Each substance has a particular volatility which determines the temperature and pressure required for a change in phase from liquid to gas. This is used in distillation which is a physical process that does not involve any change in the chemical composition of the constituents of the mixture.
Homogenous mixtures also called solutions can be separated into the constituent substances by distillation if there is a difference in the concentration of the constituent substances in the gaseous phase. In cases where this does not happen, distillation cannot be used as a means of separation. Solutions that do not allow separation by distillation are known as azeotropes. An example of this is 95.63% ethanol and 4.37% water.
For the separation of azeotropes other more complex means of separation have to be used rather than a simple process of heating the solution and collecting the gases created.