Qualitative analysis is one of the most generous topics in O-Level Chemistry. The Notes for Qualitative Analysis table is printed in your paper, so you are not being tested on memory. You are being tested on whether you can read observations carefully, phrase them the way the mark scheme wants, and reason from evidence to a conclusion.
Students still drop marks here every year, usually for the same small reasons. Here is how to stop that.
Understand what the question is really asking
Every QA question is one of three types:
- Given the test, predict the observation. You are told a reagent is added to a named solution. Read off the expected observation.
- Given the observations, identify the ion. You are handed a table of results and must work backwards.
- Design the test. You must state the reagent, the conditions, the observation and the conclusion yourself.
Type 3 is where most marks are lost, because a vague answer like "test with silver nitrate" is incomplete. Full marks need reagent, observation and inference.
Cation tests: aqueous sodium hydroxide and aqueous ammonia
Almost all cation identification comes from adding these two reagents, first a few drops, then in excess. Learn the pattern of behaviour rather than isolated facts.
Coloured precipitates give you the answer immediately
- Cu²⁺: light blue precipitate. Insoluble in excess NaOH, but dissolves in excess aqueous ammonia to give a dark blue solution.
- Fe²⁺: green precipitate, insoluble in excess of both reagents.
- Fe³⁺: red-brown precipitate, insoluble in excess of both reagents.
- Cr³⁺: green precipitate, soluble in excess NaOH to give a green solution, insoluble in excess ammonia.
Note the trap: Fe²⁺ and Cr³⁺ both give green precipitates. The behaviour in excess NaOH separates them.
White precipitates need the excess test
Zn²⁺, Al³⁺, Pb²⁺ and Ca²⁺ all give white precipitates with NaOH. Work through them systematically:
- Ca²⁺: insoluble in excess NaOH, and gives no precipitate (or only a faint one) with aqueous ammonia.
- Zn²⁺: soluble in excess NaOH and soluble in excess aqueous ammonia, both giving colourless solutions.
- Al³⁺ and Pb²⁺: soluble in excess NaOH, but insoluble in excess aqueous ammonia.
To separate Al³⁺ from Pb²⁺, add aqueous potassium iodide. A yellow precipitate of lead(II) iodide confirms Pb²⁺. Aluminium gives nothing. Adding dilute hydrochloric acid also works, giving a white precipitate of lead(II) chloride.
NH₄⁺ is the odd one out. Add aqueous sodium hydroxide and warm, then test the gas: ammonia turns damp red litmus paper blue. Forgetting the word "warm" costs a mark.
Anion tests: acidify first, then add the reagent
The biggest anion error in Paper 3 is skipping the acidification step.
- Carbonate: add dilute acid. Effervescence occurs and the gas turns limewater milky.
- Chloride (aqueous): acidify with dilute nitric acid, then add aqueous silver nitrate. White precipitate.
- Iodide (aqueous): acidify with dilute nitric acid, then add aqueous lead(II) nitrate. Yellow precipitate.
- Sulfate (aqueous): acidify with dilute nitric acid, then add aqueous barium nitrate. White precipitate.
- Sulfite: add dilute sulfuric acid and warm. Sulfur dioxide is released and decolourises acidified aqueous potassium manganate(VII) from purple to colourless.
- Nitrate: add aqueous sodium hydroxide and aluminium foil, then warm carefully. Ammonia is produced.
Why acidify? Because any carbonate present would also give a white precipitate with silver nitrate or barium nitrate. The acid removes carbonate as carbon dioxide first, so the white precipitate you see is genuinely due to chloride or sulfate.
Gas tests worth writing out perfectly
- Ammonia: turns damp red litmus paper blue.
- Carbon dioxide: gives a white precipitate with limewater.
- Chlorine: bleaches damp litmus paper.
- Hydrogen: burns with a "pop" when a lighted splint is applied.
- Oxygen: relights a glowing splint.
- Sulfur dioxide: turns acidified aqueous potassium manganate(VII) from purple to colourless.
Mark-scheme phrasing that earns the mark
Examiners reward precision. Use these habits:
- Write "soluble in excess", not "disappears" or "melts".
- Write "colourless", never "clear". A blue copper solution is clear but not colourless.
- Write "effervescence" or "bubbles of gas evolved", not "it fizzes a bit".
- Always name the colour of the precipitate: "red-brown", not "orange-ish".
- Separate observation from inference. "White precipitate soluble in excess sodium hydroxide, insoluble in excess ammonia, therefore Al³⁺ or Pb²⁺ present."
- If the evidence narrows it to two ions, say both, then state the confirmatory test.
A quick worked reasoning example
A colourless solution gives a white precipitate with a few drops of NaOH which dissolves in excess. With aqueous ammonia, a white precipitate forms and stays. Adding dilute nitric acid then aqueous barium nitrate gives a white precipitate.
Reasoning: colourless rules out Cu²⁺, Fe²⁺, Fe³⁺ and Cr³⁺. Soluble in excess NaOH but insoluble in excess ammonia gives Al³⁺ or Pb²⁺. The barium nitrate result indicates sulfate. Since lead(II) sulfate is insoluble, the solution cannot contain Pb²⁺, so it is aluminium sulfate. That kind of cross-check is exactly what the top band rewards.
How to practise
Build your own blank QA table from memory each week, then check it against the printed notes. Next, take past-year Paper 3 questions and write out full answers in the reagent, observation, inference format. Speaking your reasoning aloud is the fastest way to spot gaps.
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