How can simple test-tube reactions identify the ions present in an unknown sample?
Qualitative tests for carbonate, sulfate, halide and ammonium ions, the correct sequence of tests to avoid interference, and the observations and ionic equations for each test.
An OCR H432 module 3 answer on qualitative analysis: tests for carbonate, sulfate, halide and ammonium ions, the order of testing to avoid false results, and the relevant ionic equations.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
OCR specification point 3.1.3 wants you to carry out and interpret qualitative tests for four ions: carbonate, sulfate, halide and ammonium. You must know the reagents, the observations, the ionic equations, and crucially the order in which the tests are done so that one ion does not interfere with another. This is a Paper 3 favourite because it links practical skill to ionic equations.
The carbonate test
The sulfate test
Add dilute nitric or hydrochloric acid, then aqueous barium nitrate (or barium chloride). A white precipitate of barium sulfate that does not dissolve in excess acid confirms a sulfate:
The acid is essential: it reacts away any carbonate first, because barium carbonate is also a white solid and would be mistaken for barium sulfate. Barium sulfate is highly insoluble and does not redissolve in excess acid, which distinguishes it from a carbonate precipitate that would fizz and dissolve.
The halide test
Add dilute nitric acid, then aqueous silver nitrate. The precipitate colour identifies the halide: chloride white, bromide cream, iodide yellow, confirmed by solubility in ammonia (chloride dissolves in dilute, bromide in concentrated, iodide in neither).
The ammonium test
Add aqueous sodium hydroxide and warm gently. Ammonia gas is released, turning damp red litmus blue:
Warming drives the ammonia out of solution as a gas; ammonia is the only common alkaline gas, so a damp red litmus turning blue is a reliable confirmation. This is the one test of the four that produces a gas rather than a precipitate, which is why it can be done at any stage without interfering with the precipitation tests.
The correct order of testing
Examples in context
Example 1. Identifying an unknown salt. Given a single solid, a student dissolves it, runs the tests in order, and combines a positive ammonium result and a positive sulfate result to deduce ammonium sulfate, a common fertiliser.
Example 2. Water quality. Detecting sulfate and chloride levels in groundwater uses exactly these precipitation tests, which is why getting the order and the acidification right is a real analytical skill.
Try this
Q1. Write the ionic equation for the reaction in the sulfate test. [1 mark]
- Cue. .
Q2. Explain why dilute nitric acid, not dilute sulfuric acid, is used to acidify a sample before the halide test. [2 marks]
- Cue. Sulfuric acid would add sulfate ions; while this does not directly affect the halide test, nitric acid is the safe choice because it adds no ion that forms a silver precipitate, unlike chloride from hydrochloric acid.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksA solution may contain carbonate, sulfate and chloride ions. Describe, in the correct order, the tests you would carry out to confirm the presence of all three, explaining why the order matters.Show worked answer →
Test for carbonate first by adding dilute acid; effervescence of a gas that turns limewater milky confirms carbonate (1). Carbonate must be removed first because it also forms a precipitate with barium ions and would give a false positive for sulfate (1).
Then add barium nitrate; a white precipitate insoluble in excess acid confirms sulfate. Remove excess sulfate, then add silver nitrate; a white precipitate confirms chloride (1). Sulfate is tested before halide because silver sulfate is slightly insoluble and could mask the halide result (1).
Markers reward the carbonate-then-sulfate-then-halide order, with a reason that each earlier ion would interfere with the next test.
OCR 20203 marksDescribe a test for ammonium ions and write the ionic equation for the reaction that occurs.Show worked answer →
Add aqueous sodium hydroxide and warm gently (1). A gas is released that turns damp red litmus paper blue, confirming ammonia and therefore ammonium ions (1).
Ionic equation: (1).
Markers reward warming with hydroxide, the damp red litmus turning blue, and the correct ionic equation.
Related dot points
- Group 2 reactivity and reducing power, reactions of Group 2 elements and their oxides and hydroxides, the halogens as oxidising agents, halide displacement, disproportionation of chlorine, and tests for halide ions.
An OCR H432 module 3 answer on Group 2 and the halogens: reactivity trends, reactions with water and oxygen, halogen displacement, disproportionation of chlorine, and halide ion tests.
- The periodic table arranged by atomic number into periods and groups, the s, p and d blocks, and the periodic trends in atomic radius, first ionisation energy and melting point across Periods 2 and 3.
An OCR H432 module 3 answer on periodicity: the structure of the periodic table, the s, p and d blocks, and the trends in ionisation energy, atomic radius and melting point across Periods 2 and 3.
- Oxidation numbers and the rules for assigning them, oxidation and reduction as loss and gain of electrons, oxidising and reducing agents, and the construction of half-equations and overall redox equations.
An OCR H432 module 2 answer covering oxidation number rules, oxidation and reduction as electron transfer, oxidising and reducing agents, and building half-equations and balanced redox equations.
- Names and formulae of common ions, binary and polyatomic compounds, the use of oxidation numbers in naming, and the construction of balanced full and ionic equations including state symbols.
An OCR H432 module 2 answer covering common ion formulae, naming with oxidation numbers, writing chemical formulae from charges, and constructing balanced full and ionic equations with state symbols.
Sources & how we know this
- OCR A-Level Chemistry A (H432) specification — OCR (2015)