Pure Substances & Mixtures
The word pure means something very specific in chemistry — and it is not what it means on a carton of orange juice. Getting that distinction right, and knowing the one physical test that reveals purity, is worth easy marks — it comes up early and often in the exam.
- Pure substance (chemistry) — a single element or a single compound, with nothing else mixed in. A beaker of pure water contains only H2O molecules.
- Pure substance (everyday language) — something natural or unadulterated, that has had nothing added to it, e.g. “pure” milk or “pure” orange juice. In chemistry, milk is a mixture.
- Mixture — two or more elements or compounds physically mixed together, not chemically combined. The components keep their own properties and can be separated by physical means — the filtration, crystallisation and distillation methods from C1, plus the chromatography you meet below.
Black and red circles are two different types of atom (the same diagram you met in C1). An element and a compound are both pure — one kind of particle throughout; a mixture contains more than one kind, jumbled together and not chemically bonded.
Telling pure from impure: the melting point test
A pure substance melts and boils at one specific, sharp temperature — pure ice melts at exactly 0 °C and pure water boils at exactly 100 °C. A mixture melts and boils over a range of temperatures, and the impurities usually lower the melting point as well as spreading it out. So melting point and boiling point data can be used to distinguish a pure substance from a mixture: compare the measured value with the data-book value, and check whether the change of state is sharp or smeared over a range.
On a cooling curve, a pure substance freezes at a single sharp temperature (a flat plateau); a mixture freezes over a range, so the line only slopes.
A liquid is thought to be pure water. It is heated and boils steadily at 102 °C. Is it pure?
- Pure water boils at exactly 100 °C (the data-book value).
- The sample boils above 100 °C, so it is not pure — a dissolved solid (e.g. salt) raises the boiling point.
- For a fuller check you would look for a sharp boiling temperature: a pure liquid holds steady at one value, a mixture boils over a range.
- Using the everyday meaning. If a question says “in chemistry”, pure means a single element or compound — never “natural” or “nothing added”.
- Saying a mixture melts at one temperature. Mixtures melt and boil over a range; only pure substances have a sharp melting/boiling point.
- Forgetting impurities lower the melting point. This is why gritting salt melts ice and why a smeared melting range signals an impure drug.
🧪 Exam-style questions
Which two of these are mixtures? Tick (✓) two boxes, then press Check.
A solid is heated. It melts gradually between 68 °C and 75 °C. What does this show? Tick (✓) one box.
In chemistry, what is meant by a pure substance? Tick (✓) one box.
Formulations
Some mixtures aren’t accidental — they are designed. A formulation is a mixture that has been put together as a useful product, with every component there for a reason and present in a carefully measured quantity so the final product has exactly the properties it needs.
A formulation is a mixture that has been designed as a useful product, in which:
- each component is included for a particular purpose, and
- the components are mixed in carefully controlled (measured) quantities so the product has the required properties.
The AQA examples to remember: fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
Take paint — the example examiners use most. It is not one substance but a recipe, each part doing a job:
Each component has a purpose and a measured amount — change the recipe and you change the product. That is what makes paint a formulation, not just a mixture.
You are given information about a product and asked to decide whether it is a formulation. Look for both of these:
- it is a mixture of several components (often shown as a list or a percentage composition on a label), and
- each component is present in an exact, measured amount for a particular job.
A label reading “42% active ingredient, 31% solvent, 18% …” is the classic tell — precise proportions chosen to give required properties.
- A formulation is not a compound. The components are physically mixed, not chemically combined, so they keep their own properties and could in principle be separated.
- A single pure substance is never a formulation — pure copper, distilled water or oxygen are one substance each, not a designed mixture.
- You don’t need to know the named ingredients of any real branded product — only the idea of measured components with purposes.
🧪 Exam-style questions
Which of these is a formulation? Tick (✓) one box.
A weed-killer label lists: 36% active weed-killer, 4% wetting agent, 60% water. Which statement best explains why this product is a formulation? Tick (✓) one box.
Which statement about formulations is correct? Tick (✓) one box.
Chromatography & Rf Values
Chromatography separates the components of a mixture and gives information that helps identify them — it is how you prove that “black” ink is really a blend of dyes, or that a food colouring contains a banned additive. It works because every chromatography method has two phases, and different substances spend different amounts of time in each.
- Mobile phase — the phase that moves. In paper chromatography this is the solvent (e.g. water or ethanol) that travels up the paper.
- Stationary phase — the phase that stays put. In paper chromatography this is the chromatography paper itself.
