Overview
What you’ll learn
- Choose appropriate apparatus for measuring time, temperature, mass and volume, and judge their advantages.
- Describe an acid–base titration and identify its end-point with a suitable indicator.
- Run and interpret paper chromatograms, use locating agents, and calculate Rf values.
- Select suitable separation and purification techniques and judge purity from melting and boiling points.
- Carry out the standard tests for anions, cations and gases, including flame tests.
Syllabus checklist (0620)
Statements are paraphrased and grouped by tier. Core is studied by everyone; Supplement is examined on the Extended papers only.
Core — every candidate
- 12.1Name appropriate apparatus for the measurement of time, temperature, mass and volume, including stopwatches and the balance; thermometers; and burettes, volumetric pipettes, measuring cylinders and gas syringes.
- 12.1Suggest advantages and disadvantages of experimental methods and apparatus for a given task.
- 12.1Describe the terms solvent, solute, solution, saturated solution, residue and filtrate.
- 12.2Describe an acid–base titration, including the use of a burette, a volumetric pipette and a suitable indicator.
- 12.2Describe how to identify the end-point of a titration using an indicator.
- 12.3Describe how paper chromatography is used to separate mixtures of soluble coloured substances, using a suitable solvent.
- 12.3Interpret simple chromatograms, including the use of known substances to identify unknowns, and the identification of pure and impure substances.
- 12.4Describe and explain methods of separation and purification using: a suitable solvent; filtration; crystallisation; simple distillation; and fractional distillation.
- 12.4Suggest suitable separation and purification techniques, given information about the substances involved.
- 12.4Identify substances and assess their purity using melting point and boiling point information.
- 12.5Describe tests for the anions: carbonate (add dilute acid, then test for carbon dioxide); chloride, bromide and iodide (acidify with dilute nitric acid, then add aqueous silver nitrate); nitrate (add aluminium foil and aqueous sodium hydroxide, then test for ammonia); sulfate (acidify with dilute nitric acid, then add aqueous barium nitrate); sulfite (acidified aqueous potassium manganate(VII)).
- 12.5Describe tests using aqueous sodium hydroxide and aqueous ammonia to identify the aqueous cations Al³⁺, NH₄⁺, Ca²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Fe³⁺ and Zn²⁺.
- 12.5Describe tests for the gases: ammonia (damp red litmus paper); carbon dioxide (limewater); chlorine (damp litmus paper); hydrogen (lighted splint); oxygen (glowing splint); and sulfur dioxide (acidified aqueous potassium manganate(VII)).
- 12.5Describe the flame test for the cations Li⁺, Na⁺, K⁺, Ca²⁺, Ba²⁺ and Cu²⁺.
Supplement — Extended candidates only
- 12.3Describe how paper chromatography separates mixtures of soluble colourless substances, using a solvent and a locating agent.
- 12.3State and use the equation Rf = distance moved by substance ÷ distance moved by solvent.
In 30 seconds
- 1Choose apparatus by what it measures: stopwatch (time), thermometer (temperature), balance (mass), burette/pipette/cylinder/gas syringe (volume).
- 2A titration uses a volumetric pipette for a fixed volume and a burette for accurate, drop-by-drop addition until the indicator changes.
- 3Chromatography separates soluble substances by how far each travels; Rf = distance moved by substance ÷ distance moved by solvent (Supplement).
- 4Filtration separates insoluble solid (residue) from liquid (filtrate); distillation recovers the solvent; crystallisation recovers the solute.
- 5A pure substance has sharp, fixed melting and boiling points; mixtures melt and boil over a range.
- 6Learn the qualitative tables: anion tests, NaOH/NH₃ cation tests, gas tests and flame colours — Paper 6 lives here.
