CARA'S TIPSPractical 4: qualitative analysis of cations
WAECNECO
What the syllabus asks
WAEC: identification of cations Ca²⁺, Zn²⁺, Al³⁺, Pb²⁺, Cu²⁺, Fe²⁺, Fe³⁺, NH₄⁺ (and Na⁺, K⁺ by flame); preliminary tests; observation–inference tables; ionic equations.
NECO: identification of cations Ca²⁺, Zn²⁺, Al³⁺, Pb²⁺, Cu²⁺, Fe²⁺, Fe³⁺, NH₄⁺ (and Na⁺, K⁺ by flame); preliminary tests; observation–inference tables; ionic equations.
What you will learn
- Lay out an observation–inference table.
- Carry out and interpret preliminary tests.
- Identify cations with NaOH(aq) and NH₃(aq).
- Distinguish Al³⁺ from Pb²⁺.
- Test for NH₄⁺.
- Use flame tests.
- Write ionic equations for cation tests.
- Infer the identity of a salt from a sequence of tests.
Key points
- Dropwise, then excess; fresh portions.
- Ca²⁺: no ppt with NH₃.
- Zn²⁺: soluble in excess of both.
- Al³⁺/Pb²⁺: KI decides (yellow PbI₂).
- Cu²⁺: deep blue with excess NH₃.
- Fe²⁺ green→brown; Fe³⁺ reddish-brown.
- NH₄⁺: warm with NaOH; damp red litmus blue.
Start here: How Question 2 works
Question 2 gives you a substance (usually labelled C, D or E) and a list of tests to carry out in order. You record what you see (observation) and what it means (inference) in a three-column table. The rubric says credit is given “for observations precisely recorded and for accurate inferences”, and that entries must be made “in ink, at the time they are made”.
The three columns
| Test | Observation | Inference |
|---|---|---|
| C + distilled water, shake; filter | C partly dissolves; colourless filtrate; white residue | C is a mixture of a soluble and an insoluble salt |
| Filtrate + NaOH(aq) in drops, then in excess | white precipitate, soluble in excess | Zn²⁺, Al³⁺ or Pb²⁺ present |
- Test: copy what the paper says you did, briefly.
- Observation: colours, precipitates, gases (colour, smell, effect on test paper), solubility in excess. Use exact words: “white precipitate”, not “cloudy”; “effervescence”, not “bubbles came out” (both accepted, but precise words are safer); “colourless, odourless gas”.
- Inference: the ion or gas indicated. When a test only narrows the possibilities, list them all (“Zn²⁺, Al³⁺ or Pb²⁺ present”). When a later test decides, write the single ion.
Marks
A typical scheme gives one mark per correct observation and one per correct inference, so a test with “white precipitate (1), soluble in excess (1); Zn²⁺, Al³⁺ or Pb²⁺ (1)” is worth three. In the 2024 Nigerian paper the salt-analysis question was worth 15 marks. Writing an inference that the observation does not support (“Zn²⁺ present” after only one test) loses the mark; writing all possibilities earns it.
Golden rules
- Do the tests in the order given, on fresh portions as instructed.
- Add reagents dropwise first, then in excess, shaking after each addition.
- Test every gas, even when you think you know it.
- Record negative results too: “no visible reaction”, “no precipitate”. They are observations and often carry marks (for example, no ppt with NH₃ for Ca²⁺).
Words examiners accept, and words they do not
| Write | Avoid |
|---|---|
| white precipitate | white solid, cloudy, milky (except for lime water) |
| precipitate dissolves in excess | precipitate disappears, goes away |
| effervescence; colourless gas evolved | bubbles, fizz only |
| gas turns damp red litmus blue | gas is alkaline (that is an inference) |
| pungent / choking smell | bad smell |
| deep blue solution | blue colour |
| no visible reaction | nothing |
| solution turns from orange to green | it changed colour |
Notice the difference between an observation and an inference. “Gas turns lime water milky” is an observation; “CO₂ evolved” is an inference. Putting an inference in the observation column usually earns nothing in that column. Keep the columns honest.
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Practice questions
- Cu²⁺
- Fe³⁺
- Zn²⁺
- Ca²⁺
Show answer
A: Cu²⁺
Cu(OH)₂ is blue.
- Zn²⁺
- Al³⁺
- Fe²⁺
- Cu²⁺
Show answer
D: Cu²⁺
Tetraamminecopper(II) is deep blue.
- Pb²⁺
- Ca²⁺
- Zn²⁺
- Al³⁺
Show answer
C: Zn²⁺
Only zinc dissolves in excess ammonia.
- NH₄⁺
- Al³⁺ or Pb²⁺
- Zn²⁺
- Ca²⁺
Show answer
B: Al³⁺ or Pb²⁺
Both are amphoteric but do not dissolve in ammonia.
- a yellow precipitate with KI(aq)
- a brick-red flame
- a blue precipitate with NaOH
- effervescence with acid
Show answer
A: a yellow precipitate with KI(aq)
PbI₂ is yellow.
Key words
- Observation
- What is seen, heard or smelt during a test.
- Inference
- Conclusion drawn from an observation.
- Precipitate (ppt)
- Insoluble solid formed in a solution.
- Amphoteric
- Reacting with both acids and alkalis (Zn, Al, Pb hydroxides).
- Complex ion
- Ion of a metal bonded to molecules such as NH₃, e.g. [Cu(NH₃)₄]²⁺.
- Decrepitation
- Crackling of crystals on heating (e.g. Pb(NO₃)₂).
- Sublimate
- Solid deposited directly from a vapour.
- Effervescence
- Rapid evolution of gas bubbles from a liquid.
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