Science required practicals
How do you do the identifying ions required practical?
You run chemical tests on an unknown ionic compound to find its positive and negative ions. A flame test and sodium hydroxide solution identify the metal ion from the flame colour or precipitate colour. Dilute acid and limewater show a carbonate, acidified silver nitrate shows a halide (white chloride, cream bromide, yellow iodide), and acidified barium chloride gives a white precipitate with sulfate. It is on separate Chemistry only.
Last updated 23 September 2026 · Written and fact-checked by the GCSEwiz team
What does the identifying ions practical investigate?
How to identify the ions in an unknown ionic compound using chemical tests. You find the positive ion (the metal) with a flame test and sodium hydroxide solution, and the negative ion with the tests for carbonate, halide and sulfate ions. The practical uses one compound at a time, because in a mixture one flame colour can hide another.
It is on separate (triple) Chemistry only. If you take Combined Science: Trilogy, you do not need this practical or the ion tests in it. You do still need the gas tests for hydrogen, oxygen, carbon dioxide and chlorine, which are on both courses.
What equipment do you need?
For the flame tests you need a Bunsen burner and a nichrome wire loop. The other tests happen in test tubes, using these solutions: sodium hydroxide, dilute hydrochloric and nitric acids, silver nitrate, barium chloride and limewater.
- A Bunsen burner and a heatproof mat
- A nichrome wire loop in a handle
- Test tubes, a rack and dropping pipettes
- Dilute sodium hydroxide solution
- Dilute hydrochloric acid and dilute nitric acid
- Silver nitrate solution and barium chloride solution
- Limewater
- Distilled water, to dissolve any solid samples
- Eye protection
What is the method?
Test for the metal ion first, with a flame test and then sodium hydroxide, then test for each negative ion in turn. Use a fresh portion of the unknown for every test, so one reagent does not interfere with the next.
- Flame test: clean the wire loop (your teacher will show you how) and check it gives no colour in a blue Bunsen flame. Dip it in the sample and hold it in the flame. Lithium gives a crimson flame, sodium yellow, potassium lilac, calcium orange-red and copper green.
- Hydroxide test: add a few drops of dilute sodium hydroxide solution to a solution of the compound. Copper(II) ions give a blue precipitate (a solid forming in the solution), iron(II) a green one and iron(III) a brown one.
- If the precipitate is white, keep adding sodium hydroxide until it is in excess. Aluminium hydroxide dissolves; calcium and magnesium hydroxides stay. A flame test tells those two apart: calcium gives orange-red and magnesium gives no flame colour.
- Carbonate test: add dilute hydrochloric acid. If it fizzes, bubble the gas through limewater (or draw it up with a dropping pipette and bubble it in). Limewater turning milky shows carbon dioxide, so the compound is a carbonate.
- Halide test: add a few drops of dilute nitric acid, then silver nitrate solution. A white precipitate means chloride, cream means bromide and yellow means iodide.
- Sulfate test: add a few drops of dilute hydrochloric acid, then barium chloride solution. A white precipitate means sulfate.
What are the variables?
This is a set of identification tests, so nothing is varied on purpose. What you control is cleanliness and the order you add things, because a stray ion or a missing acid can give a misleading result.
| Variable | In this practical |
|---|---|
| What you test | One unknown ionic compound at a time, compared with known compounds |
| What you observe | Flame colour, precipitate colour, whether a white precipitate dissolves in excess, and whether a gas turns limewater milky |
| Controlled: clean equipment | A clean wire loop and clean test tubes, so no ions carry over from the last sample |
| Controlled: acid added first | Nitric acid before silver nitrate and hydrochloric acid before barium chloride, to remove carbonate ions |
| Controlled: the flame | A blue Bunsen flame, which has no strong colour of its own to hide the result |
How do you work out and present the results?
There is nothing to calculate. Record each test and what you saw in a table, then put one positive ion together with one negative ion to name the compound.
Worked example: a pale green solution gives a green precipitate with sodium hydroxide, so it contains iron(II) ions. A fresh sample gives a white precipitate with dilute hydrochloric acid and barium chloride, so it contains sulfate ions. The compound is iron(II) sulfate.
Use this table to go from what you see to the ion it shows.
| What you see | Ion present |
|---|---|
| Crimson flame | Lithium, Li⁺ |
| Yellow flame | Sodium, Na⁺ |
| Lilac flame | Potassium, K⁺ |
| Orange-red flame | Calcium, Ca²⁺ |
| Green flame | Copper, Cu²⁺ |
| Blue precipitate with sodium hydroxide | Copper(II), Cu²⁺ |
| Green precipitate with sodium hydroxide | Iron(II), Fe²⁺ |
| Brown precipitate with sodium hydroxide | Iron(III), Fe³⁺ |
| White precipitate that dissolves in excess sodium hydroxide | Aluminium, Al³⁺ |
| White precipitate that stays in excess sodium hydroxide | Calcium, Ca²⁺, or magnesium, Mg²⁺ |
| Fizzing with dilute acid, and the gas turns limewater milky | Carbonate, CO₃²⁻ |
| White precipitate with nitric acid and silver nitrate | Chloride, Cl⁻ |
| Cream precipitate with nitric acid and silver nitrate | Bromide, Br⁻ |
| Yellow precipitate with nitric acid and silver nitrate | Iodide, I⁻ |
| White precipitate with hydrochloric acid and barium chloride | Sulfate, SO₄²⁻ |
How do you make it accurate and safe?
Clean the wire loop before every flame test. Sodium's yellow is so bright that a trace left on the wire can hide another colour. Dissolve solids in distilled water, because tap water contains dissolved ions of its own.
Add the acid before silver nitrate or barium chloride. It removes carbonate ions, which would also form a precipitate and confuse the result. The choice of acid matters as well. Hydrochloric acid would add chloride ions before a halide test, and sulfuric acid would add sulfate ions before a sulfate test, so use nitric acid for halides and hydrochloric acid for sulfates.
Add sodium hydroxide a few drops at a time, so you see the precipitate form before deciding whether to add excess. The white, cream and yellow halide precipitates can look alike, so compare them side by side.
For safety, wear eye protection: sodium hydroxide solution can damage eyes. Barium chloride is harmful if swallowed and silver nitrate stains skin, so wash your hands afterwards. The wire loop stays hot after a flame test, so rest it on the heatproof mat.
What do exam questions ask about it?
Many questions give you observations and ask you to name the ions or the compound. Others ask for the result a named ion gives, or why an acid goes in first and why it has to be that particular acid. The harder ones combine tests from both halves, like this:
"A white solid gives a crimson flame. A solution of the solid, acidified with dilute nitric acid, gives a cream precipitate with silver nitrate solution. Name the solid. (2 marks)"
The crimson flame means lithium and the cream precipitate means bromide, so the solid is lithium bromide.
Common mistakes
Two of these are about which acid goes in first, so learn the reason for each acid, not just the colours.
- Adding hydrochloric acid before silver nitrate. It adds chloride ions, so you get a white precipitate whatever the compound. Use dilute nitric acid.
- Adding sulfuric acid before barium chloride. It adds sulfate ions, so every sample looks like a sulfate. Use dilute hydrochloric acid.
- Writing "red" for lithium or calcium. Lithium is crimson and calcium is orange-red, and "red" on its own does not tell them apart.
- Stopping at a few drops of sodium hydroxide when the precipitate is white. Add excess: it is the only way to pick out aluminium.
- Using a dirty wire loop. Sodium left on it from an earlier test gives a strong yellow that hides other colours.