Notes · 1 of 5
Apparatus and solution vocabulary
Match the instrument to the measurement, and use the solution words precisely.
| Measurement | Apparatus |
|---|---|
| Time | Stopwatch |
| Temperature | Thermometer (or temperature sensor) |
| Mass | Balance |
| Volume of liquid, accurately | Volumetric pipette, burette |
| Volume of liquid, roughly | Measuring cylinder |
| Volume of gas | Gas syringe |
Judging apparatus means weighing up precision against convenience: a burette reads to 0.05 cm³ but must be clamped and rinsed, while a measuring cylinder is quick but coarse. A gas syringe captures a gas for volume measurement but can leak or stick; an upside-down measuring cylinder over water fails for gases that dissolve.
| Term | Meaning |
|---|---|
| Solvent | The liquid that does the dissolving |
| Solute | The solid (or gas) that dissolves |
| Solution | The solvent with solute dissolved in it |
| Saturated solution | One holding the maximum dissolved solute at that temperature |
| Residue | The solid trapped by filter paper |
| Filtrate | The liquid that passes through the filter paper |
Notes · 2 of 5
Acid–base titrations
A pipette fixes one volume; a burette measures the other to the drop.
- 1Measure the alkali
Rinse a volumetric pipette with the alkali, then deliver exactly 25.0 cm³ into a conical flask.
- 2Add indicator
Add a few drops of a single-change indicator such as methyl orange or thymolphthalein — never universal indicator, whose colour drifts gradually.
- 3Titrate
Fill the burette with the acid, record the starting reading, and add it drop by drop, swirling, near the end point.
- 4Read the end point
Stop at the first permanent colour change and take the burette reading; repeat for concordant results.
The difference between the final and initial burette readings is the titre — the volume of acid that exactly neutralised the alkali. Repeating the titration and averaging results that agree within 0.10 cm³ makes the titre reliable.
Notes · 3 of 5
Paper chromatography
Solvent climbs the paper; each substance travels its own distance.
In paper chromatography a spot of the mixture is placed on a pencil baseline near the bottom of the paper. The paper stands in a shallow layer of solvent, which climbs the paper by capillary action and carries the dissolved substances at different speeds — those that bind strongly to the paper move slowly, those that dissolve best in the solvent travel furthest.
- A sample giving one spot is a single, pure substance.
- A sample giving several spots is a mixture.
- An unknown spot at the same height as a known sample is probably the same substance.
Supplement: colourless substances are made visible with a locating agent, such as ninhydrin for amino acids, or by shining ultraviolet light on the paper. The distance each substance travels is compared with the solvent front using the Rf value:
Notes · 4 of 5
Separation and purification
Pick the technique from the properties of what you need to keep.
| Situation | Technique |
|---|---|
| Insoluble solid in a liquid | Filtration |
| Dissolved solid (the solute) wanted | Crystallisation from the solution |
| Pure solvent wanted from a solution | Simple distillation |
| Two miscible liquids with different boiling points | Fractional distillation |
| Substances dissolved in a solvent | Chromatography |
In simple distillation the solution is boiled; the solvent evaporates, is condensed in a water-cooled condenser, and is collected as a pure liquid while the solute stays behind. Fractional distillation adds a column in which repeated condensation–evaporation cycles separate liquids whose boiling points are close, such as ethanol (78 °C) and water (100 °C).
Purity is judged from fixed temperatures. A pure substance melts and boils at one sharp temperature; an impure sample melts and boils over a range, melting lower and boiling higher than the pure substance.
Notes · 5 of 5
Qualitative analysis: ions and gases
The test tables that decide Paper 6 — learn them cold.
