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IGCSE Chemistry 0620 · Chapter 12 notes · ~15 min read

Experimental techniques & chemical analysis

The practical skills behind every Paper 6 question: which apparatus to use, how to separate and purify, how to read a chromatogram and run a titration, and every test for ions and gases.

ApparatusSeparation methodsChromatographyPurityTests for ions & gases
RSby Riyaz Sir
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Choose apparatus for the accuracy you need: a burette or pipette is more accurate than a measuring cylinder.
  • Separation depends on the mixture: filter (insoluble solid), crystallise (dissolved solid), distil (solvent), fractional distillation (liquids), chromatography (dyes).
  • A pure substance has a sharp melting or boiling point. An impure one melts over a range.
  • Tests: flame test, NaOH and NH₃ for cations, acidify then add a reagent for anions, and a test for each gas.

By the end you can

  • Choose apparatus to measure time, temperature, mass and volume, and use the key words (solvent, solute, residue, filtrate).
  • Select and describe separation methods, and use melting and boiling points to judge purity.
  • Interpret chromatograms and calculate Rf values. EXT
  • Describe an acid–alkali titration and find the end-point.
  • Carry out and interpret tests for cations, anions and gases.

1Measuring and experimental design

Choosing apparatus

beakerholds, heatsliquidsconicalflaskholds liquids;swirl safelymeasuringcylindervolume of aliquid (fairlyaccurate)pipetteone fixedvolume, veryaccurateburetteany volume,very accurategassyringevolume ofa gasthermometertemperaturein °Cstopwatchtime inseconds12.34balancemass in grams
Common apparatus. Use the one that matches the accuracy you need.
MeasurementApparatusNotes
timestopwatchread in seconds
temperaturethermometerread in °C
massbalanceread in grams
volume of liquidmeasuring cylinderfairly accurate — use for approximate volumes
exact, fixed volumepipettevery accurate, one volume only (e.g. 25.0 cm³)
any volume, accuratelyburettevery accurate — read the bottom of the meniscus
volume of gasgas syringeaccurate; or collect over water in an upturned measuring cylinder
  • A beaker or conical flask is for holding and mixing — the markings are only approximate.
  • A gas syringe is more accurate than collecting a gas over water, but can stick. Over water, a gas that dissolves (such as CO₂) cannot be collected accurately.

Fair tests

  • Change one thing (the independent variable), measure one thing (the dependent variable), and keep everything else the same (the control variables).
  • Repeat the experiment and calculate a mean to improve reliability. Ignore results that are obviously wrong (anomalies).

Key words

mixture poured inresidue(solid left behind)filtrate(liquid that passes through)separates an insoluble solid from a liquidsolvent: the liquid thatdissolves the solutesolute: the substancethat dissolvessolution: solute + solventsaturated solution: no moresolute can dissolve at thattemperature
Filtration. The insoluble solid that stays on the paper is the residue; the liquid that goes through is the filtrate. The panel on the right defines the main words.

Solvent — the liquid that dissolves a substance. Solute

Saturated solution — a solution in which no more solute can dissolve at that temperature.

Residue — the solid left on the filter paper. Filtrate

2Separating and purifying

Choosing the method

MixtureMethodWhy it works
insoluble solid in a liquid (sand in water)filtrationthe solid is too big to pass through the paper
a dissolved solid, solid wanted (salt from sea water)crystallisationheat to evaporate water, then cool
a solvent from a solution (pure water from salt water)simple distillationthe solvent boils off and condenses; the solute stays
liquids that mix, with different boiling points (ethanol and water)fractional distillationthe liquid with the lower boiling point evaporates first
mixture of dyes or colourschromatographydifferent solubilities → move at different speeds

Crystallisation and distillation

getting crystals from a solutionstep 1step 2step 3step 4crystalsheat gently toevaporate someof the watertest: dip in a coldglass rod; crystalsform = saturatedleave to coolslowly: crystalsform (bigger)filter, wash with alittle cold water,dry on filter paperstop heating when the solution is saturated: do not boil it dry(hydrated crystals would be damaged and the crystals would be tiny)
Crystallisation. Heat to evaporate some of the water, test whether the solution is saturated, leave it to cool slowly, then filter, wash and dry.
  • Crystallisation: heat gently to evaporate some water. Test by dipping in a cold glass rod: crystals form when the solution is saturated. Leave to cool slowly (bigger crystals), then filter, wash with a little cold water and dry between filter papers.
  • Do not evaporate to dryness: the crystals could be damaged or lose their water of crystallisation.
water inwater outthermometerfractionatingcolumn: beadsgive a largesurface area(fractional only)condenser: cools thevapour back to a liquiddistillatesimple distillation: no column, e.g. pure water from salt waterfractional distillation: separates liquids with different boiling points
Simple and fractional distillation. In fractional distillation the column of beads gives a large surface area, so the liquids separate better.
  • Distillation: the solution is heated, the solvent evaporates, and the vapour is condensed in a water-cooled condenser. The pure liquid (the distillate) is collected. The solute stays in the flask.
  • Fractional distillation: the mixture of liquids is heated. The one with the lowest boiling point evaporates and reaches the top of the fractionating column first. Used for ethanol from water, crude oil, and liquid air.
  • The thermometer bulb is at the side arm, level with the exit, so it reads the temperature of the vapour.
Worked example

