Separation of Components of a Mixture

Is matter pure??

For a common man, purity means that there is no adulteration in the matter.
But for scientists, purity means that all the constituent particles of that substance are same in their chemical nature.

Example: It is written "pure" on milk packs. Here, pure means no adulteration is done, but milk itself is a mixture of fats, proteins, water, etc. Hence, a chemist doesn't consider milk as pure.

*image taken from google.

For a chemist, it is necessary to separate components of a mixture. The separation of the components makes it possible to study and use the individual components of a mixture.

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In a mixture, solute and solvent can exist in following pairs:
1. Solid-solid
2. Solid-liquid
3. Liquid-liquid
4. Gas-gas
(Though we have more combinations, but the separation techniques that we use comprises mainly of these pairs)

Solid-Solid

Hand picking: Use to separate heterogeneous mixture by simple physical method.

Threshing: Generally use to separate grain seeds from their stalks.
Winnowing: use to separate heavier and lighter components by wind or blowing air.
Sieving: use to separate fine particles from bigger particles of a mixture by passing through a sieve.
Sublimation: Use to separate the mixture in which 1 of the components can sublimate.
Magnetic separation: Used to separate the mixture in which 1 of the components is of magnetic nature like iron filings.

Solid-liquid

Crystallisation: A supersaturated solution is allowed to cool down and crystals of pure solid separates out.
Evaporation: Use to separate the volatile component (solvent) from its non-volatile solute.
Sedimentation and decantation: In this, heavier components settle after water is added and then water along with lighter impurities is poured into another container.
Filtration: Components of size greater than 100 nm cannot pass through filter paper and hence can be separated using filter papers.
Chromatography: Used for separation of those solutes that dissolve in the same solvent.
Churning (or centrifugation): In this, the denser particles are forced to the bottom and the lighter particles stay at the top when spun rapidly.

Liquid-liquid

Distillation: Use for separation of components of a mixture containing two miscible liquids that boil without decomposition and have sufficient difference in their boiling points.
Fractional distillation: Temperature difference between 2 miscible liquids is less than 25 K.
Using separating funnel: The immiscible liquids separate out in layers depending on their densities.

Gas-gas

Separation of components of air: Consecutive freezing and fractional distillation is done.

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When more than 2 components are present, a combination of various techniques can be used. For example, for separating a mixture of ammonium chloride, salt and sand, we can first heat the mixture in China dish, ammonium chloride will sublimate and get separated. Next, add water to the mixture of sand and salt. Salt will dissolve and sand remain as it is. Filter the sand. Now, you are left with only salt solution. Evaporate water and you will get salt crystals. Hence, all the components of a mixture are separated.