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Separation of Mixtures

Components of mixtures are usually separated by physical means (they are therefore purified). There are different physical methods which can be employed to separate mixtures. The particular technique chosen for any given mixture depends on the nature of the constituents. Here are some separation techniques:

Evaporation

This method is used to separate components of soluble solid/liquid mixtures and volatile/involatile liquid mixtures. The principle governing this method is the fact that molecules of liquid substances when they gain heat, become gaseous and are lost from the surface. Notice that the liquid, haven vaporized is not collected but lost to the atmosphere. The other component (which is required), is then collected. Example – a mixture of sodium chloride and water.

Distillation

This is used to separate components of liquid/liquid mixtures and soluble solid/liquid mixtures. It involves heating the mixture, and the vapor formed is allowed to cool, liquefy and is collected as pure liquid. Thus, each component of the mixture is purified. The principle behind this method is based on the fact that when liquids are heated to their boiling points, they become gaseous, and when the gases are cooled, they change back to the liquid.

Note: While evaporation is mostly used for solid/liquid mixtures, distillation is mostly used to separate liquid/liquid mixtures. Both evaporation and distillation involve gain of heat, and then vaporization. In evaporation, the vapor formed is allowed to escape into the atmosphere, while in distillation, the vapor is not lost but cooled, liquefied and collected as pure liquid. Distillation is used to purify solvents. There are two kinds of distillation – simple and fractional distillation:

Simple Distillation: This is used to separate mixtures of volatile/involatile liquids, or for mixtures of liquids whose boiling points are wide apart (by at least 100^oC). An example of such mixtures is the mixture of water and ink.

Fractional Distillation: This is used to separate a mixture of liquids whose boiling points are close (boiling point difference of not more than 20-30^oC). Examples of mixtures that can be separated by this method include: petroleum; alcohol and water; liquid air (a mixture of oxygen (b.pt 90 K), nitrogen (b.pt 77 K) and water (b.pt 100⁰C)).

Sublimation

Sublimation is suitable for solid mixtures containing solid substances that can vaporize directly when heated. Examples of such substances are iodine crystals, ammonium chloride, anhydrous aluminium chloride, anhydrous iron(III) chloride and benzoic acid. The vapor is cooled away from the other component(s) and collected as solid.

Dissolution

The principle behind this technique is that some solid substances are soluble in certain  kind of solvent, while others are not. Hence, it is used generally to separate soluble substances from insoluble ones. For example, a mixture of sodium chloride crystals and sand – the sodium chloride is soluble in water while sand is not. Therefore, water is added to the mixture to dissolve sodium chloride while leaving the sand to settle.

Note: organic solvents generally dissolve organic substances, e.g. kerosene dissolves wax, grease, fats and oils. Inorganic solvents dissolve inorganic substances, and ionic solvents dissolve ionic substances. Common solvents for sulphur are: carbon(IV) sulphide, CS₂ and methylbenzene (toluene). Common solvents for iodine are: ether (ethoxyethane), alcohol, carbon tetrachloride, CCl₄ and potassium iodide.

Water soluble salts includes: All common trioxonitrates(V) of metals. All common salts of sodium, potassium and ammonium. All common tetraoxosulphates(VI), except: barium tetraoxosulphate(VI) and lead(II) tetraoxosulphate(VI). Notice that calcium tetraoxosulphate(VI) is sparingly soluble. All common chlorides except those of silver, mercury(I) and lead.

Filtration

This is used to separate liquid components of mixtures from the solid components (which are in suspension). The principle of this technique is that the particles of liquid are small enough to pass through the filter material while those of solids are not. Notice that the solid particles are in suspension.

If they were settled at the bottom, then the process would be decantation and not filtration. Decantation does not involve the use of filter materials; it is the run-off of the liquid component, leaving the solid behind. Decantation will come before filtration (depending on whether the mixture contains solid components which are large and heavy enough to settle).