A mixture contains sand, salt and iron filings. Describe how to separate all three.

  1. use a magnet to remove the iron filings
  2. add water and stir to dissolve the salt, then filter: sand is the residue
  3. evaporate the filtrate (crystallise) to get the salt

Purity

timetemperaturetimetemperaturem.p.PURE substanceIMPURE substancemelts at one fixedtemperaturemelts over a rangeand at a lowertemperaturesame idea for boiling: a pure liquid boils at one sharp temperature (water: 100 °C)impurities lower the melting point and raise the boiling point
Melting a pure and an impure substance. A pure substance melts at one fixed temperature. An impure one melts over a range and at a lower temperature.
  • A pure substance has a sharp, fixed melting point and boiling point. Impurities make a substance melt over a range of temperatures and at a lower temperature than the pure substance. They raise the boiling point.
  • Examples: pure water melts at 0 °C and boils at 100 °C. Salt water boils above 100 °C.
  • Purity matters: foods and medicines must be pure so that they are safe.
Common mistakeSaying that "pure" means "natural" or "has no additives". In chemistry, pure means it is a single substance with nothing else mixed in.

3Chromatography

Separating dyes

paper chromatographybaseline(pencil)solvent frontspots 1 and 3:same substanceab1231 draw the baseline in pencil2 put a spot of each sample on it3 stand it in solvent (below the line)4 remove when the solvent is near the top; mark the solvent frontRf = a ÷ b (never more than 1)a = distance moved by the spotb = distance moved by the solvente.g. 3.0 cm ÷ 6.0 cm = 0.50colourless spots: spray with a locating agent first (Extended)
Paper chromatography. A pencil baseline carries spots of each sample. The solvent rises and the colours separate. Spots 1 and 3 have moved the same distance, so they are the same substance.
  • Method: draw a pencil baseline (pencil does not dissolve). Put a spot of each sample on it. Stand the paper in a solvent with the baseline above the solvent level. When the solvent is near the top, remove the paper and mark the solvent front.
  • Reading it: each spot is a different substance. A pure substance gives one spot. If two spots from different samples are at the same height, they are the same substance.
  • Substances separate because they are different in how well they dissolve in the solvent and how strongly they stick to the paper.
  • EXT Colourless substances (such as amino acids) are made visible by spraying with a locating agent.
EXT Rf = distance moved by the spotdistance moved by the solvent
no units; always between 0 and 1
Worked example

EXT A spot moves 3.6 cm and the solvent front moves 7.2 cm. Calculate its Rf.

  1. Rf = 3.6 ÷ 7.2
  2. = 0.50
Common mistakeDrawing the baseline in ink, or placing the solvent level above the baseline. Both let the sample wash into the solvent.

4Acid–alkali titration

The method

burette: acidflask: alkali + indicatorwhite tile1 rinse the burette with acid, fill it, read it2 pipette 25.0 cm³ of alkali into the flask3 add 2–3 drops of indicator4 add acid slowly, swirling the flask5 stop at the first permanent colour change (the end-point), read the burette6 repeat until two titres agree (within 0.10 cm³)methyl orange: red in acid, yellow in alkaliphenolphthalein: colourless in acid, pink in alkalicalculate: moles = concentration × volume (dm³)use the equation to compare moles of acid and alkaliconcentration = moles ÷ volume
The titration apparatus and method. Acid goes in the burette. A known volume of alkali and a few drops of indicator go in the flask on a white tile.
  • 1. Rinse the burette with the acid, fill it and read the starting volume.
  • 2. Use a pipette to measure 25.0 cm³ of the alkali into a conical flask. Add 2–3 drops of indicator.
  • 3. Add the acid slowly, swirling the flask, until the end-point — the first permanent colour change.
  • 4. Read the final volume. The titre = final reading − starting reading.
  • 5. Repeat until you get two or three titres within 0.10 cm³ of each other (concordant results). Use the mean of those.
  • Indicators: methyl orange (red in acid, yellow in alkali) or phenolphthalein (colourless in acid, pink in alkali). The white tile makes the colour change easier to see.
Worked example

Titres: 24.30, 23.40, 23.50 and 23.45 cm³. Find the mean titre. Then find the concentration of 25.0 cm³ of sodium hydroxide neutralised by this volume of 0.100 mol/dm³ HCl.

