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1.3Experimental Chemistry

Separation and purification — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

5 min read 16 parts Lesson 4 of 10 1 quiz

📚 Combined Science 0653 - Chemistry Hub / Topical Revision
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Lesson summary

1.3 Separation and purification — COMBINED SCIENCE (CHEMISTRY) notes: Describe and choose separation and purification methods and use melting and boiling points to judge purity.Principle — every technique relies on a difference in a physical property of the substances.Solvent cho…

  • Solvent choice
  • Ethanol and water
  • Gas mixtures
  • Pure substances
  • Impurities
  • Identification

Part 1 of 7Choosing a technique

Principle
  • Principle every technique relies on a difference in a physical property of the substances.
Solvent choice
  • Solvent choice for a solid mixture only the wanted substance should dissolve in the solvent.
  • Sand and salt are separated with water because salt dissolves and sand does not.

Part 2 of 7Filtration

Use
  • Use separates an undissolved solid from a liquid or solution.
  • Centrifugation can also be used.
Method
  • Method pour the mixture through filter paper in a funnel.
  • The liquid passes through as the filtrate.
  • The solid is too large to pass and stays as the residue.

Part 3 of 7Crystallisation

1
Heat the solution to evaporate the solvent until it is saturated, removing no more than about half the liquid.
2
Test by dipping a cold glass rod in; crystals form on it if the solution is saturated.
3
Leave the saturated solution to cool slowly so that crystals grow.
4
Filter off the crystals.
5
Wash them with distilled water and dry them between filter papers or in an oven.
  • Use
    • separates a dissolved solid that is more soluble in hot solvent than in cold, such as copper(II) sulfate.

Part 4 of 7Simple distillation

1Use
  • Use separates a pure liquid from a solution or from a mixture of liquids.
2Process
  • Process the solution is heated and the solvent evaporates, then it cools in a condenser and is collected.
  • Only the solid solute is left behind.
3Limit
  • Limit it separates the products of fermentation but less effectively than fractional distillation.

Part 5 of 7Fractional distillation

Use
  • Use separates miscible liquids with close boiling points or when higher purity is needed, such as crude oil.
Ethanol and water
  • Ethanol and water ethanol distils first.
    • (boiling point 78 °C)
  • Heating stops when the temperature rises towards 100 °C.
Safety
  • Safety an electric heater is safer than a naked flame with flammable liquids.
Gas mixtures
  • Gas mixtures gases can be liquefied by cooling and then separated by fractional distillation.

Part 6 of 7Purity and melting and boiling points

Pure substances
  • Pure substances melt and boil at sharp, fixed temperatures.
  • Water melts at 0 °C and boils at 100 °C.
Mixtures
  • Mixtures melt and boil over a range of temperatures.
Impurities
  • Impurities lower the melting point and raise the boiling point.
Identification
  • Identification measured values are compared with data tables, and the closer they are the purer the sample.

Part 7 of 7More from the specification

1
Chromatographic development with locating agents
  • Chromatographic development with locating agents ultraviolet illumination or chemical locating agents reveal colorless chromatogram zones for reliable separation and identification.
2
Chromatography principles
  • Separation technique utilizing differential partition of dissolved components between a mobile phase and a stationary phase.
3
Chromatogram interpretation
  • The visual output showing separated solute spots whose retention factor depends on relative distance moved versus solvent front.
4
Purification by crystallisation
  • Producing high-purity crystals of soluble salts by evaporating solvent to saturation point and cooling.
    • (such as magnesium chloride)
5
Differential solubility separation
  • Separating insoluble calcium sulfate from soluble magnesium chloride via dissolution in water, followed by filtration.
6
Locating agent
  • Chemical that makes colourless spots visible.
7
Simple distillation
  • Separating a solvent from a solution by evaporating and condensing it.
8
Chromatography
  • Method separating substances by their different solubilities in a solvent.
9
Chromatogram
  • The paper showing the separated spots.
10
Crystallisation
  • Obtaining a dissolved solid from a solution by evaporating some of the solvent and cooling.
11
Solubility
  • Mass of solute that dissolves in a given mass of solvent.
12
Chloride
  • Salt of hydrochloric acid.

Key terms

TermMeaning
FiltrationSeparating an insoluble solid from a liquid.
CrystallisationFormation of crystals from a saturated solution on cooling.
DistillationEvaporating and condensing a liquid to separate it.
Fractional distillationSeparating miscible liquids by boiling point.
MiscibleAble to mix completely.
CondenserApparatus that cools vapour to liquid.

Avoid these mistakes

1
Evaporating all the water — this can decompose the crystals.
2
Using simple distillation for liquids with close boiling points.
3
Confusing filtrate (liquid) with residue (solid).
4
Saying impurities raise the melting point — they lower it.

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