Identify an Unknown Compound Using Cation, Anion & Flame Tests
Practical Skills Development
- Use of appropriate qualitative reagents and techniques to analyse and identify unknown samples or products including gas tests, flame tests, precipitation reactions
- Safe use of appropriate heating devices and techniques including use of a Bunsen burner and a water bath or an electric heater
Aim
- To carry out flame tests, cation tests and anion tests
Health & Safety Aspects
Hazard symbols to show substances that are harmful to health and toxic
- Limewater, dilute nitric acid and sodium hydroxide are harmful to health as they are irritating to the eyes and skin
- Dilute hydrochloric acid is not classified as hazardous at the concentrations typically used in this practical, however it may still cause harm to the eyes or the skin
- Barium chloride and silver nitrate solutions are not classified as hazardous at the concentrations typically used in this practical (at higher concentrations, barium chloride can be classified as toxic)
- For limewater, dilute nitric acid and dilute hydrochloric acid, avoid contact with the skin
- Safety glasses should be worn throughout the tests
- Small volumes should be used to minimise the risk of these hazardous substances
Flame Tests
Materials
- Flame test wires
- Solids samples of the chlorides of sodium, lithium, potassium, calcium and copper(II)
- Concentrated hydrochloric acid
- Bunsen burner
Method
- Dip the loop of an unreactive metal wire such as nichrome or platinum in dilute acid, and then hold it in the blue flame of a Bunsen burner until there is no colour change
- This cleans the wire loop and avoids contamination
- This is an important step as the test will only work if there is just one type of ion present
- Two or more ions means the colours will mix, making identification erroneous
- Dip the loop into the solid sample and place it in the edge of the blue Bunsen flame
- Avoid letting the wire get so hot that it glows red otherwise this can be confused with a flame colour
Diagram showing the technique for carrying out a flame test
The colour of the flame and the metal ion present
Diagram showing the colours formed in the flame test for metal ions
- Metal ions produce a colour if heated strongly in a flame
- Ions from different metals produce different colours
- The flame test is thus used to identify metal ions by the colour of the flame they produce
Cation Tests
Materials
- 0.5 mol/dm3 sodium hydroxide solution
- Solutions containing the cations: Zn2+/Al3+, Mg2+, Ca2+, Cu2+, Fe2+ & Fe3+
- Dropping pipettes
- Test tubes
Method
- A few drops of NaOH are added at first and very slowly to the test solution
- If it is added too quickly and the precipitate is soluble in excess, then you run the risk of missing the formation of the initial precipitate which dissolves as quickly as it forms if excess solution is added
- Any colour changes or precipitates formed are noted
- Then the NaOH is added in excess and the reaction is observed again
Results
Cation Tests Results
Testing Anions - Sulfate
Materials
- 0.2 mol/dm3 barium chloride solution
- 1.0 mol/dm3 hydrochloric acid
- Test tubes & dropping pipettes
Method
- Acidify the sample with dilute hydrochloric acid and then add a few drops of aqueous barium chloride
- The test can also be carried out with barium nitrate solution
- If a sulfate is present then 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
Testing Anions- Halides
Materials
- 0.05 mol dm-3 silver nitrate solution
- 1.0 mol dm-3 nitric acid
- Test tubes & dropping pipettes
Method
- Acidify the sample with dilute nitric acid, HNO3, followed by the addition of silver nitrate solution, AgNO3
- The acidification is done to remove carbonate ions that might give a false positive result
- If a halide is presents a silver halide precipitate is formed
Ag+ (aq) + X– (aq) → AgX (s)
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- 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
Testing Anions- Carbonate
Materials
- 1.0 mol dm-3 hydrochloric acid
- limewater solution
- Delivery tube assembly
Method
- Add a few cm3 of dilute hydrochloric acid and pass the gas evolved through limewater
- If a carbonate compound is present then 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 (g) + Ca(OH)2 (aq) → CaCO3(s) + H2O(l)
Results
Limewater turns milky in the presence of CO2 caused by the formation of insoluble calcium carbonate