12.4 Separation and Purification

2026 Syllabus Objectives

  1. Core: Describe and explain methods of separation and purification using:

    • (a) a suitable solvent
    • (b) filtration
    • (c) crystallisation
    • (d) simple distillation
    • (e) fractional distillation

  2. Core: Suggest suitable separation and purification techniques, given information about the substances involved

  3. Core: Identify substances and assess their purity using melting point and boiling point information

Understanding Separation Techniques

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The method of separation selected depends on the states of the chemicals involved in the mixture. Different physical properties of substances allow us to separate them effectively.

Key Terminology 🔑

Solution: Formed when a substance (solute) dissolves into another substance (solvent).

Residue: The solid left behind in the filter paper after filtration has taken place.

Filtrate: The liquid that passes through the filter paper during filtration.

Pure substance: A single chemical element or compound - it melts and boils at definite temperatures.

Mixture: Two or more substances mixed together but not chemically combined - the substances can be separated by physical means.

Separation Using a Suitable Solvent

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Principle

Differences in solubilities can be used as a simple method for separating mixtures of solids. Compounds with different types of structure show different solubilities:

  • Metallic structures are insoluble in all solvents
  • Ionic compounds (such as sodium and potassium salts) are often very soluble in water, dissociating into their ions
  • Simple molecular substances (such as halogens) are soluble in hexane and water, but others (particularly organic compounds) dissolve in hexane but not water
  • Giant covalent compounds (such as diamond and silicon dioxide) are insoluble

Method

When separating a mixture of two solids:

  1. Select a solvent in which one solid is soluble and the other is insoluble
  2. Add the solvent to the mixture
  3. Filter the mixture to obtain:
    • Residue: the insoluble material
    • Filtrate: solution containing the soluble compound
  4. Recover the soluble compound by evaporating the solvent

Example: Separating Salt and Sand

Salt and sand can be separated using water as the suitable solvent:

  • Salt dissolves in water to form a solution
  • Sand is insoluble and remains as a solid
  • The sand can be removed by filtration
  • The salt can be recovered by evaporating the water

Example: Separating Sulfur and Iron Filings

Problem: Separate a mixture of sulfur and iron filings.

Solution: Toluene is added as the solvent. The mixture is then filtered and the solvent evaporated. The iron filings are insoluble and are left behind on the filter paper (residue). The sulfur dissolves into the toluene and is produced after evaporation of the toluene.

💡 Key Point: Grinding the solid mixture increases surface area, allowing faster dissolution of the soluble component.

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