Chemistry · IGCSE 0620 · §12.1–12.5

Experimental Techniques & Chemical Analysis

Chemistry’s detective work: separate a mixture, prove it is pure, then name every ion and gas by the colour it leaves behind.

Chemistry · 0620 Topic 12 of 12
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Experimental design

solvent solvent front baseline (pencil)
FIG 12.0 Paper chromatography in outline: the solvent rises from a pencil baseline to the solvent front, carrying each component a different distance.

Good chemistry begins before any reaction: choosing apparatus that measures with the right precision, and using the vocabulary of solutions exactly.

Definition
Solvent, solute & solution
A solvent dissolves a solute; the mixture formed is a solution. A saturated solution holds the maximum concentration of solute at a specified temperature.
Definition
Residue & filtrate
The residue is the solid left after filtering; the filtrate is the liquid that passes through.

Choosing the right apparatus

QuantityApparatus
TimeStop-watch
TemperatureThermometer
MassBalance
Fixed, accurate volumeVolumetric pipette
Variable, accurate volumeBurette
Approximate volumeMeasuring cylinder
Volume of a gasGas syringe
The standard instrument for each measurable quantity

Apparatus is chosen by trade-off. A burette delivers any volume to high precision but is slower to read; a measuring cylinder is quick but coarse; a pipette fixes one accurate volume every time. To transfer exactly 25.0 cm³ of acid for a titration, a 25.0 cm³ volumetric pipette is the right tool — the volume is fixed and must be accurate, which rules out the measuring cylinder, and the burette is designed for variable volumes.

Examiner note
Match apparatus to the precision the task needs — a pipette or burette for accurate volumes, a measuring cylinder only for approximate ones.

Separation & purification

Every mixture has a technique that undoes it. The trick is reading the mixture — soluble or not, solid or liquid, one component wanted or several — and matching it to the method that exploits the difference.

Definition
Filtration & crystallisation
Filtration separates an insoluble solid (the residue) from a liquid (the filtrate) using filter paper. Crystallisation obtains a dissolved solid from its solution by evaporating to saturation, then cooling so crystals form.

Matching method to mixture

MethodSeparates
Dissolve & filterA soluble solid from an insoluble one
FiltrationAn insoluble solid from a liquid
CrystallisationA dissolved solid from its solution
Simple distillationA solvent (pure liquid) from a solution
Fractional distillationTwo or more miscible liquids
Which method separates what

In simple distillation the solution is boiled, the vapour of the pure liquid is cooled in the condenser, and the distillate is collected.

Definition
Distillation
Simple distillation separates a liquid from a solution; fractional distillation separates miscible liquids by boiling point.
thermometer condenser (cold water) distillate heat
FIG 12.1 Simple distillation: the solution is heated, the vapour is condensed by cold water and the pure distillate is collected.

Assessing purity

Melting and boiling points are the test of purity. A pure substance changes state at a single, sharp temperature; the presence of an impurity lowers and broadens the melting point and raises the boiling point.

Examiner note
A pure substance melts and boils at one sharp temperature. An impurity lowers and widens the melting point and raises the boiling point.

Titration & chromatography

Two techniques of measurement and comparison: titration finds exactly how much, and chromatography finds exactly what.

Definition
Rf value
Distance moved by the substance ÷ distance moved by the solvent. A fixed value, always less than 1, that identifies a substance.
Definition
Titration
Adding one solution from a burette to a fixed volume of another (from a pipette) until the reaction is just complete — the end-point, shown by the indicator changing colour.
Definition
Chromatography
Separating soluble substances as a solvent carries them at different rates up a paper.

Acid–base titration

A volumetric pipette measures a fixed volume of one solution into a flask with a few drops of a suitable indicator. The second solution is run in from a burette, swirling, until the indicator just changes colour — the end-point, where the reaction is exactly complete.

Paper chromatography

A drop of mixture is placed on a pencil baseline and a solvent is allowed to rise. Each component travels its own distance, so a pure substance gives one spot and a mixture gives several. Comparing an unknown’s spots with knowns identifies it; colourless substances are revealed with a locating agent.

front base known A known B unknown
FIG 12.2 A chromatogram: the unknown is run beside known substances A and B on the same baseline so its spots can be compared.

ExtendedWorked example: calculating Rf

A spot rises 3.6 cm while the solvent front rises 9.0 cm. Rf = distance moved by substance ÷ distance moved by solvent = 3.6 ÷ 9.0 = 0.40. The value has no units and is below 1, as it must be — a spot never overtakes the solvent.

Examiner note
Draw the chromatography baseline in pencil — ink would dissolve and run.

