Is Matter Around Us Pure • Topic 2 of 3

Separation of Mixtures

Because the parts of a mixture keep their own properties, we can separate them by simple physical methods that exploit a difference in one property — size, density, boiling point, solubility or attraction to a solvent. Choosing the right method is half the skill of chemistry.

Evaporation

Evaporation separates a non-volatile soluble solid from its liquid. Heating a salt solution drives off the water as vapour and leaves the salt behind. It is used to recover dyes from inks.

Centrifugation

Centrifugation spins a mixture at high speed so that the denser particles are flung to the bottom while the lighter liquid stays on top. It separates very fine suspended particles that are too small to filter — for example, cream from milk, or blood cells from plasma in a clinic.

Separating funnel

Two immiscible liquids (which do not mix), such as oil and water, are separated using a separating funnel. The denser liquid sinks to the bottom and is run off first through the stopcock; the lighter layer is collected next.

Sublimation

If a mixture contains a substance that sublimes (changes directly from solid to vapour on heating), it can be separated this way. A mixture of common salt and ammonium chloride is separated by heating: ammonium chloride sublimes and re-deposits on a cool surface, leaving the salt behind. Camphor and iodine also sublime.

Chromatography

Chromatography separates substances that dissolve in the same solvent but travel at different rates. In paper chromatography, a drop of ink is placed on filter paper and a solvent rises through it; each component moves a different distance, giving separate spots. It is used to separate the colours in a dye and even pigments from a leaf.

Distillation and fractional distillation

Distillation separates two miscible liquids whose boiling points differ by more than about 25 °C, such as acetone and water. The mixture is boiled; the vapour of the lower-boiling liquid is led through a condenser and collected. When the boiling points are close, fractional distillation is used — a fractionating column packed with beads gives repeated condensation and evaporation, sharpening the separation. It separates the gases of air and the components of petroleum.

Crystallisation

Crystallisation purifies a solid by dissolving it in a hot solvent and letting pure crystals form as the solution cools and is filtered. It gives purer results than simple evaporation and is used to obtain pure copper sulphate crystals.

Obtaining the gases of air

Air is first cooled and compressed to a liquid, then warmed in a fractionating column. The gases boil off in order of their boiling points — nitrogen first (lowest boiling point), then argon and oxygen — so the components of air are separated.

Simple distillation: a mixture is boiled, the vapour is condensed and the distillate is collectedmixtureheatthermometercondenser (water in/out)distillateDistillation separates miscible liquids by boiling pointboil → vapour → condense → collect
Solved Examples
1
Worked Example
Name the method best suited to separate (a) salt from sea water, (b) cream from milk.
Solution
  1. Salt is a non-volatile soluble solid in water, so heating drives off the water and leaves the salt — evaporation.
  2. Cream is a fine, slightly less dense suspension in milk that cannot be filtered — spinning separates it.
  3. So cream is separated by centrifugation.

Answer: (a) evaporation, (b) centrifugation.

2
Worked Example
A mixture contains common salt and ammonium chloride. How would you separate the two? Justify your choice.
Solution
  1. Ammonium chloride sublimes on heating (solid → vapour) while common salt does not.
  2. On warming the mixture, ammonium chloride vapour rises and re-deposits as solid on a cool surface.
  3. The salt is left behind in the dish.

Answer: Use sublimation; ammonium chloride sublimes and is collected, leaving the salt.

3
Worked Example
Oil and water are present together in a beaker. Which apparatus separates them, and which layer is removed first?
Solution
  1. Oil and water are immiscible liquids, so they form two separate layers.
  2. A separating funnel is used; the denser water sinks to the bottom.
  3. The water is run off first through the stopcock; the lighter oil is collected afterwards.

Answer: A separating funnel; the lower (denser) water layer is removed first.

4
Worked Example
Acetone (b.p. 56 °C) and water (b.p. 100 °C) are mixed. Which separation method should be used and why?
Solution
  1. Both are miscible liquids, so a separating funnel will not work.
  2. Their boiling points differ by 44 °C, which is more than about 25 °C.
  3. So simple distillation is suitable; acetone boils off first and is condensed and collected.

Answer: Simple distillation, since the boiling points differ by more than 25 °C.

5
Worked Example
Why is fractional distillation, rather than simple distillation, used to separate the gases of air?
Solution
  1. The main gases of air have boiling points very close together (nitrogen −196 °C, argon −186 °C, oxygen −183 °C).
  2. Simple distillation cannot separate liquids with such close boiling points.
  3. A fractionating column provides repeated evaporation and condensation, so each gas is collected as it boils off in turn.

Answer: The gases have very close boiling points, so the column of fractional distillation is needed for a clean separation.

6
Worked Example
A sample of impure copper sulphate is to be purified to pure crystals. Which method is preferred over simple evaporation, and why?
Solution
  1. Evaporation would leave behind the impurities along with the copper sulphate.
  2. In crystallisation, the solid is dissolved in hot water, filtered to remove insoluble impurities, then cooled.
  3. Pure copper sulphate forms regular crystals on cooling, while soluble impurities stay in solution.

Answer: Crystallisation is preferred because it gives purer crystals and removes both soluble and insoluble impurities.

Key Points

  • Mixtures are separated by physical methods that use a difference in size, density, boiling point or solubility.
  • Evaporation recovers a soluble non-volatile solid; centrifugation spins out fine suspended particles too small to filter.
  • Immiscible liquids are separated with a separating funnel; substances that sublime are separated by sublimation.
  • Distillation separates miscible liquids with boiling points > 25 C apart; fractional distillation is used when boiling points are close (e.g. gases of air, petroleum).
  • Crystallisation gives purer crystals than evaporation; chromatography separates substances soluble in the same solvent by their different rates of travel.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.Which method separates two immiscible liquids?
Explanation: A separating funnel lets the denser liquid be run off first; distillation is for miscible liquids.
Q2.A mixture of salt and ammonium chloride is best separated by:
Explanation: Ammonium chloride sublimes on heating while salt does not, so sublimation separates them.
Q3.The technique used to separate different pigments of a dye is:
Explanation: Chromatography separates components that dissolve in the same solvent but move at different rates.
Q4.Fractional distillation is preferred over simple distillation when the two liquids have:
Explanation: Close boiling points need a fractionating column for repeated evaporation and condensation.
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