12610574
| Question | Answer |
| What is a false positive? | A result that suggests that a particular ion is present when it actually isn't. This can happen if there are ions present that will interfere with the test you are carrying out |
| How can you prevent false positives? | You can sometimes add a substance to your sample to remove problematic ions You should also do the three tests in the following order: Carbonates --> Sulphates --> Halides |
| What is the test for carbonates? | Add a dilute strong acid (e.g. dilute nitric acid or dilute hydrochloric acid). If carbonates are present, then carbon dioxide will be released |
| Give the half equation for the test for carbonates | CO₃²⁻ + 2H⁺ --> CO₂ + H₂O |
| What is the test for carbon dioxide? | Bubble the gas through limewater - if there is CO₂ present, it'll turn the limewater cloudy |
| TRUE OR FALSE All sulphates are soluble in water | FALSE Most sulphates are soluble in water, however there are some which are insoluble. E.g. barium sulphate is insoluble |
| Describe the method for the sulphate test | To test for a sulphate ion, add a few drops of barium nitrate solution (Ba(NO₃)₂) |
| If the sulphate test is positive, a white precipitate will be formed. What is this precipitate? | Barium sulphate |
| Give the half equation for the formation of barium sulphate in the sulphate test | SO₄²⁻ + Ba²⁺ --> BaSO₄ |
| Complete the sentences: In the test for sulphates, a false positive may occur due to ____________ or ____________ ions. In the test for carbonates, you add a ________ strong acid which will react with any carbonates or sulphites present so that they won't interfere with the test. | In the test for sulphates, a false positive may occur due to CARBONATE or SULPHITE ions. In the test for carbonates, you add a DILUTE strong acid which will react with any carbonates or sulphites present so that they won't interfere with the test. |
| What is the test for halides? | Add nitric acid and silver nitrate solution To distinguish between different colours, the solubility is tested using ammonia (dilute and concentrated) |
| If the test for halides is positive, a precipitate is formed. The colour of the precipitate determines the halide present. What are the colours of silver chloride, silver bromide or silver iodide? | SILVER CHLORIDE = white SILVER BROMIDE = cream SILVER IODIDE = yellow |
| TRUE OR FALSE: Ammonia gas is acidic | FALSE Ammonia gas is alkaline |
| What is the test for ammonia? | Check it using a damp piece of red litmus paper. If ammonia is present, the paper will turn blue. Ammonia also has a distinctive, pungent smell |
| Outline the method used to test whether a substance contains ammonium ions (NH₄⁺) | 1. Add a few drops of aqueous sodium hydroxide and warm the mixture 2. Hold a piece of damp litmus paper near the top of the test-tube 3. If the paper turns blue, ammonia is being given off and there are ammonium ions in the substance |
| Give the half equation for the reaction between ammonium ions and hydroxide ions | NH₄⁺ + OH⁻ --> NH₃ + H₂O |
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