  1. the first (24.30) is not concordant, so ignore it
  2. mean = (23.40 + 23.50 + 23.45) ÷ 3 = 70.35 ÷ 3 = 23.45 cm³
  3. n(HCl) = 0.100 × 23.45 ÷ 1000 = 0.002345 mol
  4. 1 : 1 ratio, so n(NaOH) = 0.002345 mol
  5. c(NaOH) = 0.002345 ÷ 0.0250 = 0.0938 mol/dm³ (more on calculations in stoichiometry)
Common mistakeIncluding the first rough titre in the mean. Use only concordant values.

5Tests for ions and gases

Flame tests

flame tests: the colour tells you the metal ionLi⁺lithiumcrimson redNa⁺sodiumyellowK⁺potassiumlilacCa²⁺calciumorange-redCu²⁺copperblue-greenclean a wire loop in acid, dip it in the sample, hold it in a blue Bunsen flame
Flame test colours: lithium red, sodium yellow, potassium lilac, calcium orange-red and copper blue-green.
  • Clean a wire loop in acid, dip it in the sample and hold it in a blue Bunsen flame.
IonFlame colour
lithium Li⁺crimson red
sodium Na⁺yellow
potassium K⁺lilac
calcium Ca²⁺orange-red
copper(II) Cu²⁺blue-green

Cations: sodium hydroxide and ammonia

precipitates with sodium hydroxide, NaOH(aq)Al³⁺whiteZn²⁺whiteCr³⁺greenCa²⁺whiteCu²⁺light blueFe²⁺greenFe³⁺red-brownprecipitate dissolvesin excess NaOHprecipitate stays (insoluble in excess)with aqueous ammonia, NH₃(aq): the same precipitates form, butonly Zn²⁺ and Cu²⁺ precipitates dissolve in excess (Cu²⁺ gives a deep blue solution)Ca²⁺: no precipitate or only a traceammonium ions: warm with NaOH(aq): ammonia gas is given off(turns damp red litmus paper blue)
Precipitates with sodium hydroxide. The colour identifies the ion; whether it dissolves in excess separates similar ones.
  • Add NaOH(aq) drop by drop, then in excess, to the solution. Then repeat with NH₃(aq).
IonNaOH(aq)In excess NaOHNH₃(aq)In excess NH₃
aluminium Al³⁺white precipitatedissolves (colourless)white precipitatestays
calcium Ca²⁺white precipitatestaysno precipitate or trace–
chromium(III) Cr³⁺green precipitatestaysgreen precipitatestays
copper(II) Cu²⁺light blue precipitatestayslight blue precipitatedissolves → deep blue solution
iron(II) Fe²⁺green precipitatestaysgreen precipitatestays
iron(III) Fe³⁺red-brown precipitatestaysred-brown precipitatestays
zinc Zn²⁺white precipitatedissolves (colourless)white precipitatedissolves (colourless)
  • Ammonium ions (NH₄⁺): warm with NaOH(aq) → ammonia gas (pungent; turns damp red litmus blue).
Memory trickAl and Zn are the two white precipitates that dissolve in excess NaOH. To split them, add ammonia: Zn dissolves, Al does not.

Anions

test for halide and sulfate ions: acidify, then add a solutionCl⁻white precipitateadd AgNO₃(aq)Br⁻cream precipitateadd AgNO₃(aq)I⁻yellow precipitateadd AgNO₃(aq)SO₄²⁻white precipitateadd Ba(NO₃)₂(aq)halides: acidify with dilute nitric acid, then add silver nitratesulfate: acidify with dilute nitric (or hydrochloric) acid, then add barium nitrate (or chloride)(the acid removes carbonate ions, which would also give a precipitate)carbonate: add dilute acid → fizzing; the gas turns limewater milky (CO₂)nitrate: warm with NaOH(aq) and aluminium foil → ammonia gas (turns damp red litmus blue)
Tests for halide and sulfate ions. Acidify first, then add the reagent.
IonTestPositive result
carbonate CO₃²⁻add dilute acidfizzing; gas turns limewater milky
chloride Cl⁻acidify with dilute nitric acid, then add silver nitratewhite precipitate
bromide Br⁻acidify with dilute nitric acid, then add silver nitratecream precipitate
iodide I⁻acidify with dilute nitric acid, then add silver nitrateyellow precipitate
sulfate SO₄²⁻acidify with dilute nitric (or hydrochloric) acid, then add barium nitrate (or chloride)white precipitate
nitrate NO₃⁻add NaOH(aq) and aluminium foil, warmammonia gas (turns damp red litmus blue)
sulfite SO₃²⁻add dilute hydrochloric acid, warmsulfur dioxide (turns acidified potassium manganate(VII) from purple to colourless)
  • Why acidify first? The acid removes carbonate (and hydroxide) ions, which would also give a precipitate with silver or barium ions and give a false result.

