Tests for anions (AQA GCSE Chemistry): Revision Notes
Tests for anions
What are anions?
Anions are negative ions made from non-metal elements. You can identify different anions using simple lab tests. When you combine these tests with tests for metal ions, you can work out what unknown compounds are.
These anion tests are fundamental in analytical chemistry and are commonly used in both laboratory settings and exam questions. Mastering these three key tests will help you identify most common ionic compounds.
Key anion tests
There are three main groups of anions you need to know how to test for:
Testing for carbonate ions
What you do:
- Add any dilute acid to your sample
- Watch for bubbles of gas
What happens:
- Carbonate ions react with acid to make carbon dioxide gas
- You'll see fizzing and bubbles
- Test the gas with limewater - it turns cloudy white
Why this works: Most carbonates don't dissolve in water, but some do (like sodium carbonate). When acid meets carbonate ions, carbon dioxide gas always forms.
The limewater test is crucial for confirming carbon dioxide. Without this confirmation step, you cannot be certain that carbonate ions are present, as other reactions might also produce gas bubbles.
Testing for sulphate ions
What you do:
- Add dilute hydrochloric acid first
- Then add barium chloride solution
What happens:
- A white precipitate forms
- This white solid is barium sulphate
You must add the acid first to remove any carbonate ions that might interfere with the test. Carbonate ions would also form a white precipitate with barium chloride, leading to false positive results.
Testing for halide ions
What you do:
- Add dilute nitric acid first
- Then add silver nitrate solution
What happens: The colour of the precipitate tells you which halide ion is present:
- Chloride ions → white precipitate (silver chloride)
- Bromide ions → cream precipitate (silver bromide)
- Iodide ions → yellow precipitate (silver iodide)
Memory tip: Think "White, Cream, Yellow" for chloride, bromide, iodide. This sequence follows the order of the halogen group going down the periodic table.
Why these tests work
Each test works because it forms a precipitate (solid) that you can see. The key principle behind these tests is the formation of insoluble compounds with characteristic properties:
- Carbonate test: Carbon dioxide gas bubbles up and turns limewater cloudy due to the formation of insoluble calcium carbonate
- Sulfate test: Barium sulphate is insoluble so it forms a distinctive white solid
- Halide test: Silver halides are insoluble and have different colours that help distinguish between the three halide ions
Worked Example: Identifying an Unknown Compound
A white powder is used to treat drinking water. When tested:
- It makes a white precipitate with sodium hydroxide
- It doesn't fizz with acid
- It makes a white precipitate when barium chloride is added
Step 1: Analyse the acid test No fizzing with acid means no carbonate ions present
Step 2: Analyse the barium chloride test White precipitate with barium chloride indicates sulphate ions present
Step 3: Consider the sodium hydroxide test White precipitate with sodium hydroxide suggests a metal ion like aluminium
Answer: The powder contains sulphate ions (no fizzing rules out carbonate). It's likely aluminium sulphate, .
Key Points to Remember:
- Carbonate ions → add acid → fizzing and CO₂ gas → test with limewater
- Sulphate ions → add HCl then barium chloride → white precipitate
- Halide ions → add nitric acid then silver nitrate → coloured precipitates (white/cream/yellow)
- Always add acid first in sulphate and halide tests to remove interfering ions
- The colour of precipitates helps you identify which specific halide ion is present