Identifying cations

Add a hydroxide to a metal-ion solution and it precipitates — usually in a tell-tale colour. Read the colour, then test whether the precipitate dissolves in excess, and the cation names itself.

Definition
Precipitate
An insoluble solid formed when two solutions react. Its colour — and whether it dissolves in excess reagent — names the cation.

The hydroxide tests

CationWith aqueous NaOHWith aqueous ammonia
Al³⁺white, dissolves in excesswhite, insoluble in excess
NH₄⁺ammonia gas on warming—
Ca²⁺white, insoluble in excessno precipitate (or slight)
Cr³⁺green, dissolves in excessgreen, insoluble in excess
Cu²⁺light blue, insolubleblue, dissolves → deep blue
Fe²⁺green (→ brown in air)green, insoluble in excess
Fe³⁺red-brownred-brown
Zn²⁺white, dissolves in excesswhite, dissolves in excess
Precipitate with aqueous NaOH and with aqueous ammonia

Flame tests

IonFlame colour
Li⁺red
Na⁺yellow
K⁺lilac
Ca²⁺orange-red
Ba²⁺light green
Cu²⁺blue-green
Flame colour by ion
Examiner note
Add the reagent drop by drop, then in excess. “Soluble in excess” is often the deciding observation.
Why this matters
Al³⁺ and Zn²⁺ both give a white precipitate that dissolves in excess NaOH. Aqueous ammonia separates them: Zn²⁺ redissolves, Al³⁺ does not.

Identifying anions & gases

Where cations are named by hydroxide and flame colours, anions are named by the precipitate or gas they give with a chosen reagent — and gases by a quick bench test at the mouth of the tube.

Definition
Silver halides trend
Down Group VII the precipitate darkens: chloride white → bromide cream → iodide yellow.

Testing for anions

AnionTestResult
CarbonateAdd dilute acidFizzes; CO₂ turns limewater milky
ChlorideDilute HNO₃, then AgNO₃White precipitate
BromideDilute HNO₃, then AgNO₃Cream precipitate
IodideDilute HNO₃, then AgNO₃Yellow precipitate
SulfateDilute HNO₃, then Ba(NO₃)₂White precipitate
NitrateNaOH + aluminium foil, warmAmmonia (damp red litmus → blue)
SulfiteAcidified KMnO₄Purple → colourless
Reagent and result for each anion

Testing for gases

GasTestResult
HydrogenLighted splintSqueaky “pop”
OxygenGlowing splintRelights
Carbon dioxideBubble through limewaterTurns milky
AmmoniaDamp red litmus paperTurns blue
ChlorineDamp litmus paperBleached white
Sulfur dioxideAcidified KMnO₄Purple → colourless
The standard bench tests for gases
Examiner note
For chloride and sulfate, acidify with dilute nitric acid first. It removes carbonate, which would otherwise give a false white precipitate.
Why this matters
These tests are the backbone of the practical papers — most identification marks come straight from these tables.

Exam advice

Common mistakes

Not adding the reagent “in excess”
Whether a precipitate redissolves is often the deciding observation — Al³⁺ vs Zn²⁺ turns on it.
A chromatography baseline drawn in ink
Ink dissolves and runs; the baseline must be pencil. An easy mark lost.
Skipping the dilute nitric acid step
Without acidifying first, a carbonate gives a false white precipitate with silver or barium nitrate.
Naming a precipitate colour vaguely
“Changes colour” scores nothing — state the actual colour (white, green, red-brown…).
Swapping the two splint tests
Hydrogen gives a squeaky “pop” with a lighted splint; oxygen relights a glowing splint.

Model answer

A colourless solution is thought to be zinc sulfate. Describe the tests, and the results, that would confirm each of the two ions present.
[4 marks]
Mark 1
Test for the cation
Add aqueous sodium hydroxide → white precipitate.
Mark 2
Confirm it is zinc
The precipitate dissolves in excess NaOH and in excess ammonia (Al³⁺ would not).
Mark 3
Test for the anion
To a fresh portion add dilute nitric acid, then barium nitrate.
Mark 4
Confirm it is sulfate
A white precipitate forms.

Recall checklist

  • State the apparatus for measuring time, temperature, mass and volume.
  • Distinguish solute, solvent, solution and saturated.
  • Select a separation method for a given mixture.
  • Explain how melting point reveals purity.
  • Describe an acid–base titration and its end-point.
  • Calculate an Rf value from a chromatogram.
  • Identify a cation from hydroxide precipitate and flame-test results.
  • Identify an anion and a gas from their tests.

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