| Anion | Test | Positive result |
|---|---|---|
| Carbonate, CO₃²⁻ | Add dilute acid, then test the gas | Fizzing; gas turns limewater milky (CO₂) |
| Chloride, Cl⁻ | Dilute nitric acid, then aqueous silver nitrate | White precipitate |
| Bromide, Br⁻ | Dilute nitric acid, then aqueous silver nitrate | Cream (pale yellow) precipitate |
| Iodide, I⁻ | Dilute nitric acid, then aqueous silver nitrate | Yellow precipitate |
| Nitrate, NO₃⁻ | Aluminium foil and aqueous sodium hydroxide, warm | Ammonia gas released (damp red litmus turns blue) |
| Sulfate, SO₄²⁻ | Dilute nitric acid, then aqueous barium nitrate | White precipitate |
| Sulfite, SO₃²⁻ | Acidified aqueous potassium manganate(VII) | Purple colour decolourised |
The acid is added before the silver nitrate or barium nitrate to destroy carbonate ions, which would otherwise give misleading white precipitates of silver or barium carbonate.
| Cation | NaOH(aq), few drops → excess | NH₃(aq), few drops → excess |
|---|---|---|
| Al³⁺ | White ppt, dissolves in excess | White ppt, insoluble in excess |
| Ca²⁺ | White ppt, insoluble (or slightly dissolves) in excess | No precipitate |
| Cr³⁺ | Grey-green ppt, soluble in excess giving a green solution | Grey-green ppt, insoluble in excess |
| Cu²⁺ | Light blue ppt, insoluble in excess | Light blue ppt, soluble in excess to deep blue solution |
| Fe²⁺ | Green ppt, insoluble in excess | Green ppt, insoluble in excess |
| Fe³⁺ | Red-brown ppt, insoluble in excess | Red-brown ppt, insoluble in excess |
| Zn²⁺ | White ppt, dissolves in excess | White ppt, dissolves in excess |
| NH₄⁺ | Warm: ammonia gas released | No precipitate / no visible change |
| Substance | Test | Positive result |
|---|---|---|
| Ammonia, NH₃ | Damp red litmus paper | Turns blue |
| Carbon dioxide, CO₂ | Bubble through limewater | Turns milky |
| Chlorine, Cl₂ | Damp litmus paper | Bleaches it white |
| Hydrogen, H₂ | Lighted splint | Burns with a squeaky pop |
| Oxygen, O₂ | Glowing splint | Relights |
| Sulfur dioxide, SO₂ | Acidified aqueous potassium manganate(VII) | Purple colour decolourised |
| Cation | Flame colour |
|---|---|
| Li⁺ | Red |
| Na⁺ | Yellow |
| K⁺ | Lilac |
| Ca²⁺ | Orange-red |
| Ba²⁺ | Green |
| Cu²⁺ | Blue-green |
For a flame test, clean a nichrome wire with concentrated hydrochloric acid and heat it until no colour shows, then dip it into the sample and hold it in the edge of a blue Bunsen flame. Cleaning prevents one ion’s colour masking another.
Revise
Key terms
The words examiners expect you to use precisely.
- Solute
- The substance that dissolves in a solvent to form a solution.
- Solvent
- The liquid in which a solute dissolves.
- Saturated solution
- A solution containing the maximum amount of dissolved solute at that temperature.
- Residue
- The solid left on the filter paper after filtration.
- Filtrate
- The liquid that passes through the filter paper during filtration.
- Titre
- The volume of solution added from the burette to reach the end point of a titration.
Show 6 more key termsShow fewer key terms
- End point
- The point in a titration where the indicator first shows its permanent colour change.
- Chromatogram
- The pattern of spots produced on chromatography paper after separation.
- Locating agent
- A spray that makes colourless chromatography spots visible (Supplement).
- Rf value
- Distance moved by the substance divided by distance moved by the solvent front; between 0 and 1 (Supplement).
- Qualitative analysis
- Experimental tests used to identify which ions or gases are present in a sample.
- Flame test
- Identifying metal ions by the characteristic colour they give a Bunsen flame.
Revise
Common mistakes
Where students most often lose marks in this topic — and what to do instead.
Mistake: Using a measuring cylinder to measure 25.0 cm³ for a titration.
A cylinder is too imprecise for quantitative work.
Instead: Use a volumetric pipette (fixed volume) or burette (variable, precise volume).
Mistake: Adding silver nitrate without first acidifying with dilute nitric acid.
