9.5C - Tests for carbonate, sulfate and halide ions
Learn the reagent sequence and positive observation for carbonate, sulfate and halide ions. These tests identify negative ions, and using the correct acid prevents the reagent itself from causing a false positive.
Choosing the test
The ions in this lesson are all anions, which means negatively charged ions. A test is useful only if the observation helps identify one ion or one group of ions.
Anion
An anion is a negatively charged ion.
The three Edexcel 9.5C tests are:
| Ion or ion group tested | Ion formula | Required reagent or reagents | Positive observation |
|---|---|---|---|
| carbonate ion | dilute acid | carbon dioxide gas is evolved; it turns limewater cloudy | |
| sulfate ion | dilute hydrochloric acid, then barium chloride solution | white precipitate forms | |
| chloride ion | dilute nitric acid, then silver nitrate solution | white precipitate forms | |
| bromide ion | dilute nitric acid, then silver nitrate solution | cream precipitate forms | |
| iodide ion | dilute nitric acid, then silver nitrate solution | yellow precipitate forms |
The reagent order matters. For sulfate and halide tests, the acid is added before the precipitating reagent so that ions such as carbonate do not give misleading precipitates.
Testing carbonate ions
Carbonate ions can be tested in a solid sample or in solution. Add dilute acid to the sample. If carbonate ions are present, carbon dioxide gas is evolved.
The visible observation at the sample is fizzing or effervescence, because a gas is being produced. To identify the gas as carbon dioxide, pass it into limewater. Carbon dioxide turns limewater cloudy.
The acid reacts with carbonate ions to make carbon dioxide and water:
carbonate ion + acid -> carbon dioxide + water
At ionic level, this can be represented as:
CO3^2- + 2H+ -> CO2 + H2O
A strong answer does not just say "bubbles". Bubbles show that a gas forms, but the Edexcel test identifies carbonate ions by showing that the gas evolved is carbon dioxide.
| Step | What to do | What a positive result shows |
|---|---|---|
| 1 | add dilute acid to the sample | gas is evolved |
| 2 | test the gas with limewater | limewater turns cloudy, so the gas is carbon dioxide |
| 3 | link the result to the ion | carbonate ions are present |
Testing sulfate ions
The sulfate test is done on a solution. If the sample is a soluble solid, dissolve a small amount in deionised water first. An insoluble sample needs a suitable teacher-provided preparation method.
Add dilute hydrochloric acid to the solution, then add barium chloride solution. A positive result is a white precipitate of barium sulfate.
Precipitate
A precipitate is an insoluble solid that forms when two solutions react.
The useful ion reaction is:
Ba^2+(aq) + SO4^2-(aq) -> BaSO4(s)
The state symbol (s) shows that barium sulfate is an insoluble solid, which is why a white precipitate is seen.
Use dilute hydrochloric acid for this Edexcel test. Do not use sulfuric acid, because sulfuric acid contains sulfate ions and could give a false positive result. The acid also helps remove carbonate ions before barium chloride is added, because carbonate ions can also form a white precipitate with barium ions.
Testing halide ions
Halide ions are ions formed from Group 7 elements. The three halide ions in this lesson are chloride, bromide and iodide ions.
The halide test is done on a solution. If the sample is a soluble solid, dissolve a small amount in deionised water first. Add dilute nitric acid, then add silver nitrate solution.
Continue this lesson.
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