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12.3.3Experimental Techniques and Chemical Analysis

Identification of Gases

15 min read 8 parts Lesson 9 of 9 1 quiz

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Section 1 of 8Identification of Gases

1
Most metal hydroxides are insoluble, so adding OH− (from NaOH or aqueous NH3) to a solution of the metal ion forms a coloured or white precipitate that identifies the cation.
2
White precipitates (Al3+, Zn2+, Ca2+) are told apart by excess reagent: Al(OH)3 dissolves in excess NaOH but not excess NH3; Zn(OH)2 dissolves in excess of both; Ca(OH)2 dissolves in neither and gives little or no ppt with NH3.
3Coloured precipitates
Cu(OH)2 light blue (dissolves in excess NH3 to a dark blue solution); Fe(OH)2 green (oxidised to red-brown in air); Fe(OH)3 red-brown; Cr(OH)3 green (dissolves in excess NaOH).
4
Ammonium ions give no precipitate; on warming with NaOH they release ammonia, which turns damp red litmus paper blue.
5Flame tests
Flame tests dip a clean wire loop in concentrated HCl, then in the solid, hold in the hottest part of a blue Bunsen flame: Li+ red, Na+ yellow, K+ lilac, Ca2+ orange-red, Ba2+ light green, Cu2+ blue-green.
6Carbonate
Carbonate add dilute acid → effervescence; the gas turns limewater milky, showing CO2.
7Halides
Halides acidify with dilute nitric acid (removes carbonate), then add aqueous silver nitrate: Cl− white ppt, Br− cream ppt, I− yellow ppt.
8Sulfate
Sulfate acidify with dilute nitric acid, add aqueous barium nitrate → white ppt of barium sulfate.
9Sulfite
Sulfite add acidified aqueous potassium manganate(VII); colour changes from purple to colourless (sulfite is a reducing agent).
10Nitrate
Nitrate add aqueous NaOH and aluminium foil, warm; ammonia is produced (turns damp red litmus blue) because nitrate is reduced.
11Gas tests
NH3 damp red litmus → blue; CO2 limewater → milky; Cl2 bleaches damp litmus; H2 squeaky pop with lighted splint; O2 relights glowing splint; SO2 acidified KMnO4 purple → colourless.
12Water
Water anhydrous CuSO4 white → blue or anhydrous CoCl2 blue → pink shows water is present; only m.p./b.p. (0 °C/100 °C) shows it is pure.

Section 2 of 8Process & sequence

    • Dissolve the unknown solid in distilled water in a test tube (note the colour of the solution).
    • Carry out a flame test on the solid with a clean nichrome/platinum wire dipped in concentrated HCl;
      • note the flame colour.
    • Split the solution; to one portion add aqueous NaOH dropwise then in excess; note ppt colour and whether it dissolves.
      • Warm to test for NH3 with damp red litmus.
    • To a second portion add aqueous ammonia dropwise then in excess;
      • compare with the NaOH results (distinguishes Zn2+ from Al3+).
    • Test anions
      • add dilute acid (look for fizzing → carbonate);
        • acidify with dilute HNO3 and add AgNO3 (halides) or Ba(NO3)2 (sulfate).
    • Test any gas given off with the appropriate test (limewater, splints, litmus, KMnO4).
    • Combine the results to name the compound, e.g. green ppt with NaOH + white ppt with Ba(NO3)2 → iron(II) sulfate.

    Testing a solution for halide ionswrite these steps in this order; each step is a marking point.

    1. Pour about 2 cm3 of the unknown solution into a test tube.
    2. Add dilute nitric acid to remove any carbonate ions.
    3. Add a few drops of aqueous silver nitrate.
    4. Observe the precipitate colourwhite = chloride, cream = bromide, yellow = iodide.
    5. Record the result and write the ionic equation, e.g. Ag+ + Br− → AgBr.

    Telling the white-precipitate cations apartwrite these steps in this order; each step is a marking point.

    1. Divide the solution between two test tubes.
    2. To each, add aqueous NaOH dropwise with shaking, then in excess.
    3. To the second tube, add aqueous ammonia dropwise with shaking, then in excess.
    4. Al3+: dissolves in excess NaOH but not in excess ammonia.
    5. Zn2+: dissolves in excess of both, giving a colourless solution.
    6. Ca2+: dissolves in neither, and gives no or only a slight precipitate with ammonia.

