GCSE · AQA Combined Science · Chemistry Paper 2 · C8 Chemical Analysis

Chemical analysis, for the exam.

The whole of C8 — what "pure" really means, formulations, reading a chromatogram and calculating Rf, and the four gas tests you have to recall. Built for both tiers.

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Both tiers in one booklet. Everything here is for Foundation and Higher. Anything that's Higher tier only sits in a purple HT box — Foundation students can skip those. Green boxes are required practicals. Do one topic at a time; each is about 10–15 minutes.

Topic 01 · C8 · Purity & formulations

Pure substances & formulations

By the end of this topic you'll know what "pure" means in chemistry (not what it means on a milk bottle), how melting point reveals purity, and why a formulation is a mixture made on purpose.

Part 1What "pure" means in chemistry

In everyday language "pure" can mean natural or unspoilt — "pure orange juice", "pure mountain air". In chemistry it means something much stricter. A pure substance is a single element or a single compound, not mixed with anything else.

So pure orange juice is, to a chemist, a mixture — it has water, sugars, acids and more. Pure water, on the other hand, is only H2O molecules and nothing else.

Key words

Pure substance
A single element or single compound, with nothing else mixed in.
Mixture
Two or more substances not chemically joined — they keep their own properties and can be separated physically.
Formulation
A mixture made to an exact recipe, where each component has a job and a measured amount.

A handy test: a pure substance melts and boils at one fixed temperature. A mixture melts and boils over a range of temperatures, and the melting point is usually lower than that of the pure substance. So measuring the melting point tells you how pure a sample is.

MELTING: PURE vs IMPURE PURE 0 °C melts at one sharp temperature IMPURE −4 to −1 °C melts over a range, and starts lower
An impurity lowers the melting point and spreads it over a range — salt on icy roads works this way

⚠ Watch out — "pure" is a trap word

In an exam, "pure" never means "natural" or "clean". It means one element or one compound only. If a sample melts over a range of temperatures, it is not pure — and the wider the range, the more impurity is present.

Quick check

A solid melts cleanly at exactly 80 °C every time. What does this tell you?

  • AIt must be an element
  • BIt is likely a pure substance
  • CIt is definitely a mixture
  • DIt is a formulation
Show answer
B. A single, sharp melting point is the signature of a pure substance. It could be an element or a compound, so A is too narrow. A mixture would melt over a range, so C and D are wrong.

Part 2Formulations — mixtures by design

A formulation is a mixture that has been designed to a precise recipe. Each component is there for a reason and is present in a carefully measured quantity, so the product does its job exactly right.

Formulations are everywhere: paints (pigment, binder, solvent, additives), medicines (the active drug plus fillers and coatings), fuels, cleaning products, fertilisers, alloys and foods. Change the proportions and you change how the product behaves — which is why the recipe matters.

A PAINT FORMULATION paint • pigment — gives the colour • binder — sticks it to the wall • solvent — keeps it runny • additives — drying, finish
Each ingredient has a job and a measured amount — that is what makes it a formulation, not just any mixture

Worked example — is it a formulation?

A pharmacist describes a paracetamol tablet as containing 500 mg of the active drug plus measured amounts of a binder, a filler and a coating. Explain why this is a formulation.

Mixture?Yes — several substances, not chemically joined.
Designed?Each component has a purpose (drug, binder, filler, coating).
VerdictMade to an exact recipe with measured amounts → a formulation.
Quick check

Which of these is best described as a formulation?

