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AQA 5.8 / CHEMISTRY

Chemical analysis

How can you find out what is in a sample? Use melting points, chromatography and gas tests to build a clear answer from evidence.

Supported GCSE version · Clear examples, worked calculations and optional challenges.

Foundation & Higher28 main + 4 challenge questionsSave and return any time

Understand the science

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YOUR TOPIC GUIDE

Chemical analysis

Work through one section at a time. Higher-only material is labelled explicitly; optional challenges can also use Foundation content.

Words to know

Pure substance

A single element or compound, not mixed with another substance.

Impurity

Another substance mixed with the substance being tested.

Formulation

A mixture designed as a useful product, with carefully measured ingredients.

Chromatography

A method that separates substances as they move between a stationary and a mobile phase.

Stationary phase

The phase that stays in place; in paper chromatography, the paper.

Mobile phase

The phase that moves; in paper chromatography, the solvent.

Solvent

A liquid that dissolves a substance.

Baseline

The pencil starting line on which samples are placed.

Solvent front

The furthest point reached by the solvent during chromatography.

Rƒ value

Distance moved by a substance divided by the distance moved by the solvent front, both from the baseline.

Reference sample

A known substance used for comparison with an unknown.

Limewater

Calcium hydroxide solution; it turns cloudy when carbon dioxide is added.

Bleached

Having its colour removed; damp litmus turns white in chlorine.

Observation

Something directly seen or measured in a test.

1. What does “pure” mean in chemistry?

In chemistry, a pure substance contains just one element or one compound. Pure oxygen is an element. Pure sodium chloride is a compound. Both can be pure.

Everyday labels use “pure” differently. “Pure orange juice” might mean nothing has been added, but it still contains water, sugars and other substances. It is a mixture in chemistry.

At a specified pressure, a pure substance has a characteristic melting point and boiling point. A sample that melts over a range or at a different temperature may contain impurities. Impurities often lower a solid’s melting point and broaden its melting range. Compare with reliable reference data under the same conditions.

Example melting-point evidence
SampleObserved melting behaviour
Reference pure solidMelts sharply at 80 °C
Sample PMelts between 79.5 and 80.5 °C
Sample QStarts at 73 °C; finishes at 78 °C

P is more consistent with the pure reference. Q’s lower, broader range suggests impurities. Allow for measurement uncertainty: one reading does not prove identity or purity with complete certainty.

Temperature ranges: pure reference at 80 degrees Celsius, sample P 79.5 to 80.5 and sample Q 73 to 78.
Bars show the melting ranges in the table. They are not heating curves or time measurements.

2. A formulation is designed for a job

A formulation is a mixture deliberately made to have useful properties. Its ingredients are measured carefully, and each has a purpose. Not every mixture is a formulation: a puddle of muddy water was not designed to meet a recipe.

Examples of formulations
ProductWhy its ingredients are chosen
PaintPigment gives colour; other ingredients help it spread, dry and stick.
MedicineAn active ingredient has the intended effect; other ingredients may control taste or delivery.
Cleaning productIngredients help remove dirt and allow the product to be used conveniently.
Fuel blendThe proportions are adjusted to give suitable burning properties.
Alloy, fertiliser or foodMeasured proportions produce the intended strength, nutrient supply or taste.

You are not expected to memorise a manufacturer’s recipe. Use the information supplied to explain how the quantities or roles of ingredients help the product work.

Worked example: a 250 g formulation contains 15% pigment by mass. Pigment mass = 15 ÷ 100 × 250 = 37.5 g.

3. Why chromatography separates colours

A dark ink can contain several coloured substances. Chromatography separates them as a solvent moves across paper. The paper is the stationary phase; the solvent that travels is the mobile phase.

The substances spend different proportions of time in the two phases. A substance that dissolves readily in the solvent and is less strongly attracted to the paper usually travels further. A substance more strongly held by the paper travels less far. This makes the substances separate into spots.

Colour is not the only evidence: two different substances may be the same colour. Compare their positions or Rf values under the same conditions.

