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Fractionation is the separation of a chemical mixture into (nearly) pure compounds, or into mixtures with other ratios. (Completely pure compounds are not possible in practice; fractionation would be used to get as close as can be practically attained.) Some kind of fractionation commonly accompanies a phase change of a mixture, e.g., during a change from liquid to gas. Laboratory methods of fractionation include the use of a centrifuge, as well as through distillation, the former method by accelerating the mixture to the point that denser compounds "push aside" less dense compounds, and the latter method by raising the temperature to the point that one of the molecular species evaporates. Such fractionation can occur in nature (e.g., iron falling into the center of a forming planet to form an iron core) and are components of planet formation and atmosphere formation.
Isotope fractionation is such a separation that produces mixtures with new isotope ratios.