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Mixtures and Separation Techniques

  • A mixture consists of two or more components (elements, compounds or both) which are not chemically combined
  • Each component in the mixture still retains its properties before mixing
  • The different types of mixtures include air, crude oil, alloys (e.g. brass, bronze, etc.), sea water, foams, etc.
  • The main differences between mixtures and compounds are tabulated below

Separation Techniques
  • The method chosen to effect the separation of a mixture depends on the physical properties of the constituents which are retained in the mixture.
  • The various separation techniques and their applications are tabulated below

Separating an Insoluble Solid from a Liquid
  1. Filtration
    • Filtration is carried out by pouring the mixture into a funnel containing a cone of filter paper.
    • The solid particles which are too large to pass through the pores of the filter paper are held back on the filter paper as residue
    • The liquid which passes through the filter paper is collected as the filtrate
    • Filtration can be used to separate a mixture of chalk particles and water

      Photo Credit: Key Stage Wiki

  2. Decantation
    • This technique is the same as filtration but it does not make use of a funnel and filter paper because the insoluble salt or solid is heavy enough to stay at the bottom of the liquid
    • The liquid is carefully poured away from the solid at the bottom of the vessel
    • This method is often used to separate crystals from their mother liquor

      Photo Credit: Q Know Books

  3. Centrifugation
    • This technique also does filtration and decantation do
    • It is the choice if fine particles of the insoluble solid would slow down the filtration by blocking the pores of the filter paper
    • When a liquid is spun round in a centrifuge, the force of gravity acting on the suspended particles is increased by a centrifugal force
    • The particles settle down to the bottom of the liquid which is then removed by decantation
Separation of a Soluble Solid from a Solution
  1. Evaporation
    • This is effected by heating the solution in an evaporating dish
    • The solvent or liquid component is lost to the atmosphere, leaving the dissolved solute in the evaporating dish
    • Instead of heating the evaporating dish directly on a flame, it may be heated on a water-bath or oil-bath to reduce the loss of the solid by spitting and prevent fire outbreak if the solvent is inflammable
    • This method is used in salt-making industries

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  2. Crystallization
    • This method is the choice when the dissolved solute to be recovered is easily decomposed by heat
    • The solution of the solute is first heated to drive off excess solvent and produce a saturated solution
    • On cooling the saturated solution to room temperature, crystals of the solute are obtained
    • Crystal formation may be induced by seeding (addition of tiny crystals of the solute expected to the solution to serve as the nucleus for the crystals to be formed) or scratching the inside of the vessel holding the saturated solution
    • Crystallization technique is used in the manufacture of drugs and sugar production
  3. Fractional Crystallization
    • This technique is used to separate two or more solid solutes which are dissolved in the same solution
    • The solution is concentrated by heating it as above to remove excess solvent
    • On cooling, the resulting saturated solution of the solutes, crystals of the solute with a lower solubility come out first
    • Further cooling brings out the crystals of another solute with a higher solubility
Separation of a Liquid from a Solution
  1. Simple Distillation
    • This technique is used to separate a single liquid from one or more solids dissolved in a solution
    • It is therefore useful for obtaining pure water from sea water or ink solution
    • On heating the mixture, e.g. impure water, the water in the solution turns to vapor and moves out of the distillation flask
    • It then passes through a Liebig condenser, which condenses by means of the circulating water in its outer jacket
    • The condensed liquid called distillate is then collected in a receiver
    • The dissolved impurities are left behind in the flask
    • Distillation is used in the manufacture of alcohols and the production of distilled water

      Photo Credit: Pediaa

  2. Fractional Distillation
    • This technique is used for the separation of two or more miscible liquids which have different boiling points
    • When a mixture of miscible liquids is boiled, the vapors of the components move up the fractionating column
    • The component with the lowest boiling point rises to the top of the column and passes into the condenser
    • On condensing, the liquid component is collected in a receiver as distillate
    • The thermometer registers the boiling point of the component passing over and the temperature remains constant until all of the component has distilled over
    • After this, the temperature rises and the second component will pass over, register its boiling point on the thermometer and condense into another receiver
    • The last component to distil over is the one with the higher boiling point
    • Fractional distillation can be used to separate a mixture of ethanol (b.p. 78°C) and water (b.p. 100°C)

