiling component will steadily decrease. Eventually the temperature within the apparatus will begin to change; a pure compound is no longer being distilled. The temperature will continue to increase until the boiling point of the next-lowest-boiling compound is approached. When the temperature again stabilizes, another pure fraction of the distillate can be collected. This fraction of distillate will be primarily the compound that boils at the second lowest temperature. This process can be repeated until all the fractions of the original mixture have been separated.

                                 

When a mixture of two or more volatile compounds is heated, the vapor pressure of the mixture equals the sum of the vapor pressures of each compound in the mixture. The magnitude of the vapor pressure exerted by each compound is determined by the vapor pressure of that compound and the mole fraction of that compound present in the mixture. For an ideal two-compound solution, the solution vapor pressure is expressed by Raoult's law, shown in Eq 1.0

                               

                                                                                    Eq 1.0

When two liquids form a homogeneous solution, they are said to be miscible. Such a homogeneous mixture will boil at a temperature between the boiling points of the pure compounds. The exact boiling point of the mixture depends upon the relative amounts of the compounds present. When vapor is produced from such a liquid mixture, the composition of the vapor mixture is different from the composition of the liquid mixture from which it forms. As the vapors produced by the distillation move into the watercooled condenser, these vapors condense to a liquid, the distillate, which has the same composition as the vapor from which it is formed. The distillate collected in the receiver will contain more of the more volatile compound than was present in the original mixture. If one compound is much more volatile than the other, the compounds can be separated in one vaporization step. Such a step is called simple distillation

More info - http://www.chem.umass.edu/~samal/269/distill.pdf