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Identifying a compound means working out both of its ions — the positive metal ion (cation) and the negative non-metal ion (anion). The last lesson covered cations; this one covers the three groups of anions you must be able to test for: carbonates, sulfates and halides (chloride, bromide and iodide). Each has a simple test with a clear positive result, and a small but important point of method — adding an acid first — keeps the results reliable. This lesson, part of Topic C4 of OCR Gateway Science A, sets out the anion tests and how to identify a salt completely.
By the end of this lesson you should be able to describe the test and positive result for carbonate, sulfate and halide ions, explain why an acid is added first, write ionic equations for the precipitate tests, and carry out the full identification of an unknown salt.
| Anion | Test (method) | Positive result |
|---|---|---|
| Carbonate, CO32− | Add dilute acid | Fizzes, giving carbon dioxide (turns limewater milky) |
| Sulfate, SO42− | Add dilute hydrochloric acid, then barium chloride solution | White precipitate (barium sulfate) |
| Halide (Cl⁻, Br⁻, I⁻) | Add dilute nitric acid, then silver nitrate solution | Precipitate: chloride white, bromide cream, iodide yellow |
Exam Tip: Notice each precipitate test uses a different acid first: hydrochloric acid before barium chloride (for sulfate), and nitric acid before silver nitrate (for halides). Using the wrong acid can introduce the very ion you are testing for.
To test for a carbonate, add a dilute acid (such as hydrochloric acid). If a carbonate is present, the mixture fizzes as carbon dioxide is given off:
carbonate+acid→salt+water+carbon dioxide
for example CaCO3+2HCl→CaCl2+H2O+CO2. You confirm the gas is carbon dioxide by bubbling it through limewater, which turns milky (as covered in the gas-tests lesson).
Exam Tip: The carbonate test is "add dilute acid → fizzes → gas turns limewater milky". Stating all three parts (acid, fizzing, limewater) gives the full chain of evidence.
To test for a sulfate, add some dilute hydrochloric acid and then a few drops of barium chloride solution. A white precipitate of insoluble barium sulfate confirms a sulfate:
Ba2++SO42−→BaSO4
The dilute hydrochloric acid is added first to remove any carbonate ions, which would otherwise also form a white precipitate (barium carbonate) and give a false positive. Adding the acid reacts away the carbonate so that only a genuine sulfate produces the white precipitate.
Exam Tip: For sulfate, add dilute hydrochloric acid first (not sulfuric — that would add sulfate ions!), then barium chloride; a white precipitate confirms a sulfate. The acid removes interfering carbonate.
To test for a halide (chloride, bromide or iodide), add some dilute nitric acid and then a few drops of silver nitrate solution. A precipitate of the insoluble silver halide forms, and its colour identifies which halide is present:
| Halide ion | Silver halide formed | Colour of precipitate |
|---|---|---|
| Chloride, Cl⁻ | Silver chloride, AgCl | White |
| Bromide, Br⁻ | Silver bromide, AgBr | Cream |
| Iodide, I⁻ | Silver iodide, AgI | Yellow |
For chloride, the ionic equation is:
Ag++Cl−→AgCl
The dilute nitric acid is added first to remove carbonate ions, which would otherwise form a white precipitate of silver carbonate and confuse the result. Nitric acid is used (rather than hydrochloric) because hydrochloric acid would add chloride ions and give a false positive for chloride.
Exam Tip: The three silver-halide colours go white → cream → yellow as you go down the group (chloride, bromide, iodide). Memorise that order — distinguishing cream from yellow is a common exam discriminator.
In both the sulfate and halide tests, a dilute acid is added before the silver nitrate or barium chloride. The reason is the same in each case: carbonate ions also form white precipitates with these reagents (barium carbonate and silver carbonate), so a carbonate impurity could be mistaken for a sulfate or a chloride. Adding acid first reacts away any carbonate (it fizzes off as carbon dioxide), so only a genuine sulfate or halide produces a precipitate.
The choice of acid matters: use hydrochloric acid for the sulfate test (it adds only chloride, which does not interfere with barium chloride) but nitric acid for the halide test (hydrochloric acid would add chloride ions and ruin the chloride test).
Exam Tip: The reason for the acid is always "to remove carbonate ions that would also give a white precipitate". And match the acid to the test: nitric for halides, hydrochloric for sulfates.
Each of these tests works because the product is insoluble in water, so it drops out of solution as a solid precipitate you can see. Barium sulfate, silver chloride, silver bromide and silver iodide are all insoluble salts. This connects back to the solubility rules you may have met earlier: while most sulfates are soluble, barium sulfate is not, which is exactly why barium chloride is the chosen reagent; and while most chlorides are soluble, silver chloride is not, which is why silver nitrate is used. The tests are, in effect, deliberately chosen precipitation reactions — pick a reagent whose product with the target ion is insoluble, and a positive result becomes a visible solid.
This also explains the small differences in the silver-halide colours. Silver chloride, silver bromide and silver iodide are chemically similar but not identical, and the slight shift in colour from white through cream to yellow reflects that. The change is gradual, which is why telling cream from yellow can be tricky — it helps to compare the unknown side by side with known chloride and iodide samples.
Exam Tip: These tests rely on forming an insoluble precipitate. Knowing that barium sulfate and the silver halides are insoluble explains why barium chloride and silver nitrate are the chosen reagents — a useful "explain" point.
Higher tier: because cream and yellow can be hard to tell apart, the silver halides can be further distinguished by their behaviour with ammonia solution: silver chloride dissolves in dilute ammonia, silver bromide dissolves only in concentrated ammonia, and silver iodide is insoluble in ammonia altogether. This gives a second line of evidence on top of the colour. You are not usually required to recall the ammonia detail at Foundation tier, but knowing the colour order — white, cream, yellow — is essential for everyone.
Exam Tip: If a question stresses that two halide precipitates look similar, the solubility in ammonia (chloride dissolves in dilute, iodide does not dissolve at all) is a Higher-tier way to separate them beyond colour.
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