For Integrated Programme students: Your current school materials, teacher instructions, and assessment scope take precedence because IP topic sequence and depth vary by school. This is an Eclat IP guide, not the O-Level / SEC G3 exam-track guide.
The core idea is simple: Qualitative analysis is a controlled sequence of tests and observations.
Use it as a working check: Use fresh samples, test gases first, then cations, then anions. Record colour, precipitate formation, and solubility in excess reagent.
Then go one layer deeper: Example: a blue precipitate that dissolves in excess ammonia to form a deep blue solution points to copper ions. Vague wording like "colour changed" loses marks.
How this chapter applies
Eclat practical core: reagent sequence, fresh samples, precipitate colour, solubility in excess, gas tests, contamination control, and observation wording form the main route.
School-sensitive extension: flame tests, extended ion sets, and multi-stage unknown analysis should be used only where the current school teaches or supplies them.
2027 national comparison: K324 Topic C6 names the cation, anion, and gas tests to know, including sulfur dioxide decolourising acidified potassium manganate(VII).
Check your school: use the current practical notes or any data sheet supplied with the assessment. A school's permitted tests and expected table wording can differ.
Gas tests: HX2 gives a pop with a lighted splint, OX2 relights a glowing splint, COX2 turns limewater milky, NHX3 turns damp red litmus blue, ClX2 bleaches damp litmus, and SOX2 decolourises acidified KMnOX4. Carry out only tests authorised by the current laboratory instructions.
Cations with NaOH/NH₃: AlX3+ white ppt dissolves in excess NaOH; ZnX2+ white ppt dissolves in excess NaOH/NH₃; CaX2+
Anions: carbonates effervesce with acid (COX2 test), sulfates give a white precipitate after acidifying with dilute nitric acid and adding aqueous barium nitrate, K324 names chloride and iodide with acidified silver nitrate, and nitrates produce NHX3
Sequence and cleanliness: test gases first, then cation series, then anions. Acidify appropriately to remove interfering carbonates.
Observation-to-test checkpoint
Use the first clear observation to choose the next test. This prevents random reagent hopping and keeps each conclusion tied to evidence.
First clue
Next test
What conclusion needs
Gas forms when acid is added.
Test the gas immediately with limewater, damp litmus, or acidified KMnOX4.
Name the gas and the ion source, such as carbonate producing COX2.
A precipitate forms with NaOH.
Add excess NaOH, then repeat with aqueous ammonia on a fresh sample.
Colour first, then whether the precipitate dissolves in excess reagent.
No obvious cation clue appears.
Move to anion tests on fresh portions after acidifying where required.
State the acidified reagent and the precipitate colour.
Two ions seem possible from the same observation.
Add a confirmatory test rather than guessing.
A second result that separates the lookalikes, such as ZnX2+ dissolving in excess ammonia while AlX3+
Common trap: the observation is not the conclusion. "White precipitate" is only evidence; the ion name comes after the reagent sequence and confirmatory result.
White precipitate cation checkpoint
When several cations give a white precipitate, do not stop at "white ppt". Use excess reagent and a fresh confirmatory test to separate the lookalikes.
First result
Excess NaOH result
Aqueous ammonia result
What to do next
White ppt with NaOH
Dissolves in excess NaOH
Does not dissolve in excess ammonia
Suspect AlX3+ within the K324 comparison table.
White ppt with NaOH
Dissolves in excess NaOH
Dissolves in excess ammonia
Identify ZnX2+. The ammonia result separates zinc from aluminium.
White ppt with NaOH
Insoluble in excess NaOH
Insoluble in excess ammonia
Suspect CaX2+ if the rest of the data fits; check against flame test or other given clues.
Worked example: an unknown gives a white precipitate with sodium hydroxide, and the precipitate dissolves when excess sodium hydroxide is added. That result alone is not enough. If a fresh portion with aqueous ammonia gives a white precipitate that stays insoluble in excess ammonia, zinc is ruled out and aluminium is the K324 comparison-table match.
Gas and anion sequence checkpoint
For anion tests, separate the gas clue from the precipitate clue. Test any gas first, then use a fresh sample for the next anion test so one reagent does not contaminate the next result.
First observation or target
Next reagent or test
Conclusion you can make
Common trap
Effervescence when dilute acid is added
Bubble the gas through limewater
Milky limewater supports COX3X2− because COX2 was produced
Writing "carbonate" from bubbles alone without testing the gas
Choking gas from acidified sample
Test with acidified KMnOX4 only when instructed
Purple solution decolourises if SOX2
Test for sulfate
Acidify with dilute nitric acid, then add aqueous barium nitrate
White precipitate after acidification supports SOX4X2−
Skipping acidification and letting carbonate form a false white precipitate
Test for halide
Acidify with dilute nitric acid, then add AgNOX3
White or yellow precipitate points to chloride or iodide in K324
Using the same sample that already received barium reagent
Test for nitrate
Warm with aqueous NaOH and aluminium foil
Ammonia gas supports NOX3X−
Worked check: an unknown fizzes with dilute acid. The gas turns limewater milky, so the evidence supports carbonate. On a fresh sample, acidifying with dilute nitric acid before adding aqueous barium nitrate prevents leftover carbonate from giving a misleading white precipitate.
