Carbonates
Chemistry: AQA GCSE Higher, topic 8.3.5
Testing for Carbonate and Halides
There is a specific way to test for the presence of carbonates and halides.
Testing for carbonates
- Carbonates are compounds containing CO32− ions.
- Carbonates react with dilute acids to form a salt, carbon dioxide, and water.
- E.g. calcium carbonate + hydrochloric acid → calcium chloride + carbon dioxide + water
Testing for carbonates cont.
- As carbon dioxide is produced, we can use the standard carbon dioxide test to work out if an unknown substance is a carbonate.
- This test involves bubbling the gas through limewater. If the solution turns cloudy, the gas is carbon dioxide and the compound is a carbonate.
Testing for halides
- Halides are compounds containing ions of halogen atoms.
- There are two steps to halide testing:
- Add dilute nitric acid to an unknown solution.
- This removes carbonate ions. These could disrupt test results by forming a precipitate with the silver ions added in the second step.
- Add silver nitrate to the same solution.
- If halide ions are present, they will form a precipitate with the silver ions.
Testing for halides cont.
- Ag+(aq) + Cl-(aq) → AgCl(s)
- Silver chloride is white.
- Ag+(aq) + Br-(aq) → AgBr(s)
- Silver bromide is cream.
- Ag+(aq) + I-(aq) → AgI(s)
- Silver iodide is pale yellow.
Test yourself on Carbonates
Why is dilute nitric acid added to a solution before it is tested for halide ions?
What ions do carbonates contain?
A student adds silver nitrate to an unknown solution and a cream precipitate forms. Which halide ion is present in the solution?
A solid reacts with dilute hydrochloric acid to produce a gas that turns limewater cloudy. What does this show about the solid?
What type of ion do halide compounds contain?