IGCSE Chemistry Tests for Ions and Gases
Every chemical test has a fixed reagent, observation and conclusion. Writing an answer in those three parts collects every available mark, and this material is the cheapest set of marks in the whole syllabus.
These tests appear on the theory papers and on the practical paper, and they are pure recall. Ten minutes spent memorising the tables below is worth more per minute than almost anything else in Chemistry revision. Practise applying them on the experimental techniques topic page.
Tests for gases
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen | Hold a lighted splint at the mouth of the tube | A squeaky pop |
| Oxygen | Insert a glowing splint | The splint relights |
| Carbon dioxide | Bubble through limewater | Limewater turns milky |
| Ammonia | Hold damp red litmus paper in the gas | Litmus turns blue |
| Chlorine | Hold damp litmus paper in the gas | Litmus is bleached white |
| Sulfur dioxide | Add acidified potassium manganate(VII) | Purple solution turns colourless |
Tests for cations
Add the reagent a little at a time, then continue adding to excess and record whether the precipitate dissolves.
| Ion | With sodium hydroxide | With aqueous ammonia |
|---|---|---|
| Ammonium | No precipitate. On warming, ammonia gas turns damp red litmus blue | No reaction |
| Calcium | White precipitate, insoluble in excess | No precipitate, or a very slight white one |
| Copper(II) | Light blue precipitate, insoluble in excess | Light blue precipitate, dissolving in excess to a dark blue solution |
| Iron(II) | Green precipitate, insoluble in excess, turning brown at the surface | Green precipitate, insoluble in excess |
| Iron(III) | Red brown precipitate, insoluble in excess | Red brown precipitate, insoluble in excess |
| Zinc | White precipitate, dissolving in excess to a colourless solution | White precipitate, dissolving in excess to a colourless solution |
Tests for anions and flame tests
| Test for | Method | Positive result |
|---|---|---|
| Carbonate | Add dilute acid | Effervescence, and the gas turns limewater milky |
| Chloride | Add dilute nitric acid, then silver nitrate solution | White precipitate |
| Bromide | Add dilute nitric acid, then silver nitrate solution | Cream precipitate |
| Iodide | Add dilute nitric acid, then silver nitrate solution | Yellow precipitate |
| Sulfate | Add dilute nitric acid, then barium nitrate solution | White precipitate |
| Nitrate | Add sodium hydroxide and aluminium foil, then warm | Ammonia produced, turning damp red litmus blue |
| Water present | Add anhydrous copper(II) sulfate | White solid turns blue |
| Lithium flame test | Hold the sample in a hot flame on a clean wire | Red flame |
| Sodium flame test | As above | Yellow flame |
| Potassium flame test | As above | Lilac flame |
| Copper(II) flame test | As above | Blue green flame |
How to use this sheet
- Always acidify with dilute nitric acid before adding silver nitrate or barium nitrate. Carbonate ions would otherwise give a false white precipitate.
- Structure every answer as reagent, observation, conclusion. Three parts, and often three marks.
- Note which precipitates dissolve in excess. Zinc dissolves in both reagents, copper only in ammonia, and that distinction is the whole question.
- Say bleached for chlorine and turns blue for ammonia. Both use damp litmus, so the result is what separates them.
- For flame tests, the wire must be cleaned with acid between samples or the previous colour carries over.
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Frequently asked questions
Why must I add dilute nitric acid before testing for chloride or sulfate?
Carbonate ions present in the sample would also form a white precipitate with silver nitrate or barium nitrate, giving a false positive. Adding dilute nitric acid first removes any carbonate, so any precipitate that then forms must come from the ion being tested for.
How do I tell zinc and calcium apart?
Both give a white precipitate with sodium hydroxide, but zinc hydroxide dissolves in excess to give a colourless solution while calcium hydroxide does not. Continuing to add the reagent to excess is therefore essential rather than optional.
What is the difference between the two splint tests?
A lighted splint gives a squeaky pop with hydrogen. A glowing splint relights in oxygen. Using the wrong splint gives no useful result, so state which one you are using as part of the method.
How much detail do I need in a test answer?
Name the reagent precisely, describe what is observed including any colour change from and to, then state the conclusion. Vague results such as it changes colour or a precipitate forms score little, because the specific observation is what identifies the ion.
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Other reference sheets
Written to the published Cambridge IGCSE syllabuses. Always check the formula list and data sheet issued with your own paper, since the material provided differs between subjects and syllabus versions. Last reviewed 2026-08-12.