      Photo Credit: Pediaa

  3. Chromatography
    • Chromatography is a method of separating a mixture of similar compounds which may not be able to withstand high temperature treatment
    • It works on the principle that different but similar compounds move at different rates in a mobile phase because they are held to different extents on a stationary phase
    • The following are some of the types of chromatography:
      1. Column Chromatography
        • In column chromatography, a suitable adsorbent, such as a slurry of alumina, silica gel or cellulose, is used to pack the column in a burette. This serves as a stationary phase
        • The mixture of the compounds to be separated is dissolved in the least amount of solvent and the solution is allowed to run through the column with the help of a dropping funnel
        • The different components of the mixture are adsorbed at different distances from the top in the form of bands
        • Using a dropping funnel, a suitable solvent, acting as the mobile phase is run through the column
        • The strongly adsorbed substances move slowly in the mobile phase and form bands near the top
        • The weakly adsorbed components move faster and form bands near the bottom
        • The number of bands formed is equal to the number of components in the mixture
        • The column, together with the bands showing on it, is known as a chromatogram
        • The separated substances may be recovered by allowing the mobile phase to wash out each band through the jet of the burette

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      2. Paper Chromatography
        • In paper chromatography, the adsorption column is replaced by a filter paper of good quality
        • A base line is drawn with pencil on a rectangular chromatographic paper (or filter paper) at a point about 2 cm from the edge of the paper
        • The mixture of compounds to be separated is dissolved in the minimum amount of suitable solvent
        • The resulting concentrated solution is then spotted in the base line by means of a syringe or a micro pipette.
        • The paper is then suspended in a closed air-tight jar with the spotted end (but not the spot) dipping into an appropriate solvent which acts as the mobile phase
        • The solvent or mobile phase moves up the paper strip by capillary action
        • The ascending solvent carries along with it any component of the spotted mixture that is not strongly held by the paper
        • The components which are not equally held by the paper are therefore carried along by the solvent to different distances on the paper
        • The different components appear as spots of varying distances from the base line
        • When the ascending front of the solvent is approaching the top of the paper, the chromatogram (paper with separated spots) is removed and dried
        • If the solutes or spots are colorless, the paper may be sprayed with appropriate reagents to develop colored spots
        • Each solute can be identified by the distance it has traveled by comparing the distance with those of known substances
        • Another quantitative approach is to obtain the Rf value of each separated spot and compare it with standard values in order to identify the substance
        • Rf = distance traveled by solute (spot) ÷ distance traveled by solvent front
        • The number of spots on a paper chromatogram is the same as the number of components present in the mixture being analysed

          Photo Credit: Byjus

Separation of Liquids that do not Mix
  • Liquids that do not mix are immiscible
  • Immiscible liquids such as a mixture of benzene and water can be separated using a separating funnel
  • In a separating funnel, immiscible liquids separate into two clear layers with a distinct boundary between them
  • Once the tap is opened, the lower (more dense or heavier) layer is run out, leaving the upper less dense liquid layer in the funnel

    Photo Credit: GCSE Science

Separation of a Solid from a Mixture of Solids
  • Sublimation is the process by which a heated solid substance changes directly into the gaseous state without passing through the liquid state
  • Very few substances sublime
  • Ammonium chloride, anhydrous aluminum chloride and iodine are examples of substances that sublime
  • Sublimation technique is used to separate a solid substance which can sublime from a mixture of solids
  • On heating, the substance capable of subliming moves into the gaseous and recondenses on a cold surface as a solid called a sublimate

    Photo Credit: Chemistry Notes Blog