  • APure copper
  • BA lump of rock dug from the ground
  • CAn alloy made by mixing measured amounts of metals
  • DDistilled water
Show answer
C. An alloy is a mixture made to a precise recipe so it has the properties you want — a formulation. A and D are pure substances; B is a random mixture, not designed to a recipe.
Topic 1 — quick quiz
Click to reveal · 4 questions
  1. What does "pure substance" mean to a chemist?
    A single element or single compound, with nothing else mixed in. (Not the everyday meaning of "natural" or "clean".)
  2. How can a melting point tell you whether a substance is pure?
    A pure substance melts at one sharp temperature. An impure substance (a mixture) melts over a range, and the melting point is lower than the pure value.
  3. Define a formulation.
    A mixture made to an exact recipe, where each component has a particular purpose and is present in a measured amount — e.g. paints, medicines, fuels, alloys.
  4. Is sea water a pure substance? Explain.
    No. It contains water plus dissolved salts, so it is a mixture — it would boil and freeze over a range, not at the fixed points of pure water.
Topic 02 · C8 · Chromatography

Chromatography & Rf values

How a smudge of ink splits into separate colours, why each spot travels its own distance, and the one calculation you must be able to do.

Part 1How chromatography separates a mixture

Paper chromatography separates the substances in a mixture and helps you tell them apart. It works because of two "phases":

The stationary phase is the chromatography paper, which stays put. The mobile phase is the solvent (water or ethanol) that moves up through the paper, carrying the substances with it.

Each substance in the mixture is attracted to the two phases by different amounts. A substance that is more attracted to the solvent (and less to the paper) travels further up. One that clings to the paper barely moves. Because they separate, a single spot of a mixture spreads into several spots — one for each component.

A FINISHED CHROMATOGRAM solvent front start line (pencil) spot moved solvent moved
Measure both distances from the pencil start line — the spot's distance and the solvent's distance

⚠ Watch out — pencil, not pen

Always draw the start line in pencil. Pencil (graphite) is insoluble, so it won't run up the paper. If you used pen, the ink would dissolve and separate too, ruining the result. And keep the start line above the solvent — if the spot sits in the solvent it just washes off.

Part 2Pure, impure, and the Rf value

A chromatogram tells you a lot. A pure substance produces a single spot in any solvent. If you see two or more spots, the sample is a mixture. You can identify substances by comparing their spots with known reference substances run alongside.

To put a number on how far a spot travelled, we calculate its Rf value — the ratio of the distance the spot moved to the distance the solvent moved.

Equation

Rf = distance moved by substance ÷ distance moved by solvent given
Rf has no units (it's a ratio of two distances) and is always between 0 and 1. Measure both distances from the pencil start line.

Worked example — calculating Rf

A spot moves 4.0 cm up the paper. In the same time the solvent front moves 5.0 cm. Calculate the Rf value.

EquationR₌ = distance of spot ÷ distance of solvent
Sub in= 4.0 ÷ 5.0
AnswerR₌ = 0.8 (no units)

⚠ Watch out — Rf is always below 1

The spot can never travel further than the solvent that carries it, so Rf is always between 0 and 1. If you get a value above 1, you've divided the wrong way round — it's spot ÷ solvent, not solvent ÷ spot.

Investigating chromatography & Rf values

Aim: separate and tell apart the coloured substances in a mixture (e.g. food colourings or inks) and calculate their Rf values.

  1. Draw a start line in pencil about 2 cm from the bottom of the chromatography paper.
  2. Add a small spot of each mixture (and reference colours) on the line, letting each dry. Keep spots small and well separated.
  3. Pour solvent into the beaker so it sits below the start line, then stand the paper in it. Cover the beaker to stop the solvent evaporating.
  4. Let the solvent rise most of the way up, then remove the paper before it reaches the top.
  5. Immediately mark the solvent front in pencil and leave the paper to dry.
  6. Measure the distance moved by each spot and by the solvent, then calculate R₌ = spot ÷ solvent for each colour.

Control / improve: cover the beaker so the solvent doesn't evaporate, and keep spots small so they don't overlap. To compare two samples fairly, run them on the same paper in the same solvent — Rf only matches when conditions are identical.

Quick check

A spot moves 3 cm while the solvent moves 12 cm. What is the Rf value?