Gases

pop!H₂ hydrogenlighted splint→ squeaky popO₂ oxygenglowing splint→ relightsCO₂ carbon dioxidelimewater→ turns milkyCl₂ chlorinedamp litmus paper→ bleached whiteNH₃ ammoniadamp red litmus→ turns blueSO₂ sulfur dioxideacidified KMnO₄→ purple to colourless
Tests for six gases: H₂, O₂, CO₂, Cl₂, NH₃ and SO₂.
GasTestResult
hydrogen H₂lighted splintsqueaky pop
oxygen O₂glowing splintrelights
carbon dioxide CO₂bubble through limewaterturns milky
chlorine Cl₂damp litmus paperbleached white
ammonia NH₃damp red litmus paperturns blue
sulfur dioxide SO₂acidified potassium manganate(VII)purple → colourless
Worked example

A solution gives a green precipitate with sodium hydroxide, which turns red-brown on standing. A separate sample, acidified and treated with barium nitrate, gives a white precipitate. Name the compound.

  1. green precipitate → Fe²⁺ (it slowly oxidises, so goes red-brown)
  2. white precipitate with acidified barium nitrate → sulfate
  3. the compound is iron(II) sulfate, FeSO₄
Worked example

A white solid dissolves in water. NaOH gives a white precipitate that dissolves in excess. Ammonia gives a white precipitate that does not dissolve in excess. A flame test gives no colour. Which cation is present?

  1. white precipitate that dissolves in excess NaOH → Al³⁺ or Zn²⁺
  2. does not dissolve in excess NH₃ → Al³⁺ (zinc would dissolve)
Exam tipIn an unknown-solution question, make a table: test → observation → conclusion. Name the precipitate colour and say whether it dissolves in excess. Never write "turns cloudy" — write "white precipitate".
Common mistakeDoing the silver nitrate test without acidifying with nitric acid. Carbonates and hydroxides would also form precipitates.

6Test yourself

Test yourself — cover the answers first

Quick check

  1. What apparatus gives the most accurate fixed volume of 25.0 cm³?
  2. What is the filtrate?
  3. Which method separates ethanol from water?
  4. Why is the baseline in chromatography drawn in pencil?
  5. What is the colour of the precipitate of iron(III) with sodium hydroxide?
  6. What is the flame colour of potassium?
  7. What is the test for chlorine?
  8. EXT Calculate the Rf for a spot that has moved 2.4 cm when the solvent has moved 6.0 cm.

Exam-style

  1. Describe how to separate a mixture of sand and salt and obtain a pure, dry sample of each. [4]
  2. Describe the test for sulfate ions. State the observation for a positive result. [3]
  3. Describe how to carry out a titration to find the volume of acid needed to neutralise 25.0 cm³ of alkali. [5]
  4. EXT The chromatogram of a food dye shows three spots. What does this tell you about the dye, and how would you identify each spot? [3]
Show answers
  1. pipette
  2. the liquid that passes through the filter paper
  3. fractional distillation
  4. pencil does not dissolve in the solvent (ink would run)
  5. red-brown
  6. lilac
  7. damp blue litmus paper is bleached white
  8. 2.4 ÷ 6.0 = 0.40
  9. add water and stir to dissolve the salt
    filter to separate the sand (residue); wash and dry it
    evaporate the filtrate until saturated, then leave to cool so that salt crystallises
    filter, wash and dry the salt crystals
  10. acidify the solution with dilute nitric acid
    add barium nitrate solution
    a white precipitate forms
  11. pipette 25.0 cm³ of alkali into a flask and add indicator
    fill the burette with acid and record the volume
    add acid slowly with swirling until the colour changes
    record the final volume and calculate the titre
    repeat until two titres agree within 0.10 cm³
  12. the dye is a mixture of at least three substances
    calculate the Rf of each spot
    compare with Rf values (or spots on the same chromatogram) of known substances
Practise this chapter
Experimental designSeparation & purificationChromatographyConcentration & titrationIdentification of ions & gases
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