Tests for Ions and Gases

Qualitative analysis is tested in Papers 2, 3, 4, 5 and 6. The tests are finite and learnable, and every one follows the same pattern: add a specific reagent, observe a specific result. This page organises every test required by the 0620 syllabus.

Gas tests

These five gas tests appear constantly and must be known perfectly.

Gas	Test	Positive result
Hydrogen (H2)	Burning/lighted splint	Burns with a squeaky pop
Oxygen (O2)	Glowing splint	Relights the glowing splint
Carbon dioxide (CO2)	Bubble through limewater (Ca(OH)2)	Turns milky/cloudy (white precipitate of CaCO3)
Ammonia (NH3)	Hold damp red litmus paper	Turns blue (ammonia is alkaline)
Chlorine (Cl2)	Hold damp litmus paper	Bleaches it white (litmus turns red then white)

Note for chlorine: damp blue litmus may turn red first (Cl2 is acidic in solution) then white (bleaching). The bleaching is the definitive test.

Cation tests using sodium hydroxide solution

Add sodium hydroxide (NaOH) solution to a solution of the unknown substance. Observe any precipitate formed and whether it dissolves in excess NaOH.

Cation	Precipitate colour	In excess NaOH?	Ionic equation
Cu2+	Blue	Insoluble (stays)	Cu2+ + 2OH- -> Cu(OH)2
Fe2+	Green	Insoluble (stays)	Fe2+ + 2OH- -> Fe(OH)2
Fe3+	Red-brown	Insoluble (stays)	Fe3+ + 3OH- -> Fe(OH)3
Al3+	White	Dissolves (soluble in excess)	Al3+ + 3OH- -> Al(OH)3 then Al(OH)3 + OH- -> Al(OH)4-
Zn2+	White	Dissolves (soluble in excess)	Zn2+ + 2OH- -> Zn(OH)2 then Zn(OH)2 + 2OH- -> Zn(OH)4^2-
Ca2+	White	Insoluble (stays)	Ca2+ + 2OH- -> Ca(OH)2
Mg2+	White	Insoluble (stays)	Mg2+ + 2OH- -> Mg(OH)2

Distinguishing white precipitates

Three cations give white precipitates with NaOH: Al3+, Zn2+, Ca2+, and Mg2+.

• Al3+ and Zn2+ dissolve in excess NaOH (amphoteric hydroxides)
• Ca2+ and Mg2+ do not dissolve in excess NaOH

To distinguish Al3+ from Zn2+, use ammonia solution instead: both give white precipitates, but only Zn(OH)2 dissolves in excess ammonia.

Cation tests using ammonia solution

Add aqueous ammonia (NH3) to the unknown solution:

Cation	Precipitate	In excess ammonia?
Cu2+	Blue precipitate	Dissolves to give deep blue solution
Fe2+	Green precipitate	Insoluble
Fe3+	Red-brown precipitate	Insoluble
Al3+	White precipitate	Insoluble
Zn2+	White precipitate	Dissolves

The deep blue colour with excess ammonia is diagnostic for Cu2+.

Flame tests

Heat a nichrome wire in a roaring Bunsen flame to clean it, dip in concentrated HCl, then dip into the solid sample and hold in the flame.

Cation	Flame colour
Li+	Crimson (red)
Na+	Yellow
K+	Lilac
Ca2+	Orange-red
Cu2+	Blue-green

Flame tests are used when NaOH would not help (e.g. distinguishing Na+ from K+, since both give soluble hydroxides).

Anion tests

Test for carbonate (CO3^2-)

Add dilute acid (HCl or HNO3). If carbonate is present, effervescence occurs and the gas turns limewater milky.

CO3^2- + 2H+ -> H2O + CO2

Test for sulfate (SO4^2-)

Add dilute nitric acid (to remove carbonates that might interfere), then add barium nitrate solution.

A white precipitate of barium sulfate confirms sulfate ions.

Ba2+(aq) + SO4^2-(aq) -> BaSO4(s)

The acid must be added first. Without it, barium carbonate (also white and insoluble) could form and give a false positive.

Test for halide ions (Cl-, Br-, I-)

Add dilute nitric acid (to remove interfering ions), then add silver nitrate solution (AgNO3).

Halide	Precipitate	Colour
Cl-	AgCl	White
Br-	AgBr	Cream
I-	AgI	Yellow

Ag+(aq) + Cl-(aq) -> AgCl(s) (white) Ag+(aq) + Br-(aq) -> AgBr(s) (cream) Ag+(aq) + I-(aq) -> AgI(s) (yellow)

Important: Use nitric acid, not hydrochloric acid, because HCl would add Cl- ions and interfere with the test.

Test for ammonium ion (NH4+)

Add sodium hydroxide solution and warm gently. If NH4+ is present, ammonia gas is given off.

NH4+ + OH- -> NH3 + H2O

Test the gas with damp red litmus paper: it turns blue.

Test for water

Two distinct tests:

Test	Reagent	Observation
Test for any water	Anhydrous copper(II) sulfate	White powder turns blue
Test for pure water	Thermometer	Boils at exactly 100 C and freezes at 0 C

Anhydrous cobalt(II) chloride paper can also be used: it turns from blue to pink when water is present.

Decision tree for identifying an unknown substance

1. Dissolve or test solubility: Is it soluble? What colour is the solution?
2. Flame test: If solid, identify the cation from flame colour
3. NaOH test: Add NaOH solution to identify the cation from precipitate colour
4. Anion tests: Test portions of the solution with silver nitrate (halides), barium nitrate (sulfate), and acid (carbonate)
5. Gas tests: If gases are produced, test with appropriate methods

Common exam mistakes

1. Using HCl instead of HNO3 before silver nitrate test: HCl adds chloride ions, causing a false positive for chloride.
2. Not adding acid before barium nitrate: Carbonates and sulfites can give white precipitates with Ba2+, giving false positives.
3. Confusing cream and white precipitates: AgBr is cream, not white. Examiners mark this strictly.
4. Saying ammonia turns litmus “red to blue”: Ammonia turns damp red litmus blue. If the litmus was blue to start with, there is no observable change.

Worked exam questions

A white solid X dissolves in water to give a colourless solution. Adding NaOH gives no precipitate. A flame test gives a lilac colour. Adding dilute nitric acid and silver nitrate to the solution gives a white precipitate. Identify X. [3 marks]

• No precipitate with NaOH, so the cation is Na+, K+, or NH4+ [1]
• Lilac flame test identifies K+ as the cation [1]
• White precipitate with silver nitrate identifies Cl- as the anion. X is potassium chloride (KCl) [1]

A solution contains either Fe2+ or Fe3+ ions. Describe a test to distinguish between them. [2 marks]

• Add sodium hydroxide solution to a sample of the solution [1]
• Fe2+ gives a green precipitate; Fe3+ gives a red-brown precipitate [1]