Qualitative Analysis: Every Ion and Gas Test You Must Know

Qualitative analysis is the closest thing 0620 has to free marks: the questions come straight from a published list of tests, the answers never change, and the mark scheme accepts exact wording only. Students lose these marks not through difficulty but through almost-right phrasing: “cloudy” instead of “white precipitate”, “clear” instead of “colourless”.

Cation tests: sodium hydroxide and aqueous ammonia

Add aqueous sodium hydroxide dropwise, then in excess. Repeat with aqueous ammonia. The pair of results identifies the cation.

Cation	With NaOH(aq)	With NH3(aq)
Aluminium, Al3+	White ppt, soluble in excess giving a colourless solution	White ppt, insoluble in excess
Ammonium, NH4+	Ammonia produced on warming	No reaction
Calcium, Ca2+	White ppt, insoluble in excess	No ppt (or very slight white ppt)
Chromium(III), Cr3+	Green ppt, soluble in excess	Grey-green ppt, insoluble in excess
Copper(II), Cu2+	Light blue ppt, insoluble in excess	Light blue ppt, soluble in excess giving a dark blue solution
Iron(II), Fe2+	Green ppt, insoluble in excess	Green ppt, insoluble in excess
Iron(III), Fe3+	Red-brown ppt, insoluble in excess	Red-brown ppt, insoluble in excess
Zinc, Zn2+	White ppt, soluble in excess giving a colourless solution	White ppt, soluble in excess giving a colourless solution

(ppt = precipitate; write the full word in the exam.)

The discrimination logic the examiner builds questions from:

• White precipitate with NaOH, dissolves in excess → Al3+ or Zn2+. Separate them with ammonia: zinc’s precipitate dissolves in excess ammonia, aluminium’s does not.
• White precipitate with NaOH, insoluble in excess → Ca2+.
• Green precipitate → Fe2+ (with either reagent) or Cr3+ (dissolves in excess NaOH; that solubility is the separator).
• No precipitate with NaOH but a pungent gas on warming that turns damp red litmus blue → NH4+.

The Cu2+ ammonia result is the showpiece: light blue precipitate first, then excess ammonia dissolves it to a deep, dark blue solution. Schemes pay separately for the precipitate and for the dark blue solution.

Anion tests

Anion	Test	Result
Carbonate, CO32−	Add dilute acid	Effervescence; gas turns limewater milky (CO2)
Chloride, Cl−	Acidify with dilute nitric acid, add aqueous silver nitrate	White precipitate
Bromide, Br−	Acidify with dilute nitric acid, add aqueous silver nitrate	Cream precipitate
Iodide, I−	Acidify with dilute nitric acid, add aqueous silver nitrate	Yellow precipitate
Nitrate, NO3−	Add aqueous sodium hydroxide and aluminium foil, warm gently	Ammonia produced (turns damp red litmus blue)
Sulfate, SO42−	Acidify with dilute nitric acid, add aqueous barium nitrate	White precipitate
Sulfite, SO32−	Add dilute acid, warm gently	Sulfur dioxide produced; turns acidified aqueous potassium manganate(VII) from purple to colourless

Two details that carry marks. The acidification step is part of the test: for halides and sulfate, “acidify with dilute nitric acid” is in the scheme, and skipping it can drop the mark (the acid removes carbonate ions that would give a false precipitate). And for the nitrate test, the gas evolved is ammonia. Students who write “hydrogen” have remembered the aluminium and forgotten the chemistry.

Gas tests

Gas	Test	Positive result
Ammonia, NH3	Hold damp red litmus paper in the gas	Litmus turns blue
Carbon dioxide, CO2	Bubble through limewater	Limewater turns milky
Chlorine, Cl2	Hold damp litmus paper in the gas	Litmus is bleached (turns white)
Hydrogen, H2	Hold a lighted splint at the mouth of the tube	Burns with a squeaky pop
Oxygen, O2	Insert a glowing splint	Splint relights
Sulfur dioxide, SO2	Bubble through acidified aqueous potassium manganate(VII)	Turns from purple to colourless

Precision points: the litmus is damp (the gas must dissolve to act), the splint for hydrogen is lighted and for oxygen glowing. Swapping those two adjectives swaps the marks away. Chlorine may turn damp blue litmus red first, then bleach it; “bleaches” is the creditable word.