Carbonate ions would also give a white precipitate, faking a chloride result.
Instead: Always acidify first so only the halide precipitate can form.
Show 8 more mistakesShow fewer mistakes
Mistake: Calling the liquid that passes the filter paper the residue.
The residue is the solid left behind.
Instead: Solid on the paper = residue; liquid through the paper = filtrate.
Mistake: Computing Rf upside down (solvent ÷ substance).
This gives values greater than 1, which are impossible.
Instead: Rf = distance by substance ÷ distance by solvent, always less than 1.
Mistake: Saying a blue precipitate with NaOH proves Zn²⁺.
Zinc gives a white precipitate; blue means copper(II).
Instead: Zn²⁺: white ppt soluble in both excess NaOH and excess NH₃. Cu²⁺: light blue ppt.
Mistake: Judging purity by "the crystals look shiny".
Appearance says nothing about purity.
Instead: Purity is shown by sharp, fixed melting and boiling points.
Mistake: Measuring chromatography distances from the top of the paper.
Both the substance distance and the solvent distance must start from the same origin — the pencil baseline.
Instead: Measure each distance from the baseline: substance to its spot, solvent to the solvent front.
Mistake: Using a lighted splint as the test for ammonia.
The squeaky pop belongs to hydrogen; ammonia never ignites in this test.
Instead: Ammonia turns damp red litmus paper blue.
Mistake: Choosing a separating funnel to split ethanol from water.
A separating funnel only separates immiscible liquids, and ethanol and water mix in every proportion.
Instead: Miscible liquids with different boiling points are separated by fractional distillation.
Mistake: Reporting a titration result from a single trial.
One trial may overshoot the end point, and there is no way to tell without repeating.
Instead: Repeat until at least two titres agree within 0.10 cm³, then average the concordant ones.
Check yourself
Quick quiz
18 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Select measuring apparatus
Which piece of apparatus delivers a fixed, accurately known volume such as 25.0 cm³?
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Practise
Topical questions
10 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this topic, in three tiers, each with a model answer, mark scheme and examiner tip.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a Cambridge paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Topic 12 Practice Paper
30 marks · 40 min · 6 pages
10 multiple-choice questions and structured questions with answer lines. Topic 12: Experimental techniques and chemical analysis.
Mark scheme
PROTopic 12 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Which paper covers this topic?
Paper 1 / 2
Multiple choice (45 min, 40 marks)
Core and Extended versions
Paper 3 / 4
Theory, short and structured (1 h 15, 80 marks)
Core and Extended versions
Paper 5 / 6
Practical test or Alternative to Practical (40 marks)
Experimental skills
Want a full paper? Try the Paper 2-style multiple choice, Paper 4-style structured or Paper 6-style practical sets.
Questions
Frequently asked questions
Which apparatus should I name for each measurement?
Stopwatch for time, thermometer for temperature, balance for mass, volumetric pipette or burette for accurate liquid volumes, measuring cylinder for rough volumes, and a gas syringe for gas volumes.
How do I describe the end point of a titration?
Add the solution from the burette drop by drop until the indicator shows its first permanent colour change — for example the first permanent blue with thymolphthalein. Then record the final burette reading; the difference from the start is the titre.
What does an Rf value tell me?
Rf is the distance the substance moved divided by the distance the solvent moved. Values between 0 and 1 that match, in the same solvent, suggest two spots are the same substance.
How do I distinguish Fe²⁺, Fe³⁺ and Cu²⁺ in solution?
Add aqueous sodium hydroxide: iron(II) gives a green precipitate, iron(III) a red-brown precipitate and copper(II) a light blue precipitate, all insoluble in excess. Ammonia adds the extra clue that copper(II) dissolves in excess to a deep blue solution.
Why is dilute nitric acid added before testing for halides or sulfates?
The acid destroys carbonate ions, which would otherwise produce white precipitates with silver and barium ions and fake a positive result.