OxfordAQA International GCSE Chemistry: Purity, chromatography and identification of common gases (9202) – Revision Notes

These revision notes condense sections 3.4.1 Purity and chromatography and 3.4.2 Identification of common gases of the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. They cover statements a to e in each section. The qualification is untiered, so every line applies to everyone sitting the exam. The teaching and worked examples are in the study guide for this unit.

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Key definitions (3.4.1 a, c, e)

Term	Meaning
Pure substance	A single element or compound, with no other substance mixed in
Mixture	Two or more elements or compounds mixed but not chemically combined; their chemical properties are unchanged
Physical method	A separation that does not involve a chemical reaction, such as distillation, filtration or crystallisation
Stationary phase	The phase that does not move; in paper chromatography, the paper
Mobile phase	The phase that moves; in paper chromatography, the solvent
Chromatogram	The dried paper showing the separated spots
Solvent front	The furthest point the solvent reached
Rf value	Distance moved by a substance ÷ distance moved by the solvent

Chemistry “pure” versus everyday “pure”. In everyday language “pure” often means nothing has been added (pure fruit juice, pure wool). In chemistry it means one substance only. A natural product can be “pure” on a label and still be a mixture.

Purity from melting and boiling points (3.4.1 a)

	Pure substance	Impure substance
Melting point	Sharp, fixed value	Lower, and over a range
Boiling point	Sharp, fixed value	Higher, and over a range

Method in steps: judging a sample from melting point data

1. Compare the measured value with the data value for the pure substance.
2. Check whether it melts sharply or over a range.
3. Sharp and matching: likely the pure substance. Lower and spread out: impure. Sharp but different: a different pure substance.

Why purity matters (3.4.1 b)

• Drugs: correct dose of the active compound; no harmful impurities.
• Foodstuffs: no harmful contaminants; contents match the label.

Separating mixtures (3.4.1 c)

Mixture	Method	What you collect
Insoluble solid + liquid	Filtration	Solid as residue, liquid as filtrate
Soluble solid in solution	Crystallisation	Crystals of the solid
Liquid from a solution, or liquids with different boiling points	Distillation	The liquid (distillate) from the condenser

Crystallisation in steps: warm to drive off part of the solvent until saturated → cool so crystals form → filter → dry the crystals.

Distillation in brief: heat the mixture → the liquid with the lower boiling point evaporates → vapour cools and condenses in the condenser → collect the distillate.

Paper chromatography (3.4.1 d)

Method in steps

1. Pencil baseline near the bottom of the paper.
2. Small, concentrated spot of each sample and reference on the line.
3. Stand the paper in solvent with the solvent level below the baseline; lid on.
4. Lift the paper out before the solvent nears the top edge; pencil the solvent front straight away.
5. Dry, measure from the baseline, calculate Rf.

Why each step matters

Step	Reason
Pencil, not pen	Pencil does not dissolve in the solvent; ink would run up the paper
Solvent below the line	Otherwise the samples dissolve into the solvent in the beaker
Lid on	Stops the solvent evaporating from the paper
Small spots	Spots stay compact and separate cleanly
Mark the front quickly	The solvent dries and the front becomes invisible

Solvents other than water can be used. Pick a solvent the substances dissolve in. Water suits many food colourings; ethanol or another solvent suits inks that are insoluble in water. Rf values depend on the solvent, so compare only results from the same solvent.

Rf values and how separation happens (3.4.1 d, e)

Formula	Rearranged
Rf = distance moved by substance ÷ distance moved by solvent	distance moved by substance = Rf × distance moved by solvent

• Measure both distances from the baseline. Measure to the centre of a spot.
• Rf is a ratio with no units, and it is never more than 1.
• A spot on the baseline (Rf = 0) did not dissolve in the solvent.

Worked reminder. Solvent front 5.2 cm from the baseline; spot centre 3.9 cm from the baseline. Rf = 3.9 ÷ 5.2 = 0.75.

How separation works: each component is shared between the paper and the solvent. The more soluble a component is in the solvent, the further it moves. Different relative solubilities give different distances, so the components separate.

Pure or impure by chromatography (3.4.1 e)

• Pure: one spot.
• Impure: two or more spots.
• One spot in one solvent is not proof of purity. Run it in a second solvent, alongside a pure reference. One spot each time, level with the reference, is strong evidence.