Flame colours

Cation	Flame colour
Lithium, Li+	Red
Sodium, Na+	Yellow
Potassium, K+	Lilac
Calcium, Ca2+	Orange-red
Barium, Ba2+	Light green
Copper(II), Cu2+	Blue-green

Method, if asked: dip a clean wire (nichrome/platinum) in concentrated hydrochloric acid, then in the sample, and hold it in a non-luminous (blue) Bunsen flame. “Lilac” for potassium, not “purple”: schemes list lilac.

Test for water

Two different questions hide here, and students merge them:

• Test for the presence of water: anhydrous copper(II) sulfate turns from white to blue, or anhydrous cobalt(II) chloride turns from blue to pink.
• Test for purity of water: pure water boils at exactly 100 °C and freezes at 0 °C (at standard pressure). A boiling-point check tests purity; the colour tests only show water is present.

A question asking how to show a liquid is pure water needs both ideas: copper sulfate test for water, boiling point for purity.

How these are asked in P4 and P6

Paper 6 builds multi-step identification questions: an unknown solid, a printed test sequence, observations to complete, conclusions to draw. The skill is two-directional: given the ion, state the observation; given the observation, name the ion. Practise both directions, because the paper switches between them mid-question. Paper 6’s broader demands are covered in the Paper 6 guide.

Paper 4 embeds single tests in topic questions: a salt-preparation question from Acids, Bases and Salts ends with “describe a test to show the salt contains chloride ions” [2]: one mark for the test (acidify with dilute nitric acid, add aqueous silver nitrate), one for the result (white precipitate). Test and result are always separate marks. The experimental context for all of it sits in Experimental Techniques and Chemical Analysis.

Worked exam question

Compound X is a white solid. A student dissolves X in water and divides the solution into two parts. Test 1: aqueous sodium hydroxide is added until in excess. A white precipitate forms which dissolves in excess to give a colourless solution. Test 2: aqueous ammonia is added until in excess. A white precipitate forms which is insoluble in excess. Test 3: dilute nitric acid and aqueous silver nitrate are added to a fresh solution of X. A yellow precipitate forms. Identify compound X. Explain how the results support your answer. [4]

Model answer with mark breakdown:

• Tests 1 and 2: the cation is aluminium, Al3+ [1]: a white precipitate soluble in excess NaOH means Al3+ or Zn2+, and insolubility in excess ammonia rules out zinc [1]
• Test 3: a yellow precipitate with acidified silver nitrate identifies iodide, I− [1]
• X is aluminium iodide, AlI3 [1]

The four marks are the reasoning chain, not just the name. Writing “aluminium iodide” alone earns 1; showing how each test narrows the field earns all 4.

The mistakes that cost marks

• “Cloudy” for “precipitate”. The scheme’s noun is precipitate, with its colour. Cloudy describes the sky.
• “Clear” for “colourless”. Copper sulfate solution is clear AND blue. The creditable word for no colour is colourless.
• Dropping “in excess”. Half the cation table’s information is what happens in excess reagent. “White precipitate” alone cannot separate Al3+, Zn2+ and Ca2+.
• Splint adjectives swapped. Lighted splint for hydrogen, glowing splint for oxygen. Reversed, both marks die.
• Missing the acidification step. Halide and sulfate tests start with dilute nitric acid. The test mark includes it.

How to phrase it for full marks

Student language: “It fizzed and made a gas.” Mark-scheme language: “Effervescence; the gas turned limewater milky, so the gas is carbon dioxide and the compound contains carbonate ions.”

Student language: “The copper one went blue with ammonia.” Mark-scheme language: “A light blue precipitate formed, which dissolved in excess aqueous ammonia to give a dark blue solution.”

The grammar of every full-mark observation: colour + precipitate/gas/solution + behaviour in excess (or the confirming test). Three slots, every time. Fill all three and the mark cannot escape.

The Malaysia note

Malaysian students sitting the May/June series meet qualitative analysis twice: embedded in Paper 4 and dominating Paper 6. And since nearly all Malaysian schools enter Paper 6, this list is effectively 15+ marks of the final grade. The local failure mode is familiarity without precision: students have seen every test in school labs, so they revise it last, then write “cloudy” and “clear” in the exam. Our fix is mechanical: both tables drilled in both directions until recall is word-perfect, usually inside two sessions. If you want to see where your child stands, the free 1-hour trial lesson can run a Paper 6 identification question live, marked against the official scheme, before you pay anything. No forms. WhatsApp us; we reply the same day.