Section 3 of 8Compare

Tests for aqueous cations
CationWith aqueous NaOHWith aqueous NH3
Al3+White ppt, soluble in excess → colourless solutionWhite ppt, insoluble in excess
NH4+Ammonia gas on warming (turns damp red litmus blue)—
Ca2+White ppt, insoluble in excessNo ppt or very slight white ppt
Cr3+Green ppt, soluble in excess (green solution)Green ppt, insoluble in excess
Cu2+Light blue ppt, insoluble in excessLight blue ppt, soluble in excess → dark blue solution
Fe2+Green ppt, insoluble in excess (turns red-brown on standing in air)Green ppt, insoluble in excess
Fe3+Red-brown ppt, insoluble in excessRed-brown ppt, insoluble in excess
Zn2+White ppt, soluble in excess → colourless solutionWhite ppt, soluble in excess → colourless solution

Flame tests

  • Li+ red;
    • Na+ yellow;
    • K+ lilac;
    • Ca2+ orange-red;
    • Ba2+ light green;
    • Cu2+ blue-green.

Tests for anions and gases
Ion / gasTestPositive result
CO32−Add dilute acid; bubble gas through limewaterEffervescence; CO2 turns limewater milky
Cl−Acidify with dilute HNO3, add aqueous AgNO3White ppt
Br−Acidify with dilute HNO3, add aqueous AgNO3Cream ppt
I−Acidify with dilute HNO3, add aqueous AgNO3Yellow ppt
NO3−Add aqueous NaOH then aluminium foil; warmNH3 produced, turns damp red litmus blue
SO32−Add acidified aqueous KMnO4Purple → colourless
SO42−Acidify with dilute HNO3, add aqueous Ba(NO3)2White ppt
NH3Damp red litmusTurns blue
CO2LimewaterTurns milky
Cl2Damp litmus paperBleaches it (blue litmus goes red then white)
H2Lighted splintSqueaky pop
O2Glowing splintRelights
SO2Acidified aqueous KMnO4Purple → colourless
The anions you must be able to test for
FamilyIonsTest reagent
Carbon and nitrogenCarbonate CO32−, nitrate NO3−Dilute acid (carbonate); NaOH + aluminium foil, warm (nitrate)
Halides (Group 7)Cl−, Br−, I−Dilute HNO3 then aqueous AgNO3
SulfurSulfate SO42−, sulfite SO32−Acidified Ba(NO3)2; acidified KMnO4

Section 4 of 8Worked examples

A solution gives a white precipitate with NaOH that dissolves in excess, and a white precipitate with NH3 that does NOT dissolve in excess. With acidified AgNO3 it gives a white ppt. Identify the compound.
  • White ppt soluble in excess NaOH but insoluble in excess NH3 → Al3+.
    • White ppt with AgNO3 → Cl−.
    • Compound: aluminium chloride, AlCl3.
A green solid gives a green ppt with NaOH, insoluble in excess, turning brown at the surface. With acidified Ba(NO3)2 a white ppt forms. Name it and write the ionic equation for the precipitate with NaOH.
  • Green ppt → Fe2+; white ppt with Ba2+ → SO42−. Iron(II) sulfate, FeSO4. Fe2+(aq) + 2OH−(aq) → Fe(OH)2(s).
Excess dilute acid is added to 2.10 g of MgCO3 (Mr = 84). Calculate the volume of CO2 (at r.t.p.) that would turn limewater milky.
  • moles MgCO3 = 2.10 ÷ 84 = 0.0250 mol;
    • 1:1 ratio → 0.0250 mol CO2;
    • volume = 0.0250 × 24 dm3 = 0.600 dm3 (600 cm3).
Bromide
A white solid gives a lilac flame and, with aqueous NaOH added dropwise, a white precipitate that is insoluble in excess. With dilute HNO3 then aqueous AgNO3 it gives a cream precipitate. Identify it.
  • Lilac flame → K+.
    • White precipitate, insoluble in excess NaOH → Ca2+;
      • a compound can carry two cations, both being K+ and Ca2+ salts.
  • Cream precipitate with silver nitrate → Br− (white = chloride, cream = bromide, yellow = iodide).
  • Anion conclusionthe compound is a bromide, i.e. potassium bromide or calcium bromide.

