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

This study guide teaches sections 3.4.1 Purity and chromatography and 3.4.2 Identification of common gases (statements a to e of each) from the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. The qualification is untiered, so all of it is for every candidate. Either written paper can assess content from any part of the specification.

Links: revision notes for this unit · practice questions for this unit · course hub · printable checklist · free diagnostics

Coverage at a glance

Statement	Skills you need
3.4.1 a	State what a pure substance is; identify substances and judge purity from melting and boiling points
3.4.1 b	Explain why measures of purity matter for foodstuffs and drugs
3.4.1 c	Define a mixture; separate mixtures by distillation, filtration and crystallisation
3.4.1 d	Describe how to carry out paper chromatography; identify components using Rf values; know that solvents other than water can be used
3.4.1 e	Explain separation in terms of a stationary phase, a mobile phase and relative solubility; suggest chromatographic ways to tell pure from impure
3.4.2 a to e	Describe the tests and positive results for hydrogen, oxygen, carbon dioxide, ammonia and chlorine

Section 6.2 of the specification lists no required practical for these two sections. The chemical analysis required practical (flame tests) belongs to 3.4.3, which is outside this guide.

Pure substances and how to recognise them (3.4.1 a)

In chemistry, pure means a sample contains only one substance: one element or one compound, with nothing else mixed in. This is stricter than everyday use. A jar labelled “pure honey” has nothing added, yet honey is a mixture of sugars, water and other compounds.

A pure substance has a fixed melting point and a fixed boiling point at a given pressure, so comparing measured values with known data helps identify it.

Impurities change these temperatures in a predictable way:

• An impure solid usually melts at a lower temperature than the pure substance, and over a range of temperatures rather than sharply.
• An impure liquid usually boils at a higher temperature than the pure substance, and over a range.

For example, pure water boils at 100 °C at normal atmospheric pressure, while salty water boils above 100 °C.

Worked example 1. You are given these data: pure compound M melts at 132 °C. Three samples are heated slowly in a melting point apparatus.

Sample	Observation
A	Melts sharply at 132 °C
B	Starts to melt at 121 °C, fully melted at 127 °C
C	Melts sharply at 95 °C

Step 1. Sample A matches the data and melts sharply, so A is likely to be pure M. Step 2. Sample B melts below 132 °C and over a 6 °C range: M with impurities. Step 3. Sample C melts sharply but at the wrong temperature: a pure substance that is not M.

A sharp melting point tells you about purity; the value itself tells you about identity. Use both.

Why purity matters (3.4.1 b)

Measures of purity matter for anything eaten, drunk or taken as medicine.

• Drugs. Each tablet must contain the stated amount of the active compound so the dose is correct. An impurity could be harmful or make the medicine less effective.
• Foodstuffs. Ingredients and additives must be free from harmful contaminants, and the product must match its label.

Mixtures and physical separation (3.4.1 c)

In a mixture, two or more elements or compounds are present but are not chemically combined. Each component’s chemical properties are unchanged.

Because nothing is chemically bonded, mixtures can be separated by physical methods:

Method	Separates	How it works
Filtration	An insoluble solid from a liquid	The liquid (filtrate) passes through filter paper; the solid (residue) stays behind
Crystallisation	A soluble solid from its solution	Warm the solution to drive off part of the solvent until saturated, cool so crystals grow, then filter and dry
Distillation	A liquid from a solution, or liquids with different boiling points	The liquid with the lower boiling point evaporates, the vapour is cooled in a condenser, and the liquid (distillate) is collected

Worked example 2. How could you get dry crystals of potassium nitrate from a mixture of potassium nitrate and powdered charcoal?

Step 1. Add warm water and stir. Potassium nitrate dissolves; charcoal does not. Step 2. Filter. Charcoal is the residue; the solution is the filtrate. Step 3. Warm the filtrate to drive off part of the water, then cool it; collect the crystals by filtering and dry them.

Paper chromatography (3.4.1 d and e)

Paper chromatography separates and identifies the substances in a solution, such as the dyes in an ink or a food colouring.

How to carry out a paper chromatography separation

1. Draw a start line (baseline) in pencil about 1 cm to 2 cm from the bottom of the chromatography paper. Pencil does not dissolve in the solvent; ink would run and spoil the result.
2. Use a fine capillary tube to put a small, concentrated spot of each sample on the line. Add reference substances (known dyes) on the same line, spaced apart.
3. Stand the paper in a beaker containing a shallow layer of solvent. The solvent level must be below the baseline, otherwise the spots dissolve into the solvent in the beaker.
4. Put a lid on the beaker to stop the solvent evaporating from the paper.
5. Remove the paper when the solvent has risen most of the way up. Mark the solvent front in pencil straight away, before it evaporates.
6. Dry the paper and measure the distances.

Solvents other than water. Water suits water-soluble food colourings. Permanent marker inks do not dissolve in water, so a solvent such as ethanol is used.

Stationary phase, mobile phase and relative solubility

Every chromatography method has two phases:

• The stationary phase does not move. In paper chromatography it is the paper.
• The mobile phase moves through the stationary phase. In paper chromatography it is the solvent.

