Ion Identification
The chemical tests that reveal which ions are in a substance: flame test colours for metal ions, the coloured precipitates made with sodium hydroxide, the carbonate test, and the halide and sulfate tests, each learned as an ion, a reagent and a result.
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Which ion is it?
Chemists identify the ions in an unknown substance using simple tests, each giving a clear result. A flame test shows some metal ions by the colour they turn a flame. Adding sodium hydroxide makes coloured solid precipitates that reveal other metal ions. A dilute acid shows a carbonate by fizzing off carbon dioxide. Silver nitrate reveals halides and barium chloride reveals sulfates, each as a coloured precipitate. The skill is to learn each test as a matched set: the ion, the reagent added, and the result seen. Instrumental methods are covered in a separate module.
Ion test words
Learn these before you sort any result, and always tie an ion to its reagent and its colour.
Two ways to find a metal ion
Some metal ions are found by a flame test and others by the precipitate they make with sodium hydroxide. Learn which test suits which.
How to record a test
Never give just a colour. Record every test as three linked things: the ion you are looking for, the reagent you add, and the exact result you see, whether a flame colour, a coloured precipitate or a gas. One result is a special marker to remember: the white precipitate that dissolves again when you add more sodium hydroxide points to aluminium, because the others that look the same do not dissolve in excess. Match the ion, the reagent and the result together.
Match the metal ion to its flame colour
- lithium ion, Li+
- sodium ion, Na+
- potassium ion, K+
- copper ion, Cu2+
- a crimson flame
- a yellow flame
- a lilac flame
- a green flame
How do you test for a sulfate?
Which test correctly identifies a sulfate ion, with its result?
- Add acidified barium chloride and a white precipitate forms.
- Add silver nitrate and a cream precipitate forms.
- Hold it in a flame and it turns lilac.
- Add dilute acid and it fizzes off carbon dioxide.
True of the halide tests?
Select the TWO statements that correctly describe the silver nitrate halide test.
- Chloride gives a white precipitate
- Iodide gives a yellow precipitate
- Bromide gives a green precipitate
- Halides give no precipitate at all
Count the halides left
The silver nitrate test identifies 3 halides: chloride, bromide and iodide. A student can already name the result for 1 of them. Subtract to find how many halide results are still to be learned. What is the answer?
Order the flame test
Put the steps of carrying out a flame test in order, earliest first.
- Clean the test wire in acid to remove any traces
- Dip the clean wire into the sample
- Hold the wire in a blue Bunsen flame
- Observe and record the flame colour
Complete the tests
A metal ion can be identified by the colour it turns a flame in a _____ test. Adding sodium hydroxide can form a coloured solid called a _____. Halides are identified using acidified silver _____. Sulfates give a white precipitate with acidified _____ chloride.
Match the ion to its test and result
- a carbonate
- a sulfate
- a chloride
- a copper ion
- dilute acid gives fizzing carbon dioxide
- acidified barium chloride gives a white precipitate
- acidified silver nitrate gives a white precipitate
- sodium hydroxide gives a blue precipitate
Spot the true ion test facts
Tap the TWO statements that describe an ion test correctly.
- Lithium gives a crimson flame
- Iodide gives a yellow precipitate with silver nitrate
- Sodium gives a green flame
- A sulfate fizzes with dilute acid
Identify the ion
Read each test result and choose the ion it identifies.
- A sample is held in a flame and the flame turns lilac. Which ion is present?
- Adding sodium hydroxide makes a white precipitate that then dissolves when more sodium hydroxide is added. Which ion is present?
- Adding acidified silver nitrate to a solution gives a cream precipitate. Which ion is present?
Explain the ion tests
Explain how you would identify metal ions, a carbonate, a halide and a sulfate using chemical tests, giving the reagent and the result each time.
- Explain how a flame test identifies a metal ion, with two example colours
- Explain how sodium hydroxide identifies a metal ion by precipitate colour
- Explain the test for a carbonate and its result
- Explain the tests for a halide and a sulfate and their results