- 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
- 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
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.