4. Required practical 12: separate and compare coloured substances

Glass chromatography vessel with a lid, paper, pencil baseline above shallow solvent, and small starting spots.
The starting spots must not sit in the solvent at the bottom of the vessel.
  1. Draw a pencil baseline near the bottom of chromatography paper. Place small spots of the samples and known reference dyes on the line. Label the positions in pencil and let the spots dry.
  2. Add a shallow layer of a suitable solvent to a beaker. Suspend the paper so its lower edge touches the solvent but the sample spots remain above the liquid level. The paper must not lean against the wall.
  3. Cover the vessel with a lid. Allow the solvent to rise without moving the apparatus.
  4. Remove the paper before the solvent reaches the top. Immediately mark the solvent front in pencil, then let the paper dry.
  5. Measure from the baseline to the centre of each spot and from the baseline to the solvent front. Calculate Rf values.
  6. Compare unknown samples with references run using the same solvent, paper and conditions. Repeat if spots overlap or a result is unclear.

Why these details matter: pencil does not dissolve like many inks; spots above the liquid travel up the paper instead of dissolving into the beaker; small spots reduce overlap; a lid limits evaporation; the front must be marked before the solvent evaporates.

Safety: wear eye protection, avoid contact with dyes and use only teacher-approved solvents. Some solvents are flammable: keep them away from flames and follow ventilation instructions. Do not assume every solvent is safe because it looks like water.

5. Rƒ: compare distances fairly

The Rf value compares how far a substance moves with how far the solvent front moves. Measure both distances from the pencil baseline, not from the bottom edge of the paper.

Chromatogram with spot centre 48 mm above the baseline and solvent front 80 mm above baseline, giving Rf 0.60.
Use the centre of a spot, not its top or bottom edge.

Rf = distance moved by substance ÷ distance moved by solvent front

Use the same distance unit in the numerator and denominator. The units cancel, so Rf has no unit. It is normally between 0 and 1. An answer above 1 usually means the wrong distances were used or the division was reversed.

Worked example: spot distance = 3.6 cm; front distance = 6.0 cm. Rf = 3.6 ÷ 6.0 = 0.60 (2 significant figures). Rearrangement: spot distance = Rf × front distance. A spot with Rf 0.25 and a 60 mm front distance travels 15 mm.

A substance’s Rf can change if the solvent changes. Only compare reference values from the same conditions. A matching Rf supports an identification but does not make it certain: different substances can sometimes travel together.

6. What can a chromatogram tell you?

Several spots in one sample show that it contains a mixture. A pure coloured substance should give one spot in suitable solvents. One spot in just one solvent is not enough to prove purity: two substances might overlap or a substance might not be visible.

To compare an unknown with reference dyes, run them on the same paper. Matching heights give matching Rf values because they share the same baseline and solvent front. A second suitable solvent can provide extra evidence.

Chromatogram with reference samples A, B and C and unknown X. Baseline at zero mm, solvent front at 80 mm.
These are invented practice results. All four samples used the same solvent and paper; each visible spot is a coloured substance.

7. Four gas tests to remember

A gas test needs both a method and a positive result. “Use a splint” is not enough: you must say whether it is burning or glowing and what happens.

Detailed test tubes showing a burning splint at the mouth for hydrogen, a glowing splint inserted for oxygen, bubbling through limewater for carbon dioxide and damp litmus bleaching in chlorine.
Use small samples under teacher supervision. Do not identify gases by breathing them in.
Gas tests
GasTestPositive result
HydrogenHold a burning splint at the open end of a tube containing the gas.A squeaky pop.
OxygenInsert a glowing splint into the gas.The splint relights.
Carbon dioxideBubble the gas through limewater or shake it with limewater.The limewater becomes milky/cloudy.
ChlorineExpose damp blue litmus paper to the gas.It is bleached white; it may turn red briefly first.

Chlorine is toxic: testing it is teacher-controlled, with suitable extraction such as a fume cupboard. Never smell it. A flame going out is not a specific test for carbon dioxide—other gases can also fail to support burning.

8. Build a conclusion from evidence

Separate what you observe from what you infer. “The splint relit” is an observation. “The gas is oxygen” is a conclusion based on that test.

If a result is unclear, repeat with a fresh sample and suitable apparatus. For chromatography, do not compare distances from different solvent runs without calculating Rf and checking conditions. For melting points, compare with a reference and account for the thermometer’s resolution.

Use calculations carefully: record units for measured distances, but not for Rf. Quote sensible significant figures. All questions here are original exam-style practice. Topic 5.8 has no Higher-only specification content; optional challenges use the same science in less familiar situations.