This mini-lesson walks you through the whole of AQA Topic 4.8 — Chemical analysis: telling pure substances from mixtures and formulations, separating colours by paper chromatography and calculating Rf values, the four tests for gases, and how to identify ions by flame tests, precipitates, halide & sulfate tests and flame emission spectroscopy.
Make a guess: is 'pure milk' pure to a chemist?
Mission 1What does 'pure' mean to a chemist?
Mission 2How can a lab name a mystery powder?
Work through each screen, answer the questions as you go (some wordy, some calculations) and collect ⭐ stars. Press Start when you're ready.
Pure substances
Pure means one substance
In chemistry, a pure substance is a single element or a single compound, not mixed with anything else. Everything else is a mixture.
A pure substance melts and boils at a specific, sharp temperature.
A mixture melts/boils over a range of temperatures; impurities usually lower the melting point.
So melting-point and boiling-point data let you distinguish pure from impure substances.
Heating curves: a pure substance stays at one sharp melting point (flat line); a mixture melts over a range.
Watch out: in everyday language "pure" can mean "nothing added", eg "pure milk". But milk is a mixture! In chemistry, pure = a single element or compound — not "clean" or "natural".
Quick check
Pure or impure?
?Pure ethanoic acid melts sharply at 17 °C. A sample of "acid X" is found to melt gradually between 9 °C and 15 °C. What does this tell you about sample X?
Formulations
Formulations: useful mixtures
A formulation is a mixture that has been designed as a useful product. Each component is added in a carefully measured quantity (a precise composition) so the product has exactly the properties needed.
A formulation is still a mixture — it is not a pure substance.
You should be able to identify a formulation from information given (a list of components, each with a stated amount and purpose).
Paint is a formulation: pigment, solvent, binder and additives in measured amounts.Quick check
Which is a formulation?
?Which one of these is best described as a formulation?
Mission 1 check
What does 'pure' mean to a chemist?
?Two white solids are both labelled 'pure'. Solid A melts sharply at 122 °C. Solid B melts gradually between 110 °C and 118 °C. Which conclusion is correct?
Chromatography · Required practical 6
Paper chromatography & Rf
Chromatography separates a mixture into its components. It has two phases:
Stationary phase — the chromatography paper (it does not move).
Mobile phase — the solvent that moves up the paper, carrying the substances with it.
Substances that are more attracted to the solvent (mobile phase) travel further. A pure substance gives one spot in every solvent; a mixture separates into several spots.
Measure both distances from the baseline; the substance distance goes to the centre of the spot.
Rf = distance moved by substance ÷ distance moved by solvent
Both distances are measured from the baseline (origin). Different compounds have different Rf values in a given solvent, so Rf helps identify them.
Watch out:Rf has no units (it is a ratio of two lengths) and is always less than 1, because a spot can never travel further than the solvent front. Always measure to the centre of the spot, and draw the baseline in pencil so it doesn't dissolve.
Calculate
Your turn — Rf
1On a chromatogram the solvent front moved 8.0 cm from the baseline. A green spot's centre moved 6.0 cm. Calculate its Rf value.
(no units)
Hint: Rf = 6.0 ÷ 8.0. Give a decimal — it should be less than 1.
Calculate
One more Rf
2In the same experiment a yellow dye's centre moved 3.0 cm while the solvent moved 12 cm. Calculate the Rf value of the yellow dye.
(no units)
Hint: Rf = 3.0 ÷ 12. The smaller Rf, the less the substance was attracted to the solvent.
Mission 2 of 2
How can a lab name a mystery powder?
Make a guess: how can a few tests name an unknown powder?
Tests for gases
The four gas tests
You must know these four qualitative tests for common gases:
Hydrogen: pop · Oxygen: relights · Carbon dioxide: limewater milky · Chlorine: bleaches damp litmus.Match it
Match the gas to its test
Tap a gas on the left, then tap its correct test result on the right.