Section 5 of 8Keep going

  1. Identification of Ions & Gases IGCSE Chemistry 0620: what to do next

    These Identification of Ions & Gases IGCSE Chemistry 0620 notes covered the key ideas, worked examples and exam technique for this topic. Check your Identification of Ions & Gases IGCSE Chemistry 0620 understanding with the practice questions below, then reread any Identification of Ions & Gases IGCSE Chemistry 0620 point you were unsure about before moving on.

    Identification of Ions & Gases IGCSE Chemistry 0620: revision checklist

    • Revise the main Identification of Ions & Gases IGCSE Chemistry 0620 definitions until you can write them from memory.
    • Practise Identification of Ions & Gases IGCSE Chemistry 0620 exam questions under timed conditions.
    • Link each Identification of Ions & Gases IGCSE Chemistry 0620 idea to a diagram or a worked example.
    • List the common Identification of Ions & Gases IGCSE Chemistry 0620 mistakes and how to avoid them.
    • Explain one Identification of Ions & Gases IGCSE Chemistry 0620 idea out loud as if teaching a friend.
    • Summarise Identification of Ions & Gases IGCSE Chemistry 0620 in five key points on a revision card.
    • Check which command words appear most often in Identification of Ions & Gases IGCSE Chemistry 0620 questions.
    • Come back to Identification of Ions & Gases IGCSE Chemistry 0620 after a week for spaced practice.
    • Mark your answers to Identification of Ions & Gases IGCSE Chemistry 0620 questions against the key ideas above.
    • Highlight the Identification of Ions & Gases IGCSE Chemistry 0620 keywords that examiners expect to see.
    • Draw a mind map that connects Identification of Ions & Gases IGCSE Chemistry 0620 to earlier lessons.

Section 6 of 8Key terms

Section 7 of 8Command terms

Command terms
State / Name
Give a short answer only; no explanation is needed (1 mark = 1 fact).
Define
Give the precise meaning. Learn the syllabus definitions word-for-word: every keyword in the definition is a marking point.
Describe
Say what happens / what the data show, with key features and figures. Do not give reasons.
Explain
Give reasonswhat happens AND why/how, linked with “because”, “so”, “therefore”.
Suggest
Apply knowledge to an unfamiliar situation; any sensible, scientifically-based answer can score.
Compare
Give similarities AND differences, each as a direct comparison (“X is … whereas Y is …”).
Outline
Set out the main points briefly.
Calculate
Work out from given facts; show working, give the unit and the requested significant figures.
Identify
Name/select from given information.
Predict
Give an expected result without a full justification.
Evaluate
Judge the importance or quality of the evidence/method, with reasons.

Section 8 of 8Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Qualitative analysis · Cation · Anion · Precipitate · Amphoteric hydroxide · Flame test · Excess · Limewater · Litmus · Acidified · Test for water · Complex ion (tetraammine copper)
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Use the exact scientific term and learn syllabus definitions word-for-word.
2
Write “rate of … increases”, not “… increases”.
3
Say “concentration gradient”, never just “gradient”; name particles precisely (atoms, molecules, ions) — never “it”.
4
Give units with every numerical answer and use the significant figures asked for; show all working.
5
Compare = a similarity AND a difference; describe = what happens; explain = why it happens.
6
Repeats let you identify anomalies and calculate a mean — they do not make an experiment “valid”.

Words to avoid

1
limewater goes cloudy with oxygencarbon dioxide turns limewater milky
2
hydrogen makes a lighted splint relighthydrogen gives a squeaky pop with a lighted splint; oxygen relights a glowing splint
3
a blue solution forms with NaOHa light blue precipitate forms (a solid, not a solution)
4
acidify with hydrochloric acid before adding silver nitrateacidify with dilute nitric acid (HCl would add chloride ions)
5
chlorine turns litmus redchlorine bleaches damp litmus paper
6
potassium burns purplepotassium ions give a lilac flame
7
ammonium and ammonia are the sameNH4+ is the aqueous ion, NH3 is the gas
8
the gas is carbon dioxide because it extinguished the splintnitrogen also extinguishes a flame; only limewater turning milky confirms CO2
9
the gas smells like swimming pools, so it is chlorinesmell is a property, not a confirmatory test; chlorine bleaches damp litmus
10
the solution is clear, so it is colourlessclear means see-through; colourless means no colour (dilute copper sulfate is clear but blue)
11
put the litmus paper into the solutiondamp litmus paper is held in the gas, without touching the sides of the tube
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