  • A4.0
  • B0.25
  • C9 cm
  • D0.36
Show answer
B — 0.25. Rf = 3 ÷ 12 = 0.25 (no units). Answer A divides the wrong way (12 ÷ 3) and gives an impossible value above 1.
Topic 2 — quick quiz
Click to reveal · 5 questions
  1. Name the stationary phase and the mobile phase in paper chromatography.
    Stationary phase = the paper (it stays still). Mobile phase = the solvent (it moves up, carrying the substances).
  2. Why is the start line drawn in pencil, not pen?
    Pencil (graphite) is insoluble, so it won't run up the paper. Pen ink would dissolve and separate, spoiling the chromatogram.
  3. How can you tell from a chromatogram that a substance is pure?
    A pure substance gives one spot only. Two or more spots means it's a mixture.
  4. Write the equation for Rf and state its range.
    Rf = distance moved by substance ÷ distance moved by solvent. It has no units and is always between 0 and 1.
  5. A spot travels 6 cm and the solvent travels 8 cm. Calculate the Rf value.
    Rf = 6 ÷ 8 = 0.75.
Topic 03 · C8 · Tests for gases

The four gas tests

Four gases, four tests, four results — learn them as matched pairs and you'll never lose these easy marks.

Part 1The tests you must recall

You need to recall the test and the positive result for four gases. These are guaranteed marks if you've learned them — get them word-perfect.

The four gas tests

Hydrogen — H2
Hold a lit splint at the mouth of the tube. A positive result is a squeaky pop.
Oxygen — O2
Put a glowing splint into the tube. Oxygen relights the glowing splint.
Carbon dioxide — CO2
Bubble the gas through limewater. It turns the limewater cloudy / milky.
Chlorine — Cl2
Hold damp litmus paper in the gas. It bleaches the paper white (it may turn red first).
FOUR TESTS, FOUR RESULTS Hydrogen test: lit splint result: squeaky pop Oxygen test: glowing splint result: splint relights Carbon dioxide test: bubble through limewater result: limewater goes cloudy Chlorine test: damp litmus paper result: litmus bleaches white
Learn each test paired with its result — examiners want both halves

⚠ Watch out — don't swap the splints

The two splint tests are easy to mix up. Hydrogen = lit splint → squeaky pop. Oxygen = glowing splint → relights. A lit splint in oxygen just keeps burning (not a "pop"), and a glowing splint won't pop in hydrogen. For chlorine, the litmus must be damp — dry litmus won't show the bleaching.

Worked example — identifying an unknown gas

A colourless gas turns damp blue litmus paper red and then bleaches it white. Name the gas and justify your answer.

ClueIt bleaches damp litmus paper white.
MatchOnly chlorine bleaches litmus (it's also acidic, hence red first).
AnswerThe gas is chlorine (Cl₂).
Quick check

A glowing splint is lowered into a test tube of gas and it relights. Which gas is present?

  • AHydrogen
  • BCarbon dioxide
  • COxygen
  • DChlorine
Show answer
C — Oxygen. A glowing splint that relights is the oxygen test. Hydrogen needs a lit splint and gives a squeaky pop; carbon dioxide would put the splint out; chlorine would bleach litmus.
Topic 3 — quick quiz
Click to reveal · 5 questions
  1. Describe the test for hydrogen and the positive result.
    Hold a lit splint at the mouth of the tube — hydrogen gives a squeaky pop.
  2. How do you test for oxygen?
    Put a glowing splint into the gas — oxygen relights it.
  3. What is the test for carbon dioxide and what do you see?
    Bubble the gas through limewater — it turns cloudy / milky.
  4. How is chlorine identified?
    Damp litmus paper is held in the gas — chlorine bleaches it white (it may go red first).
  5. A gas turns limewater cloudy. What is it, and why must the limewater be involved rather than a splint?
    It is carbon dioxide. CO₂ doesn't pop or relight a splint — it actually puts a splint out — so the limewater test is the one that identifies it.
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