Quick check
Name that gas
?A colourless gas is collected. When a glowing splint is held in it, the splint relights. Which gas is it?
Flame tests · Chemistry only
Flame tests for metal ions
Chemistry only. Some metal ions (cations) give a distinctive flame colour. Clean a wire loop, dip it in the sample and hold it in a blue Bunsen flame:
The five AQA flame-test colours. In a mixture of ions, a strong colour (eg sodium's yellow) can mask the others.
Watch out: learn the colours exactly — potassium is lilac (not blue), calcium is orange-red (not plain orange), and copper is green. A bright sodium yellow can mask the other colours in a mixture.
Quick check · Chemistry only
Which metal ion?
?A compound is heated on a clean wire loop and the flame turns lilac. Which metal ion does this show is present?
Metal hydroxides · Chemistry only
Sodium hydroxide precipitate tests
Chemistry only. Adding a few drops of sodium hydroxide solution to a metal-ion solution can form a coloured (or white) insoluble hydroxide precipitate:
Al, Ca, Mg → white (only Al re-dissolves in excess); Cu²⁺ blue, Fe²⁺ green, Fe³⁺ brown.
Watch out: Al³⁺, Ca²⁺ and Mg²⁺ all give a white precipitate — to tell aluminium apart, add excess NaOH and only Al(OH)₃ re-dissolves. Don't confuse Fe²⁺ (green) with Fe³⁺ (brown).
Sort it · Chemistry only
Re-dissolve in excess NaOH?
Tap an ion, then tap the box: does its hydroxide precipitate dissolve when you add excess sodium hydroxide?
✅ Dissolves in excess
❌ Stays as a precipitate
Carbonates · Halides · Sulfates · Chemistry only
Tests for negative ions
Chemistry only. Three anion (negative-ion) tests to know:
Halides (Cl⁻, Br⁻, I⁻): add dilute nitric acid then silver nitrate solution → white, cream or yellow precipitate.
Sulfates (SO₄²⁻): add dilute hydrochloric acid then barium chloride solution → white precipitate.
Silver halides: Cl⁻ white, Br⁻ cream, I⁻ yellow. Sulfate: white precipitate with barium chloride.
Watch out: use the right acid — dilute nitric acid for the silver-nitrate (halide) test, dilute hydrochloric acid for the barium-chloride (sulfate) test. The acid removes carbonate ions that would otherwise give a false white precipitate.
Quick check · Chemistry only
Identify the halide
?Dilute nitric acid and silver nitrate solution are added to an unknown salt. A yellow precipitate forms. Which halide ion is present?
Mission 2 check
How can a lab name a mystery powder?
?A white powder gives a lilac flame in a flame test. A solution of it, with dilute nitric acid then silver nitrate added, gives a yellow precipitate. What is the powder?
Sort it · Chemistry only
Read the result, name the ion
Tap the ion that matches each test result.
Flame emission spectroscopy · Chemistry only
An instrumental method
Chemistry only.Flame emission spectroscopy is an instrumental method used to analyse metal ions in solution. The sample is put into a flame and the light given out is passed through a spectroscope. The output is a line spectrum.
Each metal ion gives its own pattern of lines — compare with a reference to identify it.
The line positions identify the ion; the intensity can measure its concentration.
The unknown's lines match the reference for metal X (not Y), identifying the metal ion.
Advantages of instrumental methods over chemical tests: they are more sensitive (detect tiny amounts), faster and more accurate — useful when only a small sample is available.
Quick check · Chemistry only
Why use the instrument?
?Give the best reason a forensic scientist would choose flame emission spectroscopy over a simple flame test to identify a tiny sample.
Recap
The facts to know
Pure substance: single element/compound; sharp melting & boiling points.
Formulation: a useful mixture with a precise composition (fuels, paints, medicines, alloys, fertilisers, foods).
Rf = distance moved by substance ÷ distance moved by solvent (no units, < 1).