Identification of common gases (3.4.2 a to e)

Gas	Test	Positive result
Hydrogen	Lighted splint near the gas	Pop
Oxygen	Glowing splint in the gas	Relights
Carbon dioxide	Bubble through limewater (calcium hydroxide solution)	Cloudy white
Ammonia	Damp red litmus paper	Turns blue
Ammonia	Hydrogen chloride from concentrated hydrochloric acid held near it	White smoke of ammonium chloride
Chlorine	Damp blue litmus paper	Turns red, then bleached white

Smell: ammonia and chlorine both have a sharp, choking smell. Smell does not tell them apart; litmus does. Both are toxic, so they are handled in a fume cupboard.

Equations worth knowing

• 2H₂ + O₂ → 2H₂O (the pop)
• Ca(OH)₂ + CO₂ → CaCO₃ + H₂O (insoluble calcium carbonate makes the cloudiness)
• NH₃ + HCl → NH₄Cl (the white smoke is solid ammonium chloride)

Must-know distinctions

• Lighted splint (hydrogen: pop) versus glowing splint (oxygen: relights).
• Red litmus to blue (ammonia, alkaline) versus blue litmus to red then white (chlorine, acidic then bleaching).
• Melting point lowered by impurities versus boiling point raised by impurities.
• Mixture (not chemically combined; separate by physical methods) versus compound (chemically combined in fixed proportions).
• Stationary phase (paper) versus mobile phase (solvent).
• Baseline (pencil, start) versus solvent front (where the solvent stopped).

Quick self-test

1. What does a chemist mean by a pure substance?
2. A sample of a solid melts between 74 °C and 79 °C. The pure solid melts at 82 °C. What can you conclude?
3. Name the method used to obtain sugar crystals from sugar solution.
4. Name the method used to collect water from ink.
5. Give the reason a pencil, not a pen, is used for the start line.
6. A spot moves 3.3 cm and the solvent front 5.5 cm from the baseline. Calculate Rf.
7. A dye has Rf 0.40. The solvent front moves 8.5 cm. How far does the dye move?
8. A spot is 1.4 cm from the baseline; the solvent front is 5.6 cm from the baseline. Calculate Rf.
9. Name the stationary phase and the mobile phase in paper chromatography.
10. Describe the test and result for oxygen.
11. Which gas turns damp red litmus blue?
12. Damp blue litmus turns red and then white. Name the gas.

Answers

1. One element or one compound only, with nothing else mixed in.
2. The sample is impure: it melts below 82 °C and over a range.
3. Crystallisation.
4. Distillation.
5. Graphite is insoluble in the solvent, so the line stays put; pen ink would dissolve and move.
6. 3.3 ÷ 5.5 = 0.60.
7. 0.40 × 8.5 = 3.4 cm from the baseline.
8. 1.4 ÷ 5.6 = 0.25.
9. Stationary phase: the paper. Mobile phase: the solvent.
10. Put a glowing splint into the gas; it relights.
11. Ammonia.
12. Chlorine.

Where marks are usually lost

• Saying impurities raise the melting point. They lower it and spread it over a range.
• Describing a “sharp” melting point as proof of identity. It shows purity; the value has to match data to identify the substance.
• Choosing filtration to get a dissolved solid out of a solution. Filtration only removes insoluble solids.
• Taking Rf measurements from the paper’s lower edge rather than the baseline, or to the leading edge of a spot rather than its middle.
• Writing Rf with units, or as a number above 1 (the division was upside down).
• Saying one spot in one solvent proves a substance is pure.
• Swapping lighted and glowing splints between hydrogen and oxygen.
• Leaving out “damp” for litmus, or stopping at “turns red” for chlorine without the bleaching.
• Writing “limewater turns cloudy” without saying the gas was bubbled through it.
• Using smell as the test that tells ammonia and chlorine apart.

Next steps

Work through the practice questions, then go back to the study guide for anything you got wrong. The exam preparation guide covers technique across the course, and the identification of ions guide continues chemical analysis with flame tests and precipitates. Find weak spots with the free 10-minute diagnostics.

Official syllabus

OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, published by OxfordAQA. These notes cover section 3.4.1 Purity and chromatography and section 3.4.2 Identification of common gases.