Tests for Cations, Anions and Gases
Pure recall, and worth a great many marks across the papers. The requirement is exact: the right reagent, the right observation, and the right conclusion.
1. Flame tests (cations)
Method: dip a clean nichrome or platinum wire in concentrated hydrochloric acid, then in the solid, and hold it in a blue Bunsen flame.
| Ion | Flame colour |
|---|---|
| Lithium, Li⁺ | red |
| Sodium, Na⁺ | yellow |
| Potassium, K⁺ | lilac |
| Calcium, Ca²⁺ | orange-red |
| Copper(II), Cu²⁺ | blue-green |
Sodium gives a YELLOW flame — not orange. Confusing the two was recorded, and it matters because calcium is the orange-red one.
Not every metal gives a flame colour. Aluminium, zinc and iron do not — so a colourless flame is itself information. Uncertainty about which cations give a positive flame test was recorded.
A flame test identifies METAL ions, not gases. A recorded error applied a flame test to hydrogen — that is the squeaky pop test.
2. Cation tests with sodium hydroxide
Add aqueous sodium hydroxide dropwise, then in excess. The precipitate colour and its behaviour in excess identify the ion.
| Ion | With a little NaOH | In excess |
|---|---|---|
| Ca²⁺ | white precipitate | insoluble — stays |
| Al³⁺ | white precipitate | DISSOLVES — colourless solution |
| Zn²⁺ | white precipitate | DISSOLVES — colourless solution |
| Cu²⁺ | light blue precipitate | insoluble |
| Fe²⁺ | green precipitate | insoluble |
| Fe³⁺ | red-brown precipitate | insoluble |
| NH₄⁺ | no precipitate — ammonia gas on warming | — |
The three white precipitates — Ca²⁺, Al³⁺, Zn²⁺ — are told apart by ADDING EXCESS. Only aluminium and zinc dissolve. This is the whole point of the test, and uncertainty between zinc and aluminium was recorded.
Iron(II) is GREEN; iron(III) is RED-BROWN. Misidentifying the ion giving a green precipitate was recorded.
Distinguishing Al³⁺ from Zn²⁺: use aqueous ammonia instead. With excess ammonia, the zinc precipitate dissolves but the aluminium one does not.
Ammonium test: add NaOH and warm. Ammonia gas is released — turning damp red litmus BLUE.
The gas released is AMMONIA, not hydrogen. Recorded as an error.
3. Anion tests
| Ion | Test | Positive result |
|---|---|---|
| Carbonate, CO₃²⁻ | add dilute acid | effervescence; gas turns limewater milky |
| Sulfate, SO₄²⁻ | add dilute nitric acid, then barium nitrate | WHITE precipitate |
| Chloride, Cl⁻ | add dilute nitric acid, then silver nitrate | WHITE precipitate |
| Bromide, Br⁻ | same as chloride | CREAM precipitate |
| Iodide, I⁻ | same as chloride | YELLOW precipitate |
| Nitrate, NO₃⁻ | add NaOH and aluminium foil, then warm | ammonia gas — damp red litmus turns blue |
Sulfate uses BARIUM nitrate; halides use SILVER nitrate. Confusing barium nitrate with silver nitrate was recorded, as was stating the wrong reagent for chloride.
Both give a WHITE precipitate — so the reagent is what distinguishes them. Read the question carefully.
The halide precipitates go white → cream → yellow as you go down the group (chloride, bromide, iodide). Learn them in that order.
Add dilute acid FIRST in the sulfate and halide tests, to remove carbonate ions that would otherwise give a false white precipitate.
The nitrate test uses NaOH and aluminium, not iron sulfate. A recorded error reached for iron(II) sulfate — that belongs to a different (brown ring) test not required here.
Sulfate (SO₄²⁻) and sulfite (SO₃²⁻) are different ions, as are nitrate and nitrite. Confusion between them was recorded twice.
4. Gas tests
| Gas | Test | Result |
|---|---|---|
| Hydrogen, H₂ | lighted splint | squeaky pop |
| Oxygen, O₂ | glowing splint | relights |
| Carbon dioxide, CO₂ | bubble through limewater | turns milky / cloudy |
| Ammonia, NH₃ | damp red litmus | turns BLUE |
| Chlorine, Cl₂ | damp litmus paper | BLEACHED white |
| Sulfur dioxide, SO₂ | acidified potassium manganate(VII) | purple → colourless |
Hydrogen uses a LIGHTED splint; oxygen uses a GLOWING one. Swapping them is the classic error.
Limewater turns MILKY with carbon dioxide — it does not form a “white precipitate” in the way an ion test does. A recorded error described it that way; the correct observation is milky or cloudy (it is calcium carbonate forming, but the observation asked for is the cloudiness).
Chlorine BLEACHES litmus; ammonia turns red litmus BLUE. Both use damp litmus, so state the colour change precisely.
5. Answering analysis questions
Give the REAGENT, the OBSERVATION and the CONCLUSION — all three are usually separate marks.
“Add dilute nitric acid then aqueous barium nitrate (reagent). A white precipitate forms (observation). This shows sulfate ions are present (conclusion).”
Say “effervescence” or “bubbles”, not “it fizzes”; “white precipitate”, not “it goes cloudy” — unless the test genuinely is a cloudiness (limewater).
Learn the solubility rules alongside these tests — knowing which salts are insoluble explains why the precipitates form. Not remembering the solubility chart was recorded.
6. Mistakes that cost marks
Sodium’s flame given as orange rather than yellow.
Not adding NaOH in excess to separate the white precipitates.
Swapping the iron(II) and iron(III) colours.
Using silver nitrate for sulfate or barium nitrate for chloride.
Omitting the dilute acid before the precipitate tests.
Confusing sulfate with sulfite, or nitrate with nitrite.
Swapping the lighted and glowing splint tests.
Saying CO₂ gives a white precipitate with limewater.
Giving an observation without a conclusion, or vice versa.
Applying a flame test to a gas.
Frequently asked questions
What flame colour does sodium give? Yellow. Potassium is lilac, lithium red, calcium orange-red, copper blue-green.
How do I tell Ca²⁺, Al³⁺ and Zn²⁺ apart? All give white precipitates with NaOH; only Al³⁺ and Zn²⁺ dissolve in excess. Use ammonia to separate those two — only zinc dissolves.
What colour precipitate does Fe³⁺ give? Red-brown. Fe²⁺ gives green.
How do I test for sulfate? Dilute nitric acid, then barium nitrate → white precipitate.
How do I test for chloride? Dilute nitric acid, then silver nitrate → white precipitate.
What colour is the iodide precipitate? Yellow (bromide is cream).
Why add acid first? To remove carbonate ions, which would give a false positive.
How do I test for carbon dioxide? Bubble through limewater — it turns milky.
How do I test for oxygen? A glowing splint relights.
What does chlorine do to damp litmus? Bleaches it white.
Quick revision checklist
- I know all five flame colours
- I know which metals give no flame colour
- I know the NaOH precipitate colours for all six cations
- I know which dissolve in excess
- I can separate Al³⁺ from Zn²⁺ using ammonia
- I know the ammonium test and that the gas is ammonia
- I know the carbonate, sulfate, halide and nitrate tests
- I know barium for sulfate and silver for halides
- I know the white/cream/yellow halide sequence
- I add dilute acid first and know why
- I know all six gas tests
- I give reagent, observation and conclusion
- I use precise observation wording
These notes cover tests for cations, anions and gases in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and the qualitative analysis notes supplied with your own exam papers.
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