Separation depends on how each substance is distributed between the two phases. A component that is more soluble in the solvent (and less attracted to the paper) spends more time in the mobile phase and is carried further up the paper; a less soluble component lags behind. Because the components travel at different speeds, they spread out into separate spots.
How a paper chromatogram is made
A pencil start line (baseline) is drawn near the bottom of the paper and a small spot of each mixture is placed on it. The paper is stood in solvent so that the solvent level is below the start line. The solvent rises up the paper by capillary action, carries the soluble components with it, and is left to run until it is near the top. The result is a chromatogram: the developed paper with its separated spots.
Rf = d1d2 — the distance the substance moved over the distance the solvent moved. Always measure from the start line to the centre of the spot.
A single spot of three-dye “black” ink sits on the pencil start line, just above the solvent.
Pure or impure? What the spots tell you
A chromatogram is also a quick purity test:
- a pure compound produces a single spot — and it stays a single spot in every solvent;
- a mixture separates into two or more spots;
- two samples that are the same substance produce spots that travel the same distance (the same Rf) in the same solvent.
That last point is how chromatography identifies things: run your unknown next to a known reference and compare the spots.
Rf = distance moved by the substancedistance moved by the solvent
- Both distances are measured from the start line — to the centre of the spot, and to the solvent front.
- Rf is a ratio, so it has no units and is always less than 1.
- The same compound gives the same Rf in the same solvent; change the solvent and the Rf changes. Comparing to reference Rf values (measured under the same conditions) identifies an unknown.
Worked example. A spot moves 3.0 cm; the solvent moves 6.0 cm. Rf = 3.06.0 = 0.50.
Required practical 6: paper chromatography
Investigate how paper chromatography can be used to separate and identify a mixture of coloured substances (e.g. food colourings), and calculate Rf values.
The one rule the practical is built on: the start line and spots must sit above the solvent, or the samples wash straight off into the beaker.
- Draw a pencil start line about 2 cm from the bottom of the chromatography paper (pencil is insoluble, so it won’t run).
- Use a fresh capillary tube to put a small spot (2–3 mm) of each known colouring on the line, plus a spot of the unknown mixture. Label each in pencil.
- Add solvent to the beaker to a depth of less than 1 cm, so the level is below the start line. Hang the paper so its bottom edge dips in.
- Leave it undisturbed until the solvent has risen about three-quarters of the way up.
- Remove the paper, immediately mark the solvent front in pencil, and let it dry.
- Measure, in mm, the distance from the start line to the centre of each spot, and from the start line to the solvent front. Calculate Rf for each.
- Independent variable: the substance spotted on the start line (each different colouring or ink).
- Dependent variable: the distance each spot moves — used to work out its Rf value.
- Control variables: the same solvent, the same paper and the same start line for every sample; always measure to the centre of the spot.
- Compare each colouring’s Rf (and spot position) with the unknown to see which dyes the unknown contains.
- A colouring that gives a single spot is a pure substance; one that separates into several spots is a mixture.
- Rf values let you compare results between papers and labs — but only if the same solvent was used, since Rf depends on the solvent.
- Start line in ink. The ink dissolves and travels up with the samples, ruining the chromatogram. Always pencil.
- Solvent above the start line. The spots dissolve straight into the solvent and are lost.
- Measuring to the top of a spot. Always measure to the centre of the spot.
- An Rf greater than 1, or with units. Rf is a ratio of two distances — it has no units and must be less than 1. If yours is bigger than 1, you’ve divided the wrong way round.
🧪 Try the common mistakes
Run the chromatography three ways and watch what goes wrong with each mistake.
Correct setup: a pencil start line, solvent below the spots. Press play.
🧪 Exam-style questions
On a chromatogram, a colour moves 7.2 cm and the solvent moves 9.0 cm. Calculate the Rf value of the colour.
Show answer
- Rf = distance moved by substance ÷ distance moved by solvent = 7.2 ÷ 9.0 1 mark
- = 0.80 (no units, less than 1). 1 mark
A student set up their chromatography incorrectly. Which two of these are mistakes? Tick (✓) two boxes, then press Check.
On a chromatogram, a sample produces a single spot in two different solvents. What does this show? Tick (✓) one box.
A colour has an Rf value of 0.65 and moves 3.2 cm up the paper. Calculate the distance moved by the solvent.
Show answer
- Rearrange Rf = substance ÷ solvent → solvent = substance ÷ Rf = 3.2 ÷ 0.65 1 mark
- = 4.9 cm (4.92 cm). 1 mark