OCR A level chemistry - Ion tests

describe the test for carbonate ions.

- Add dilute acid (e.g. dilute HCl) to your sample.

- If carbonates are present, carbon dioxide will be released.

describe the test for sulphate ions.

- Most sulphates are soluble in water, but barium sulphate is insoluble.

- So, to test for a sulphate ion, add dilute HCl followed by barium chloride (or barium nitrate) solution (BaCl2).

- If you get a white precipitate, it'll be barium sulphate (BaSO4), which tells you your sample is a sulphate.

describe the test for halide ions

- Add nitric acid, then silver nitrate solution.

- If chloride, bromide or iodide is present, a precipitate will form.

Silver chloride (AgCl) - white precipitate.

Silver bromide (AgBr) - cream precipitate.

Silver iodide (AgI) - yellow precipitate.

You can then test the solubility of these precipitates in ammonia to help you tell them apart.

describe the test for ammonium ions:

Add some sodium hydroxide to the sample in a test tube and warm the mixture. If there is ammonia given off, this means there are ammonium ions in the substance.

how can we test to see if ammonia gas is present?

Ammonia gas is alkaline, so you can check for it using a damp piece of red litmus paper. If there's ammonia present, the paper will turn blue.

how should the test be ordered?

1. carbonate.

2. sulphate.

3. halide.

why should carbonate ions be tested for first?

neither sulphate nor halide ions produce bubbles with acid, so this test can be carried out without the possibility of an incorrect conclusion.

If there are no bubbles, there is not carbonate and you can move onto the next test.

why should sulphate ions be tested for next?

barium carbonate is white and insoluble in water. So, if you carry out a sulphate test on a carbonate, you will get a white precipitate too. So carry out the carbonate test first and only proceed when you know there is no carbonate.

why should halide ions be tested for last?

silver carbonate and silver sulphate are insoluble in water and will form precipitates in this test, so it is important to carry out this test last.

describe what should be done when testing one solution for multiple ions:

Carbonate.

- If there are bubbles, then continue adding the acid until the bubbling stops - this will ensure all of the carbonate ions have been removed.

Sulphate.

- To the above solution, add an excess of barium nitrate (instead of barium chloride as this may interfere with the next test).

- Any sulphite ions will precipitate out as barium sulphate.

- Filter this out.

Halide.

- To the solution left, add silver nitrate and record if there is a precipitate formed.