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1.2Principles of Chemistry

Elements, Compounds or Mixtures — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

10 min read 12 parts Lesson 10 of 25 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Lesson summary

Elements, Compounds, Mixtures and Separation Techniques — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Classify substances, judge purity from melting and boiling points and choose the correct separation method for a mixture.All substances can be classified as an element, a compound or a mixture.

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Using "pure" to mean natural or clean
  • Saying compounds can be separated by filtration

Part 1 of 5Elements, compounds and mixtures

All substances can be classified as an element, a compound or a mixture.

TypeDefinitionExamples
ElementMade of atoms with the same number of protons; cannot be split into anything simpler (118 known)Copper, iron, magnesium
CompoundPure substance of two or more elements chemically combined; cannot be separated by physical meansCuSO4, CaCO3, CO2
MixtureTwo or more substances not chemically combined; separated by physical methodsSand and water, iron filings and sulfur

Always classify by asking

  • is it one kind of atom (element), more than one kind chemically joined in a fixed ratio (compound), or different substances simply mixed in any proportion (mixture)?
  • Air, sea water and alloys such as brass are mixtures, even though they look uniform.

Part 2 of 5Pure substances and purity

In chemistry a pure substance contains only one element or compound; tap water is a mixture because it also contains dissolved ions and chlorine.

1
Pure substances melt and boil at sharp, specific temperatures (water 0°C and 100°C).
2
Impure substances melt and boil over a range, usually with a lower melting point and higher boiling point.
3
Sulfur melts at 114°C; a sample that begins melting at 100°C and finishes at 113°C is impure because it melts over a range.
4
Melting point analysis is used to check the purity of drugs, and purity is also important in foodstuffs.

Part 3 of 5Separation techniques

TechniqueUsed to separateKey points
FiltrationInsoluble solid from a liquidSolid stays as residue; liquid passes through as filtrate
CrystallisationDissolved solid from solutionEvaporate some solvent to a saturated solution (test with a cold glass rod), cool slowly, filter, wash and dry crystals
Simple distillationSolvent from a solution, or a pure liquid from a mixtureHeat, vapour condenses in the condenser; cold water in at the bottom and out at the top
Fractional distillationMiscible liquids with different boiling pointsEthanol (78°C) distils before water (100°C)
Paper chromatographySubstances with different solubilities, such as inksPencil line above the solvent; solvent rises by capillary action; more soluble substances travel further

Choose a method by asking

  • is the solid dissolved or not, are the liquids miscible, and what are the boiling points?
  • To separate the soluble solid from a solution (such as sodium chloride), use crystallisation;
    • to obtain the liquid, use distillation.

Part 4 of 5Chromatograms and Rf

A pure substance gives one spot; a mixture gives more than one.

Chromatograms and Rf
Identical substances give spots at the same height, and a known reference spot helps identify an unknown.
The retention factor Rf = distance moved by the substance / distance moved by the solvent; it has no units, is always less than 1 and depends on the solvent.
Measure from the baseline to the centre of the spot, and to the solvent front.

Remember the conditions where each separation method is not suitable: filtration cannot separate dissolved substances, crystallisation is wasteful if the solid decomposes on heating, simple distillation cannot separate liquids with similar boiling points, and chromatography is used when the quantities are tiny and the substances are coloured or can be made visible. Choosing a method is usually the first mark; describing how it works is the second.

Part 5 of 5Worked examples

  1. Sulfur melts at 114°C. A sample begins to melt at 100°C and finishes at 113°C. Is it pure? [2]2 marks

  2. Name the technique used to obtain sodium chloride from salt solution. [1]1 mark

  3. Describe how ethanol is separated from a mixture with water. [3]3 marks

  4. A spot travels 3.0 cm while the solvent moves 6.0 cm. Calculate Rf. [1]1 mark

Key terms

TermMeaning
ElementSubstance made of atoms with the same atomic number.
CompoundSubstance of two or more elements chemically combined.
MixtureSubstances not chemically combined.
ResidueSolid left on the filter paper.
FiltrateLiquid that passes through filter paper.
Rf valueDistance moved by substance divided by distance moved by solvent.
MiscibleAble to mix completely.

Avoid these mistakes

1
Using "pure" to mean natural or clean.
2
Saying compounds can be separated by filtration.
3
Putting the pencil line below the solvent level.
4
Mixing up simple and fractional distillation.

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