Both filtration and decantation usually follow the process of dissolution. E.g. after the sodium chloride component of a mixture of sodium chloride and sand is dissolved in water, the liquid component (sodium chloride solution) is decanted (separation from sand), and then filtered to obtain clear sodium chloride solution.

Crystallization

The principle of this method is based on the fact that soluble salts are only soluble to certain concentrations at a given temperature. Decrease in the temperature of their saturated solutions will see the salts forming out of the solution. It is used to obtain a soluble salt from its solution, and it involves heating the solution up to the point of saturation (for salts which crystallize with water of crystallization, e.g., ZnSO₄ . 7H₂O).

Cooling the solution below this point results in the formation of the crystals from the solution. For salts which do not crystallize with water, e.g., NaCl, their solutions are heated to dryness to produce them. Notice that salts which crystallize with water are not heated to dryness, otherwise, their crystalline nature will be lost. To purify further, the salt can be recrystallized. I.e., the crystals obtained is dissolved in hot distill water and the process of crystallization is repeated.

Notice that crystallization needs evaporation (by heating) for the solution to become saturated. It is possible to separate a mixture of more than one water-soluble salt by crystallization. This is because the solutions of different substances attain saturation at different temperatures. A solution containing a mixture of different substances therefore crystallizes its components separately when cooled below the saturated points of the different components in solution - this is known as fractional crystallization.

Chromatography

This method is mostly popular for the separation of colored components of pigments (e.g. ink and paints). However, it is useful also in separating certain non-colored components of mixtures.

Note: All chromatographic methods involve two phases, namely: stationary phase and mobile phase. Separation is based on the relative speed of the components of the mixture in-between the two phases.

If the stationary phase is a solid, the process is called adsorption chromatography. If the stationary phase is a liquid, the process is called partition chromatography.

Column chromatography (adsorption chromatography). The stationary (adsorbent) is a solid, e.g. finely divided alumina and silica gel. The column is usually a glass tube with a tap at the bottom packed with the adsorbent and the mobile phase (the eluting solvent).

As the solvent travels down the column, it carries with it the different components, which travel down at different rates depending on the extent to which they are adsorbed. More strongly adsorbed components travel down more slowly than less adsorbed ones. Hence, components are separated based on their different degree of adsorption on to the stationary phase as they move down the column (which causes them to move at different speeds).
 

Paper chromatography (partition chromatography).

Paper chromatography, also known as partition chromatography is a technique that involves the use of strips of filter paper. Notice that the stationary phase in paper chromatography is the moisture in the paper, and not the paper itself. This is an example of partition chromatography. Separation depends on the different degree of motion (i.e. speed) of the components of the mixture between the stationary water phase and the mobile chromatographic solvent (due to the different affinity the components have for both the stationary and mobile phases).

The material to be separated is applied as a spot near the bottom of the strip of paper. It is dipped into the solvent and the chromatogram left to develop. The solvent (e.g. propanone or ethanol) ascend the strip of paper by capillary action, and carries the solute along with it, different components travel at different rates depending on their relative affinity for both the mobile and stationary phrases.

This is ascending paper chromatography. A descending technique can be made by allowing the solvent to flow down the strip from a tray containing the solvent. Components of mixtures with greater affinity for the mobile phase than the stationary phase are separated first.
 

Precipitation is used to separate a salt which is soluble in one solvent, forming a mixture with that solvent, but become insoluble when another liquid which mixes well with the mixture but which does not dissolve the salt is added. The salt will therefore be precipitated from the solution and collected by filtration. For example, iron(II) tetraoxosulphate(VI) is soluble in water to form a mixture (i.e. a solution). When ethanol is added to the solution (ethanol is miscible with water), the iron(II) tetraoxosulphate(VI) will be precipitated from the solution as it is insoluble in ethanol.

Sieving

Sieving is used to separate solid mixtures whose components’ particle size differ greatly. A sieve is used to make the separation. The particles of one component are small enough to pass through the sieve, while those of the other are not, and are therefore held onto the sieve, separated from the first. Notice that the principle of separation used here is the large difference in the particle size of the components of the mixture.