Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

4.1.2Atomic Structure and the Periodic Table

Separating Mixtures

5 min read 6 parts Lesson 1 of 10 Unit quiz

0%read

Section 1 of 6Physical Separation

4.1.2 Separating Mixtures

Mixtures are separated by physical means, which leave the chemical identity of each component unchanged.

Mixture
  • Two or more substances that are not chemically joined, which can be separated by physical means.
Basis of separation
  • Each method depends on a difference in a physical property, such as boiling point, solubility or particle size.
Choice of method
  • The method chosen depends on the substances present, such as an insoluble solid, a soluble solid or a mixture of liquids.
Chemical identity
  • Each component keeps its chemical nature after separation, because no chemical reaction has taken place.

Section 2 of 6Choosing a Method

Choosing a Method
Distillation
  • Separates a solvent from a solution;
    • water boils at 100 °C, and its vapour is condensed and collected as pure water.
Filtration
  • Separates an insoluble solid such as sand from water; the liquid that passes through is the filtrate.
Crystallisation
  • Separates a dissolved solid that is much more soluble in hot solvent, such as copper(II) sulfate from solution.
Rule of thumb
  • Use filtration for insoluble solids, distillation to recover a solvent, and crystallisation for soluble solids.

Section 3 of 6Crystallising a Solution

Crystallisation recovers a dissolved solid by controlling how it leaves solution.

1
Heat the solution to evaporate some of the solvent until the solution becomes saturated.
2
Test for saturation by dipping a cold, clean glass rod into the solution; crystals forming on the rod show that it is saturated.
3
Allow the saturated solution to cool slowly, so that crystals grow as the solid becomes less soluble.
4
Collect the crystals by filtration, wash them with a little cold distilled water to remove impurities, and leave them to dry.

Section 4 of 6Paper Chromatography

Chromatography separates components by how they move up paper in a solvent.

Principle
  • Components of a mixture have different solubilities in the solvent, so they travel different distances up the paper.
Phases
  • The stationary phase is the chromatography paper, and the mobile phase is the solvent, often water or ethanol.
Start line
  • A pencil line is used for the start, because ink would run into the sample spots and contaminate the result.
Rf value
  • Rf equals the distance moved by the substance divided by the distance moved by the solvent;
    • it has no units and is less than 1.

Section 5 of 6Worked Example

Question

  • A substance travels 2.4 cm up a chromatogram while the solvent front travels 6.0 cm.
  • Calculate the Rf value.

  1. Rf = distance moved by substance ÷ distance moved by solvent.
    What the symbols mean: Rf has no unit and is always between 0 and 1 · both distances are measured from the baseline
  2. Rf = 2.4 cm ÷ 6.0 cm = 0.40.

Answer The Rf value is 0.40, which has no units.

Section 6 of 6Exam Checklist & Tips

4.1.2 – Key Points for the Exam

  • Distillation Separating a liquid from a mixture by evaporating it and then condensing the vapour.
  • Filtrate The liquid that passes through filter paper.
  • Saturated solution A solution that cannot dissolve any more solute at that temperature.
  • Rf value The ratio of the distance moved by a substance to the distance moved by the solvent in chromatography.

Common Mistakes

  • Using ink instead of pencil for the start line of a chromatogram.
  • Using the distance from the origin to the sample without comparing it with the solvent front distance.
  • Saying that filtration separates dissolved solids from a solution.
  • Forgetting that the Rf value of a substance changes if the solvent is changed.
\n