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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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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
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Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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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

Section 1 of 12Elements, 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.

Section 2 of 12Pure 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.

Section 3 of 12Separation 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.

Section 4 of 12Chromatograms 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.

Section 5 of 12Worked 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

Section 6 of 12Principle of paper chromatography

Paper chromatography separates substances that have different solubilities in a solvent.

A spot of the mixture is placed on a pencil baseline; as the solvent travels up the paper by capillary action it carries the substances with it.
More soluble substances travel further; less soluble ones, or those that stick to the paper, travel less.
Chromatogram
The output is called a chromatogram.

Section 7 of 12Method: separating food colourings

Apparatus250 cm3 beaker, wooden spill, chromatography paper, four known colourings (A to D), an unknown mixture (U), five capillary tubes, paper clip, ruler and pencil.

1
Draw a horizontal pencil line 2 cm from the bottom of the paper.
2
Use a different capillary tube for each colouring to put a tiny spot of A, B, C and D on the line, and a spot of the unknown mixture U.
3
Keep each spot no more than 2-3 mm wide and label them in pencil.
4
Pour water into the beaker to a depth below 1 cm; clip the top of the paper to the spill and rest it on the beaker so the bottom edge dips into the solvent, with the pencil line above the solvent level.
5
Let the solvent rise at least three quarters of the way up the paper, undisturbed.
6
Remove the paper and draw a pencil line at the solvent front.
7
Measure the distance from the baseline to the solvent front, and from the baseline to the centre of each spot.

Section 8 of 12Interpreting the results

Pure substances give one spot, mixtures give several.

If the spots of U line up horizontally with the spots of known colourings, those colourings are in U.
In exams chromatograms are black and white, so judge by spots in line.
What the symbols mean:
The Rf value identifies a substance: Rf = distance moved by the substance / distance moved by the solvent.
What the symbols mean: Rf has no unit and is always between 0 and 1 · both distances are measured from the baseline
It is a ratio with no units, always less than 1, and constant for a given substance in a given solvent.
Example: a spot moves 3 cm and the solvent 6 cm, so Rf = 0.5.
  • Record results in a table with the distance moved by each spot, the distance moved by the solvent and the Rf.

Section 9 of 12Common problems

If the pencil line is below the solvent level the samples wash into the solvent.

1
If the solvent front runs off the top of the paper, the solvent distance cannot be measured accurately, so remove the paper before this happens.
2
Large spots overlap and are hard to measure.
3
Handling the paper with dirty hands can add marks.
4
A different solvent gives different Rf values, so only compare chromatograms run in the same solvent.
5
Ink dyes may be insoluble in water; in that case another solvent such as ethanol is used.

For evaluation questions

  • reliability improves if the investigation is repeated and Rf values are averaged;
    • accuracy improves by measuring to the centre of spots with a ruler in millimetres;
    • validity needs the same type of paper, solvent and starting spot size for every sample.
  • If a dye does not move from the baseline it may be insoluble in the solvent, and a different solvent should be tried.

Section 10 of 12Worked examples

  1. Why must the baseline be drawn in pencil and above the solvent level? [2]2 marks

  2. A dye spot is 4.2 cm from the baseline, the solvent front 7.0 cm. Calculate Rf. [2]2 marks

  3. A chromatogram of ink shows three spots. What does this tell you? [1]1 mark

  4. Why should the same solvent be used for unknown and reference samples? [1]1 mark

Section 11 of 12Key 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.
TermMeaning
ChromatogramPaper with separated components after the solvent has run.
Solvent frontHighest point reached by the solvent.
RfDistance moved by substance divided by distance moved by solvent.
Capillary actionMovement of liquid up a narrow space or fibres.
BaselinePencil line where samples are spotted.

Section 12 of 12Exam technique

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.
1
Measuring to the edge of a spot rather than its centre.
2
Letting the solvent run off the paper.
3
Using ink to draw the baseline.
4
Giving Rf units or a value greater than 1.

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