Tests for Cations
Specification Point 9.3C:
a) aluminium ion, Al3+
b) calcium ion, Ca2+
c) copper ion, Cu2+
d) iron(II) ion, Fe2+
e) iron(III) ion, Fe3+
f) ammonium ion, NH4+
- Some metal cations form insoluble hydroxides and can be identified by the colour of the precipitate they form on addition of sodium hydroxide solution.
- If only a small amount of NaOH is used then normally the metal hydroxide precipitates.
- In excess NaOH some of the precipitates may dissolve.
- For this reason just a few drops of NaOH is added at first and any colour changes or precipitates formed are noted.
- Then the NaOH is added in excess and the reaction is observed again.
- To test for the ammonium ion (NH4+), gentle heating is required after adding the NaOH solution.
- If the ammonium ion is present, ammonia gas is produced which can be tested:
NH4+(aq) + OH– (aq)→ NH3(g) + H2O(l)
Test for Ammonia
Specification Point 9.4C:
Describe the chemical test for ammonia
- Ammonia is a gas with a pungent smell that turns damp red litmus paper blue.
Diagram showing how to perform the test for ammonia gas using damp red litmus paper
- Another test for ammonia is to react the gas with HCl.
- A white smoke of ammonium chloride is formed if ammonia gas is present:
NH3 + HCl → NH4Cl
Test for Anions
Specification Point 9.5C:
Describe tests to identify the following ions in solids or solution as appropriate:
a) carbonate ion, CO32–, using dilute acid and identifying the carbon dioxide evolved
b) sulfate ion, SO42–, using dilute hydrochloric acid and barium chloride solution
c) chloride ion, Cl–, bromide ion, Br–, iodide ion, I–, using dilute nitric acid and silver nitrate solution
- Add dilute acid and test the gas released.
- Effervescence should be seen and the gas produced is CO2 which forms a white precipitate of calcium carbonate when bubbled through limewater:
CO32-(aq) + 2H+(aq) → CO2(g) + H2O(l)
CO2 + Ca(OH)2 → CaCO3(s) + H2O(l)
Limewater turns milky in the presence of CO2 caused by formation of insoluble calcium carbonate
- Acidify with dilute hydrochloric acid and add aqueous barium chloride.
- A white precipitate of barium sulfate is formed:
Ba2+(aq) + SO42-(aq) → BaSO4(s)
A white precipitate of barium sulfate is a positive result for the presence of sulfate ions
- Acidify with dilute nitric acid (HNO3) followed by the addition of silver nitrate solution (AgNO3).
- This forms a silver halide precipitate:
Ag+(aq) + X–(aq) → AgX(s)
- Depending on the halide present, a different coloured precipitate is formed, allowing for identification of the halide ion.
- Silver chloride is white, silver bromide is cream and silver iodide is yellow.
Each silver halide produces a precipitate of a different colour
Core Practical: Identification of Unknown Salts
Specification Point 9.6C:
Identify the ions in unknown salts, using the tests for the specified cations and anions in 9.2C, 9.3C, 9.4C, 9.5C
Identification of Unknown Salts
- To identify the ions in unknown salts
- Various samples of salts and salt solutions
- Necessary reagents to carry out the analysis
- Test tubes, Bunser burner, teat pipette, nichrome / platinum loops, litmus paper, splints
- There are a number of strategies you could choose in order to identify the ions in unknown salts.
- The tests you decide on can be carried out in any particular order, and you will probably not need to carry them all out on any one sample.
- Only small amounts of each sample and reagent are needed.
- You may need to dissolve a sample of salt in a little distilled water if the salt you are given is in the solid state.
- Record your observations carefully in a table of results.
- Repeat any tests that do not provide a clear result i.e. a colour change that was difficult to identify.
Analysis of Results:
- Once you have tabulated the results from the tests you performed, use them to infer the identity of the ions that the salt contains.
- Once you have completed the tests you can identify the salt from the cation and anion present.
A student was given two salts labelled A and B. The following set of results were obtained from a series of qualitative tests performed by the student on the samples. Using the results, identity the salts.
Specification Point 9.7C:
Identify the ions in unknown salts, using the results of the tests above
- Salt A contains lithium and bromine so it must be lithium bromide, LiBr
- Salt B contains iron(II) and sulfate ion so it must be FeSO4
Edexcel GCSE Chemistry Notes
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Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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