As we know naturally occurring substances are often mixed with some unwanted substances. In order to obtain a pure sample, one must remove these unwanted substances from such mixtures. The substances in a mixture are separated by the differences in their physical properties, such as their particle size. The more different the properties are, the easier it is to separate the substances.
Sublimation technique is used to separate solid impurities from solid substances. It is a process in which a solid changes directly into its vapours on heating. Some compounds which sublimate on heating are ammonium chloride, camphor, iodine, dry ice, etc.
Evaporation and distillation techniques are used to separate the solid-liquid mixtures. Evaporation is the process of converting a liquid into its vapours, either by exposure to air or by heating is called evaporation. It is used for obtaining common salt from the sea water. Distillation is the process of converting a liquid into its vapours by heating and then condensing or cooling the vapours in the pure liquid state.
Separating funnel and fractional distillation are used to separate the liquid-liquid mixtures. Fractional distillation is the process of separating the mixture of two miscible liquids by distillation process carried out with the help of a long fractionating column. For example alcohol and water, etc. It works on the principle that in a mixture, the liquids have an appreciable difference in their boiling points. The mixture of two immiscible liquids can be separated by using separating funnel. When a mixture of immiscible liquids is added to a separating funnel, the components of mixture separate out in layers depending on their densities, the lighter liquid will float above the heavier liquid. For example, a mixture of water and kerosene, petrol and water, diesel and water, etc. are separated by using separating funnel.
Chromatography is the most modern technique which is used for obtaining pure substances from a mixture. It is used to separate the components that are soluble in the same solvent. It involves two phases which are stationary phase and mobile phase. The absorbent material in which the mixture is placed called stationary phase. The gas or liquid solvent that flows over the stationary phase is called mobile phase. It is based on the difference in the rates of absorption of different components of mixture on the surface of a suitable adsorbent.