What are we testing for?
A salt such as copper(II) sulfate is made of two parts: a positive metal ion (Cu²⁺, the cation) and a negative ion (SO₄²⁻, the anion). To name an unknown salt, we test for each part separately.
Many tests make a precipitate: an insoluble solid that suddenly appears when two solutions are mixed. Its colour is the clue.
Always use small amounts, clean equipment and eye protection. Some solutions (silver nitrate, barium chloride, sodium hydroxide) are harmful or corrosive.
Flame tests for metal ions
Dip a clean nichrome or platinum wire loop in dilute hydrochloric acid, then in the solid sample. Hold it in the hot, blue part of a Bunsen flame and look at the colour.
| Ion | Flame colour |
|---|---|
| Lithium, Li⁺ | crimson (deep red) |
| Sodium, Na⁺ | yellow |
| Potassium, K⁺ | lilac |
| Calcium, Ca²⁺ | orange-red |
| Copper, Cu²⁺ | green (blue-green) |
Why colours? Heat lifts electrons to higher energy levels. When they fall back, they give out light of fixed colours for that element.
Limits: in a mixture one colour can hide another (sodium's strong yellow hides lilac potassium), and a dirty loop gives false results.
Metal hydroxide precipitates with sodium hydroxide
Add a few drops of sodium hydroxide solution to a solution of the salt. Many metal ions form an insoluble hydroxide:
- Cu²⁺ → blue precipitate: Cu²⁺ + 2OH⁻ → Cu(OH)₂
- Fe²⁺ → green precipitate, Fe(OH)₂
- Fe³⁺ → brown (orange-brown) precipitate, Fe(OH)₃
- Al³⁺, Ca²⁺, Mg²⁺ → white precipitates
Telling the white ones apart: keep adding sodium hydroxide (excess). Aluminium hydroxide dissolves into a colourless solution; calcium and magnesium hydroxides stay. Then a flame test separates them: calcium gives orange-red, magnesium no colour.
Some courses also test ammonium (NH₄⁺): warm with sodium hydroxide; ammonia gas turns damp red litmus blue. Zinc (Zn²⁺) gives white, soluble in excess.
Tests for carbonates, halides and sulfates
Carbonate, CO₃²⁻
Add dilute acid. Bubbles (effervescence) of carbon dioxide appear. Bubble the gas through limewater: it turns milky. CO₃²⁻ + 2H⁺ → CO₂ + H₂O.
Halides: chloride, bromide, iodide
Add dilute nitric acid, then silver nitrate solution. Chloride gives a white precipitate (AgCl), bromide cream (AgBr), iodide yellow (AgI). Ag⁺ + Cl⁻ → AgCl.
Sulfate, SO₄²⁻
Add dilute hydrochloric acid, then barium chloride solution. A white precipitate of barium sulfate forms. Ba²⁺ + SO₄²⁻ → BaSO₄.
Why add acid first?
The acid removes carbonate ions. Carbonates would also give a precipitate with silver or barium ions and fool you. Use nitric acid (not hydrochloric) for halides, because hydrochloric acid itself adds chloride ions.
Instrumental methods and flame emission spectroscopy
Instrumental methods use machines instead of test tubes. Compared with chemical tests they are accurate, sensitive (find tiny amounts) and rapid.
In flame emission spectroscopy, a sample is put into a flame and the light it gives out goes through a spectroscope. The light is split into a line spectrum: bright lines at fixed wavelengths. Every metal ion has its own pattern, like a barcode.
- Compare the lines with reference spectra to identify the metal ions, even in a mixture.
- The brightness of the lines shows the concentration, using a calibration graph made from known solutions.
Try it: in the last 3D step pick Na⁺ and then K⁺. The flame colours look different, and the lines sit in different places.
Key formulas and definitions
- Flame: Li⁺ crimson, Na⁺ yellow, K⁺ lilac, Ca²⁺ orange-red, Cu²⁺ green
- NaOH: Cu²⁺ blue, Fe²⁺ green, Fe³⁺ brown, Al³⁺/Ca²⁺/Mg²⁺ white (Al(OH)₃ dissolves in excess)
- Carbonate: acid → CO₂ fizz → limewater milky
- Halide: HNO₃ + AgNO₃ → Cl⁻ white, Br⁻ cream, I⁻ yellow
- Sulfate: HCl + BaCl₂ → white BaSO₄
- Ag⁺ + Cl⁻ → AgCl; Ba²⁺ + SO₄²⁻ → BaSO₄; Cu²⁺ + 2OH⁻ → Cu(OH)₂
Worked examples
1. A white solid gives a lilac flame. Its solution with nitric acid and silver nitrate gives a cream precipitate. Name the solid.
Lilac flame → K⁺. Cream silver halide → Br⁻. The solid is potassium bromide, KBr.
2. A green solution gives a green precipitate with sodium hydroxide. After some time in air the precipitate turns brown. Explain.
Green Fe(OH)₂ shows Fe²⁺. Air oxidises iron(II) to iron(III), so the precipitate slowly becomes brown Fe(OH)₃.
3. Two white salts both give a white precipitate with a little sodium hydroxide. With excess, salt A's precipitate dissolves, B's does not. B gives an orange-red flame. Identify the metal ions.
A dissolves in excess: Al³⁺. B stays and gives an orange-red flame: Ca²⁺.
4. A solution fizzes with dilute hydrochloric acid. After the fizzing stops, barium chloride gives a white precipitate. Which ions are present?
Fizzing (CO₂) → carbonate. The white precipitate after acid → sulfate. Both carbonate and sulfate ions are present.
5. Why can a flame test not show both sodium and potassium in a mixture, while flame emission spectroscopy can?
Sodium's bright yellow hides the faint lilac of potassium to the eye. A spectroscope splits the light, so the separate lines of each metal are all seen.
Common mistakes
- Adding hydrochloric acid before silver nitrate in the halide test. It adds chloride ions and always gives a white precipitate. Use nitric acid.
- Forgetting the acid before barium chloride. Carbonate would also give a white precipitate and be mistaken for sulfate.
- Calling all white hydroxides the same. Only aluminium hydroxide dissolves in excess sodium hydroxide; use a flame test for calcium.
- Mixing up iron ions: iron(II) gives green, iron(III) gives brown.