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Separation and purification

3 learning objectives

1. Overview

In chemistry, substances are rarely found in a pure state. This topic explores the physical methods used to separate mixtures into their individual components based on their physical properties, such as solubility and boiling points. These techniques are vital for industrial processes, medical applications, and ensuring the quality of chemical products.

Key Definitions

  • Solute: The substance (usually a solid) that is dissolved in a solvent.
  • Solvent: The liquid in which the solute dissolves to form a solution.
  • Solution: A mixture formed when a solute dissolves in a solvent.
  • Saturated Solution: A solution containing the maximum amount of solute that can be dissolved at a specific temperature.
  • Residue: The solid trapped in the filter paper during filtration.
  • Filtrate: The liquid that passes through the filter paper during filtration.
  • Distillate: The liquid collected after it has been condensed during distillation.
  • Pure Substance: A substance consisting of only one element or compound with fixed melting and boiling points.

Core Content

(a) Suitable Solvents

Separation often depends on choosing a solvent that dissolves only one component of a mixture.

  • Water: Used for most ionic compounds (e.g., Sodium Chloride) and some covalent ones (e.g., Sugar).
  • Organic Solvents: Ethanol, propanone, or cyclohexane are used for substances that do not dissolve in water (e.g., fats, oils, or sulfur).

(b) Filtration

Used to separate an insoluble solid from a liquid (e.g., sand from water).

  • The mixture is poured through filter paper in a funnel.
  • The solid remains on the paper (residue).
  • The liquid passes through into a flask (filtrate).
  • 📊A glass funnel lined with filter paper sitting in a conical flask, with solid residue caught in the paper and liquid filtrate in the flask.

(c) Crystallisation

Used to separate a soluble solid from its solution (e.g., Copper(II) sulfate crystals from a solution).

  1. Heat the solution in an evaporating dish to remove some solvent until the "saturation point" is reached.
  2. Test for saturation by dipping a cold glass rod; crystals will form on the rod.
  3. Allow the solution to cool slowly. Solubility decreases as temperature drops, causing crystals to grow.
  4. Filter the crystals, wash with a small amount of cold distilled water, and dry with filter paper.

Example: Formation of Hydrated Copper(II) Sulfate

  • Word Equation: Copper(II) oxide(s) + Sulfuric acid(aq) → Copper(II) sulfate(aq) + Water(l)
  • Symbol Equation: CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

(d) Simple Distillation

Used to separate a solvent from a solution (e.g., obtaining pure water from sea water).

  • The solution is heated until the solvent boils.
  • The vapor rises, enters a condenser (cooled by a water jacket), and turns back into a liquid.
  • The solute remains in the original flask.
  • 📊A distillation flask connected to a Liebig condenser with a thermometer at the junction. A receiving flask collects the distillate.

(e) Fractional Distillation

Used to separate miscible liquids with different boiling points (e.g., ethanol and water, or crude oil).

  • A fractionating column (filled with glass beads to increase surface area) is placed above the flask.
  • The liquid with the lower boiling point evaporates first and reaches the top of the column to be condensed.
  • Liquids with higher boiling points condense on the beads and fall back into the flask until their boiling point temperature is reached at the top of the column.

Suggesting Techniques

  • Solid + Solid (one soluble): Dissolve in solvent → Filter → Crystallise filtrate.
  • Insoluble solid + Liquid: Filtration.
  • Soluble solid + Liquid (want the solid): Crystallisation.
  • Soluble solid + Liquid (want the liquid): Simple distillation.
  • Two or more liquids: Fractional distillation.

Assessing Purity

  • Pure substances: Melt and boil at specific, "sharp" temperatures (e.g., pure water boils at exactly 100°C).
  • Impure substances:
    1. Melting Point: Is lowered and occurs over a range of temperatures.
    2. Boiling Point: Is raised and occurs over a range of temperatures.

Extended Content (Extended Only)

There are no specific Supplement-only learning objectives for this sub-topic in the current IGCSE syllabus.

Key Equations

While separation is largely physical, understanding the dehydration of crystals is often tested in this context:

Dehydration of Copper(II) Sulfate Crystals:

  • Word Equation: Hydrated copper(II) sulfate(s) ⇌ Anhydrous copper(II) sulfate(s) + Water(g)

  • Symbol Equation: CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g)

  • Symbols:

    • CuSO₄·5H₂O: Blue crystals (Hydrated)
    • CuSO₄: White powder (Anhydrous)
    • ⇌: Reversible reaction

Common Mistakes to Avoid

  • ❌ Wrong: Using "evaporation to dryness" to get large crystals. ✓ Right: Heat to the point of crystallisation and then cool slowly to allow large, well-defined crystals to form.
  • ❌ Wrong: Putting the thermometer bulb into the liquid during distillation. ✓ Right: Place the thermometer bulb at the entrance to the condenser to measure the temperature of the vapor.
  • ❌ Wrong: Confusing the terms filtrate and distillate. ✓ Right: Filtrate is for filtration; Distillate is for distillation.

Exam Tips

  • Command Word: "State": If asked to "state" a method, a single word or short phrase like "Fractional distillation" is sufficient.
  • Command Word: "Suggest": Look at the properties provided (solubility and boiling points). If boiling points are close together (e.g., 78°C and 100°C), you must suggest fractional distillation, not simple.
  • Purity Questions: If an exam table shows a substance melting at 122°C–126°C, it is impure because it melts over a range rather than at a single point.
  • Real-world contexts: Expect questions on separating gases from the air (fractional distillation of liquid air) or separating crude oil. Remember that nitrogen ($N_2$) has a lower boiling point than oxygen ($O_2$).