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.

Metal cation table, Edexcel GCSE Chemistry

  • 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.

Litmus Test for Ammonia, Edexcel GCSE Chemistry

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

Carbonate Ion

  • 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)

Test for CO2 Gas, Edexcel GCSE Chemistry

Limewater turns milky in the presence of CO2 caused by formation of insoluble calcium carbonate

Sulfate Ion

  • Acidify with dilute hydrochloric acid and add aqueous barium chloride.
  • A white precipitate of barium sulfate is formed:

Ba2+(aq) + SO42-(aq) → BaSO4(s)

Test for Sulfate Ion, Edexcel GCSE Chemistry

A white precipitate of barium sulfate is a positive result for the presence of sulfate ions

Halide 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.

Silver Halide Precipitates, Edexcel GCSE Chemistry

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

Aim: 

  • To identify the ions in unknown salts

Materials:

  • 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

Method:

  • 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.

Conclusion:

  • Once you have completed the tests you can identify the salt from the cation and anion present.

Example

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.

Salts ID test table, Edexcel GCSE Chemistry

Identifying Ions

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

Need help?

Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

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.