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SingaporeChemistrySyllabus dot point

How can we separate a mixture into its parts using differences in their physical properties?

Describe filtration, evaporation, crystallisation, simple and fractional distillation and chromatography, and choose the right method to separate a given mixture

A focused answer to the N(A) Chemistry outcome on separating mixtures. Filtration, evaporation, crystallisation, simple and fractional distillation and chromatography, the property each one uses, and how to pick the right method.

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

The syllabus wants you to describe the common methods used to separate mixtures, filtration, evaporation, crystallisation, simple distillation, fractional distillation, and paper chromatography, and to choose the right method for a given mixture. Each method works because the parts of the mixture differ in a physical property such as solubility, particle size, or boiling point. You need to know what each method separates and the property it relies on.

The answer

Filtration

Filtration separates an insoluble solid from a liquid. Pour the mixture through filter paper in a funnel. The solid is trapped as the residue; the liquid that passes through is the filtrate. This works because the solid particles are too big to pass through the tiny holes in the paper. It is used to separate sand from water or to collect a precipitate.

Evaporation and crystallisation

To get a dissolved solid back from its solution you cannot filter it, because it has dissolved. Instead you remove the water.

  • Evaporation to dryness boils off all the water, leaving the solid. It is used when the solid does not break down on heating, such as common salt.
  • Crystallisation heats the solution to remove some water, then cools it so that pure crystals form and separate from the remaining liquid. It is gentler and gives better crystals, used for solids that would decompose if heated to dryness.

Simple distillation

Simple distillation separates a liquid (the solvent) from a dissolved solid, and collects the pure liquid. The solution is heated, the liquid boils and turns to vapour, the vapour passes into a condenser where it cools back to liquid, and the pure liquid drips into a flask. It is used to get pure water from salt water.

Fractional distillation

Fractional distillation separates two or more liquids that mix together but have different boiling points, such as ethanol and water, or the liquids in crude oil. A fractionating column above the flask gives a tall surface where vapour condenses and re-evaporates many times. The liquid with the lowest boiling point reaches the top and comes off first; the others follow as the temperature rises.

Paper chromatography

Paper chromatography separates a mixture of dissolved coloured substances, such as the dyes in an ink. A spot of the mixture is placed on paper and a solvent soaks up through it. Substances that dissolve well and are weakly held by the paper travel further, so the components spread into separate spots.

Examples in context

Example 1. Getting drinking water from sea water. Sea water is salt dissolved in water. Simple distillation boils off the water, the condenser cools it, and pure water is collected while the salt stays behind. This is the chemistry behind desalination plants, which produce fresh water in places where rainfall is low.

Example 2. Crude oil in industry. Crude oil is a mixture of many liquids with different boiling points. A fractionating column at a refinery separates it into useful fractions such as petrol, diesel, and bitumen, each coming off at a different height where its boiling point matches the temperature. This is fractional distillation on an industrial scale.

Try this

Q1. Name the method used to obtain pure water from a solution of sugar in water. [1 mark]

  • Cue. Simple distillation, which boils off the water and condenses it back to a pure liquid.

Q2. A student wants to recover dissolved copper(II) sulfate crystals from its solution without the crystals breaking down. State the best method and why. [2 marks]

  • Cue. Crystallisation, because it removes only some water and lets crystals form on cooling, which is gentler than evaporating to dryness and avoids decomposing the solid.

Q3. Black ink is spotted on chromatography paper and separates into three coloured spots. Explain what this tells you about the ink. [2 marks]

  • Cue. The ink is a mixture of three dyes; they separate because each dye dissolves in the solvent and is held by the paper to a different extent, so they travel different distances.

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.

Original4 marksA mixture contains sand and salt. (a) Describe how you would obtain pure, dry salt crystals from the mixture. (b) Name the property of sand that lets you separate it from the salt solution.
Show worked answer →

(a) Add water and stir so the salt dissolves but the sand does not. Filter the mixture: the sand stays on the filter paper as residue and the salt solution passes through as the filtrate. Heat the salt solution to evaporate most of the water, then leave it to cool so salt crystals form. Filter or pour off and dry the crystals.

(b) Sand is insoluble in water, so it does not dissolve and can be filtered out while the salt stays in solution.

What markers reward: dissolving then filtering to remove sand, evaporating and crystallising to get the salt, and naming insolubility as the key property.

Original3 marksExplain why fractional distillation, rather than simple distillation, is used to separate a mixture of two liquids such as ethanol and water that have boiling points fairly close together.
Show worked answer →

The two liquids have different but fairly close boiling points, so simple distillation would let some of both vapours through together and the product would be impure. A fractionating column gives a large surface where the rising vapour repeatedly condenses and re-evaporates. The liquid with the lower boiling point reaches the top first while the higher boiling liquid drips back, so the column separates them far better than a single distillation.

What markers reward: close boiling points making simple distillation poor, the fractionating column allowing repeated condensing and evaporating, and the lower boiling liquid coming off first.

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