Qualitative analysis means finding out which substances are present in a sample (as opposed to quantitative analysis, which finds out how much). In CSEC Chemistry this comes down to a set of standard tests for gases and for the ions in solution. Marks are easy to gain here if you learn the test, the observation and the conclusion for each — examiners want all three.
Tests for gases
| Gas | Test | Positive result |
|---|---|---|
| Oxygen | insert a glowing splint | the splint relights |
| Hydrogen | insert a lighted splint | a squeaky "pop" |
| Carbon dioxide | bubble through limewater | limewater turns milky/cloudy |
| Ammonia | hold damp red litmus at the mouth of the tube | litmus turns blue (and pungent smell) |
| Chlorine | hold damp blue litmus | it is bleached (turns white) |
Tip: oxygen relights a glowing splint; hydrogen pops a lighted splint — keep those two apart.
Tests for cations (positive ions)
Two approaches are used: adding sodium hydroxide solution, and adding ammonia solution, then noting the colour of any precipitate and whether it dissolves in excess.
| Cation | With sodium hydroxide (NaOH) | Note |
|---|---|---|
| Calcium, Ca²⁺ | white precipitate, insoluble in excess | flame test: brick-red |
| Aluminium, Al³⁺ | white precipitate, dissolves in excess | |
| Zinc, Zn²⁺ | white precipitate, dissolves in excess | |
| Iron(II), Fe²⁺ | green precipitate | turns brown on standing |
| Iron(III), Fe³⁺ | red-brown precipitate | |
| Copper(II), Cu²⁺ | blue precipitate | |
| Ammonium, NH₄⁺ | no precipitate, but ammonia gas released on warming | turns damp red litmus blue |
The white precipitates (Ca²⁺, Al³⁺, Zn²⁺) are told apart by what happens in excess NaOH: calcium stays insoluble, while aluminium and zinc dissolve. Ammonia solution can then separate aluminium from zinc (zinc hydroxide dissolves in excess ammonia, aluminium does not).
Flame tests also help identify some metal ions: sodium gives a yellow flame, potassium lilac, calcium brick-red, copper blue-green.
Tests for anions (negative ions)
| Anion | Test | Positive result |
|---|---|---|
| Carbonate, CO₃²⁻ | add dilute acid | fizzes, releasing CO₂ (turns limewater milky) |
| Chloride, Cl⁻ | add dilute nitric acid, then silver nitrate | white precipitate |
| Sulfate, SO₄²⁻ | add dilute hydrochloric acid, then barium chloride | white precipitate |
| Nitrate, NO₃⁻ | add NaOH and aluminium foil, warm | ammonia gas released (damp red litmus → blue) |
For chloride and sulfate, the acid is added first to remove carbonate ions that would otherwise give a misleading precipitate — a detail worth a mark.
Testing for water and for purity
- Water turns white anhydrous copper(II) sulfate blue, and turns blue cobalt chloride paper pink. (These show something is water, not that it is pure.)
- Pure water boils at exactly 100 °C and freezes at 0 °C at normal pressure; impurities raise the boiling point and lower the freezing point.
Instrumental methods
Modern laboratories also use instrumental methods (such as chromatography and spectroscopy). Their advantages over simple chemical tests are that they are faster, more accurate/sensitive, and work on very small samples.
Why the precipitates form
The cation tests work because metal hydroxides are insoluble, so when you add sodium hydroxide (a source of OH⁻ ions) the metal ion and hydroxide ion combine to form a coloured, insoluble precipitate. For example:
Cu²⁺ + 2OH⁻ → Cu(OH)₂ (blue precipitate) Fe³⁺ + 3OH⁻ → Fe(OH)₃ (red-brown precipitate)
The colours come from the metal ion itself (transition-metal ions are coloured, which is why iron and copper give colours while calcium, aluminium and zinc give white). The "soluble in excess" behaviour of aluminium and zinc hydroxides happens because they are amphoteric — they react with extra alkali to form a soluble compound — whereas calcium hydroxide is not, so it stays as a white solid. Understanding why a precipitate forms, rather than just memorising the colour, makes the results far easier to recall.
Working through an unknown salt
A typical practical question gives you an unknown salt and asks you to identify both ions. A sensible routine is:
- Note the colour of the solid and solution — a blue solution hints at copper, a green one at iron(II).
- Test for the cation by adding sodium hydroxide and watching the precipitate colour and its behaviour in excess; confirm a metal with a flame test where useful.
- Test for the anion — add dilute acid (fizzing → carbonate), then the acidified silver nitrate test for a halide or the acidified barium chloride test for a sulfate.
- Combine the results: e.g. a blue precipitate with NaOH plus a white precipitate with acidified barium chloride means copper(II) sulfate.
Setting out your reasoning step by step, with the observation that led to each conclusion, is exactly what examiners reward in these structured questions.
Common exam mistakes
- Giving only the test or only the result — always write test → observation → conclusion.
- Confusing the splint tests (glowing vs lighted).
- Forgetting to add acid before silver nitrate or barium chloride, giving a false positive from carbonate.
- Not using the "in excess" step to tell apart the white hydroxide precipitates.
Quantitative versus qualitative analysis
Although this topic is about qualitative analysis (what is present), it is worth knowing how it differs from quantitative analysis (how much is present). Qualitative tests give a yes/no or which-ion answer from a colour change, precipitate or gas. Quantitative methods, such as titration, give a numerical amount — for example, finding the exact concentration of an acid by reacting a measured volume with an alkali of known concentration until an indicator just changes colour. Both are important: a chemist first identifies the substances qualitatively, then measures their quantities. Being clear about this distinction prevents the common mistake of describing a titration when a question only asks you to identify an ion.
Key terms to remember
- Qualitative analysis — finding out which substances/ions are present.
- Cation — a positive ion (e.g. a metal ion or NH₄⁺); tested with sodium hydroxide.
- Anion — a negative ion (e.g. Cl⁻, SO₄²⁻, CO₃²⁻); tested with specific reagents.
- Precipitate — an insoluble solid formed when two solutions react.
- Flame test — identifying a metal ion by the colour it gives a flame.
- Amphoteric hydroxide — one (e.g. aluminium, zinc) that dissolves in excess alkali.
- Limewater — used to test for carbon dioxide (turns milky).
- Instrumental method — a fast, sensitive technique (e.g. chromatography) used on tiny samples.
Quick recap
- Qualitative analysis identifies what is present; learn each test with its observation and conclusion.
- Gases: glowing splint relights (O₂), squeaky pop (H₂), limewater milky (CO₂), damp litmus for NH₃ and Cl₂.
- Cations: NaOH (and ammonia) give coloured precipitates; use "soluble in excess" and flame tests to distinguish.
- Anions: acid for carbonate, acidified silver nitrate for chloride, acidified barium chloride for sulfate.
- Anhydrous copper sulfate / cobalt chloride paper detect water; instrumental methods are faster and more sensitive.