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SingaporeChemistrySyllabus dot point

How do chemists choose the right method to separate and purify the components of a mixture?

Describe and select separation methods (filtration, crystallisation, simple and fractional distillation, use of a separating funnel) according to the properties of the substances, and test for purity using melting and boiling points

A focused answer to the O-Level Chemistry outcome on separation. Choosing filtration, crystallisation, simple and fractional distillation or a separating funnel from the properties of the mixture, and using melting and boiling points as purity tests.

Generated by Claude Opus 4.89 min answer

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

SEAB wants you to choose a separation method by looking at the properties of the substances in a mixture, then describe how the method works. The four main techniques are filtration, crystallisation, distillation (simple and fractional) and using a separating funnel. You also need to recognise that a pure substance melts and boils at a sharp, fixed temperature, so melting and boiling points are tests for purity.

The answer

Choosing a method from the properties

The whole topic is one decision tree based on what is mixed with what:

  • An insoluble solid in a liquid (sand in water): use filtration.
  • A soluble solid dissolved in a liquid (salt in water), where you want the solid: use crystallisation (or evaporation).
  • A dissolved solid where you want the liquid back (pure water from seawater): use simple distillation.
  • Two miscible liquids with different boiling points (ethanol and water): use fractional distillation.
  • Two immiscible liquids (oil and water): use a separating funnel.
  • A mixture of soluble coloured substances (dyes in ink): use chromatography (covered separately).

Filtration

Pouring the mixture through filter paper in a funnel separates an insoluble solid (the residue, trapped on the paper) from the liquid that passes through (the filtrate). It works because the solid particles are too large to fit through the pores of the paper.

Crystallisation

To recover a soluble solid, heat the solution to evaporate some of the water and make it concentrated, then leave it to cool. As it cools the solid becomes less soluble and forms crystals, which are filtered off and dried. Crystallisation is gentler than boiling to dryness, which is important if the solid decomposes on strong heating.

Simple distillation

The solution is heated; the solvent (such as water) boils off, its vapour passes into a condenser where cooling water turns it back to liquid, and the pure liquid (the distillate) is collected. The dissolved solid is left behind in the flask. This gives pure water from a salt solution.

Fractional distillation

When two liquids dissolve in each other and have different boiling points, a fractionating column is added above the flask. Vapour rising up the column repeatedly condenses and re-evaporates, so the liquid with the lower boiling point reaches the top first and distils over, while the higher-boiling liquid runs back down. This is how ethanol is separated from water and how crude oil is split into fractions.

Separating funnel and purity tests

Two liquids that do not mix (immiscible), such as oil and water, settle into layers. A separating funnel lets the lower layer run out through the tap, leaving the upper layer behind. Finally, a pure substance has a sharp melting point and a sharp boiling point; impurities lower and broaden the melting point and raise the boiling point, so these temperatures are used to check purity.

Examples in context

Example 1. Crude oil refining. Fractional distillation on an industrial scale separates crude oil into fractions such as petrol, kerosene and diesel, each collected at a different height in a tall column according to its boiling range. The same principle that separates ethanol and water in the lab runs continuously in a refinery.

Example 2. Obtaining drinking water. Simple distillation turns seawater into pure water by boiling off the water and leaving the dissolved salts behind in the flask. It is energy-hungry, which is why it is used mainly where fresh water is scarce, but it cleanly demonstrates separating a solvent from a dissolved solid.

Try this

Q1. Name the method used to separate an insoluble solid from a liquid, and name the solid collected. [2 marks]

  • Cue. Filtration; the solid collected on the filter paper is the residue.

Q2. Explain why crystallisation, rather than boiling to dryness, is used to obtain crystals of a salt that decomposes when strongly heated. [2 marks]

  • Cue. Boiling to dryness would decompose the salt; crystallisation evaporates only some water then cools, so the salt comes out as crystals without being overheated.

Q3. A liquid is suspected to contain dissolved impurity. State how its boiling point would differ from that of the pure liquid. [2 marks]

  • Cue. The impurity raises the boiling point above that of the pure liquid, and the liquid boils over a range rather than at one sharp temperature.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksA mixture contains sand, salt and water. (a) Describe how you would obtain pure, dry samples of sand and of solid salt from the mixture. (b) Name the technique used to separate the sand and explain why it works.
Show worked answer →

(a) Add more water and stir to dissolve all the salt. Filter the mixture: the sand stays on the filter paper as the residue and the salt solution passes through as the filtrate. Wash the sand with distilled water and dry it in an oven. Heat the salt solution to evaporate most of the water, then leave it to crystallise, and dry the crystals between filter paper.

(b) Filtration separates the sand. It works because sand is insoluble and its particles are too large to pass through the pores of the filter paper, while the dissolved salt and water pass through.

Markers reward dissolving then filtering, identifying sand as residue and salt solution as filtrate, recovering salt by evaporation and crystallisation, and explaining filtration by insolubility and particle size.

Original4 marksExplain why fractional distillation, rather than simple distillation, is used to separate a mixture of ethanol (boiling point 78 ∘C78\ ^\circ\text{C}) and water (boiling point 100 ∘C100\ ^\circ\text{C}), and name the part of the apparatus that makes the separation possible.
Show worked answer →

The two liquids have boiling points that are fairly close, so simple distillation would let some water vapour pass over with the ethanol, giving an impure distillate. Fractional distillation uses a fractionating column packed with glass beads or surfaces. Vapour repeatedly condenses and evaporates up the column, so by the top only the substance with the lower boiling point (ethanol) passes through.

The fractionating column is the part that makes the separation possible.

Markers reward the idea that close boiling points need repeated condensation and evaporation, naming the fractionating column, and the conclusion that ethanol distils over first because it has the lower boiling point.

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