Separation depends on the relative solubility of each component. A substance that is more soluble in the solvent spends more time in the moving phase and travels further up the paper. A less soluble substance is held back by the paper and travels less far. An insoluble substance stays on the baseline.

Rf values

The Rf value of a substance is

Rf = distance moved by substance ÷ distance moved by solvent

Take both measurements from the baseline: to the middle of the spot, and to the solvent front. Rf is a ratio with no units, less than 1. In a given solvent each substance has its own Rf value, so matching values identifies components.

Worked example 3. A black felt-tip ink is run in ethanol. The solvent front is 7.5 cm above the baseline. The ink gives two spots, 1.5 cm and 4.8 cm above the baseline. Reference dyes in the same solvent have Rf values: P 0.20, Q 0.64, R 0.45. Which dyes are in the ink?

Step 1. Rf of lower spot = 1.5 ÷ 7.5 = 0.20. Step 2. Rf of upper spot = 4.8 ÷ 7.5 = 0.64. Step 3. Compare with the references. The ink contains P and Q. It does not contain R.

Worked example 4. In a different solvent, a dye has Rf 0.36. The solvent front moves 5.0 cm. How far does the dye spot move?

Rearrange: distance moved by substance = Rf × distance moved by solvent = 0.36 × 5.0 = 1.8 cm above the baseline.

Using chromatography to tell pure from impure

• A pure substance gives one spot on a chromatogram.
• An impure substance (a mixture) gives two or more spots, as long as the components have different Rf values in that solvent.

One spot in one solvent is good evidence of purity but not proof, because two substances could share an Rf value in that solvent. Run the sample in a second, different solvent too; a pure substance still gives one spot. Running a known pure reference alongside also helps: one spot level with the reference in two solvents strongly suggests the sample is that pure substance.

Worked example 5. A supplier claims food colouring K is a single dye. In water (solvent front 6.0 cm) it gives one spot 3.0 cm above the baseline, Rf 0.50. In ethanol (solvent front 6.0 cm) it gives spots at 1.2 cm and 4.2 cm, Rf 0.20 and 0.70. What do you conclude?

The ethanol run shows two spots, so K contains at least two substances and the claim is false. In water the two dyes travelled the same distance.

Identification of common gases (3.4.2 a to e)

Each test has a method and a positive result. You need both.

Gas	What you do	What you see or hear
Hydrogen, H₂	Hold a lighted splint close to the gas	A pop
Oxygen, O₂	Lower a glowing splint into the tube	The splint relights
Carbon dioxide, CO₂	Pass the gas into limewater, which is calcium hydroxide solution	The limewater goes cloudy white
Ammonia, NH₃	Place damp red litmus paper in the gas; also bring the stopper of a concentrated hydrochloric acid bottle close	Red litmus goes blue; white smoke of ammonium chloride forms
Chlorine, Cl₂	Place damp blue litmus paper in the gas	The litmus goes red, then is bleached white

Why each test works

• Hydrogen. The flame ignites hydrogen mixed with air and it burns rapidly, making the pop: 2H₂ + O₂ → 2H₂O.
• Oxygen. A glowing splint is barely burning; in oxygen it bursts back into flame.
• Carbon dioxide. Insoluble calcium carbonate forms in the limewater, so it goes cloudy: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.
• Ammonia. It dissolves in the water on damp litmus to give an alkaline solution. With hydrogen chloride gas from concentrated hydrochloric acid it forms solid ammonium chloride, seen as white smoke: NH₃ + HCl → NH₄Cl.
• Chlorine. It dissolves in the water on damp litmus to form an acidic solution, which turns blue litmus red; chlorine then bleaches the dye, so the paper turns white.

The litmus must be damp because the gases act as acids or alkalis only once dissolved in water.

Ammonia and chlorine both have a sharp, choking smell, so smell cannot tell them apart. Both are toxic: work in a fume cupboard and rely on litmus, which separates them clearly.

Worked example 6. A gas from a reaction has no smell. A burning splint at the open end of the tube goes out with no pop. What should you test for next, and what result would confirm it?

No pop rules out hydrogen. Bubble the gas through limewater; cloudy white limewater confirms carbon dioxide. A splint going out is not itself a test for carbon dioxide, because other gases also put out flames.

Common errors

• Saying an impure solid melts at a higher temperature. Impurities usually lower the melting point and spread it over a range.
• Drawing the baseline in ink, or letting the solvent cover it.
• Measuring from the bottom edge of the paper instead of the baseline, or giving Rf with units or above 1.
• Saying “lighted splint” for oxygen or “glowing splint” for hydrogen.
• Forgetting the word “damp” for litmus.
• Stopping at “blue litmus turns red” for chlorine. The full result is red, then bleached white.

Next steps

Condense this unit with the revision notes, then test yourself with the practice questions. For melting points explained through bonding, see the bonding, structure and properties guide; for exam technique, the exam preparation guide. Use the course hub, the checklist and the free diagnostics.

Official syllabus

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