Misconception check: a single unknown usually needs several fresh portions. Reusing the same portion after adding silver nitrate or barium nitrate can create precipitates from the reagent you just added, not from the original ion.
Observation wording checkpoint
Write the observation before the inference. In qualitative analysis, the marker should be able to see exactly what you saw, what you added next, and only then which ion the result supports.
Test situation
Observation sentence to write
Inference sentence to write
Common trap
NaOH is added dropwise, then in excess
"A blue precipitate forms and remains insoluble in excess sodium hydroxide."
"This result is consistent with copper(II) ions, but check the aqueous ammonia result too."
Writing only "copper(II) is present" without colour or excess result
Aqueous ammonia is added dropwise, then in excess
"A blue precipitate forms and dissolves in excess aqueous ammonia to give a deep blue solution."
"This confirms copper(II) ions when paired with the NaOH result."
Saying "solution turns blue" and missing the precipitate step
A gas is produced on warming with sodium hydroxide
"A pungent gas is produced and turns damp red litmus paper blue."
"The gas is ammonia, so ammonium ions are present."
Naming ammonium ions before proving the gas is ammonia
Acidified silver nitrate gives a precipitate
"A white precipitate forms after acidifying and adding silver nitrate."
"This supports chloride ions."
Omitting the acidification step or writing only "halide present"
Worked check: for a blue precipitate with sodium hydroxide that stays insoluble in excess, followed by a fresh sample giving a blue precipitate that dissolves in excess aqueous ammonia to a deep blue solution, write both observations first. Only after those two sentences should you conclude copper(II) ions are present.
Misconception check: the ion name is the conclusion, not the observation. "Copper(II) ions observed" is not an observation; "blue precipitate forms" is.
Detailed notes
Order of work: gas tests when appropriate → cation tests (NaOH then NH₃) → anion tests. Always acidify before AgNOX3 or aqueous barium nitrate to avoid carbonate false positives.
Amphoteric vs basic ppt: Al(OH)X3 and Zn(OH)X2 dissolve in excess NaOH; only Zn(OH)X2 dissolves in excess NH₃.
Gas confirmation: ClX2 bleaches damp litmus; NHX3
Nitrate reduction: warm with NaOH + aluminium foil → pungent NHX3 (test with red litmus). Sulfate is checked by acidifying with dilute nitric acid, then adding aqueous barium nitrate.
Cation flow: white ppt with NaOH soluble in excess; white ppt with NH₃ insoluble → AlX3+. If soluble in excess NH₃ too → ZnX2+. Green ppt with NaOH turning brown on standing → FeX2+ oxidising to FeX3+.
Anion flow: effervescence with acid → pass gas through limewater. If limewater turns milky, carbonate is supported. No effervescence? Acidify with dilute nitric acid, then add aqueous barium nitrate for sulfate. Use a separate sample acidified with dilute nitric acid, then add aqueous silver nitrate for chloride or iodide.
Gas test sequencing: warm with NaOH. If a gas turns damp red litmus blue, NHX3 from ammonium ion is supported. If damp litmus is bleached, consider ClX2
Pitfalls and fixes
Not acidifying before AgNOX3 or aqueous barium nitrate → carbonate gives false ppt.
Vague observations: always specify colour and whether ppt dissolves in excess reagent.
Confusing FeX2+ and FeX3+: note initial ppt colour and any change on standing.
Practice drills
Draft a flowchart to identify an unknown containing one cation and one anion; include confirmatory steps.
State observations + equations when adding dilute acid to sodium carbonate.
Design a sequence to distinguish AlX3+ and ZnX2+ in solution.
Explain why acidified KMnOX4 must be acidified (KMnOX4
Quick applications
Identify an unknown: blue ppt with NaOH, dissolves in excess NH₃ to deep blue solution → CuX2+; gas from warming unknown with NaOH turns damp red litmus blue → NHX4X+.
Construct a safe order: check for carbonate (add acid, watch effervescence), test sulfate (acidify with dilute nitric acid then add aqueous barium nitrate), test halide (acidify then AgNOX3), cation tests with NaOH/NH₃ on fresh samples.
Explain acidification before AgNOX3: removes COX3X2−
Exam cues
Observations need colour + solubility in excess (e.g., “white ppt, soluble in excess NaOH to colourless solution”).
Name the reagent and the result for each gas. For SOX2, state that acidified KMnOX4 is decolourised, but perform the test only when authorised by the current practical instructions.
Use fresh samples to avoid contamination; warm gently for NHX4X+ to avoid driving off other volatiles.
