Scientific Principles & Theory
Scientific Background: Qualitative analysis is used to identify the ions present in unknown compounds. Inorganic compounds contain cations (positive metal ions or ammonium) and anions (negative non-metal or polyatomic ions).
Cations are identified by characteristic flame colours or precipitate colours when reacted with sodium hydroxide solution. Anions are identified by displacement and precipitation reactions with specific reagents:
- Carbonates (CO32-): React with acid to produce carbon dioxide gas (turns limewater cloudy).
- Halides (Cl-, Br-, I-): React with silver nitrate in nitric acid to form insoluble silver halide precipitates.
- Sulfates (SO42-): React with barium chloride in hydrochloric acid to form a white precipitate of barium sulfate.
Experimental Variables
Independent Variable
The inorganic salt solution under test.
Dependent Variable
The observed flame colour, precipitate formation, or gas evolution.
Control Variables
The volume of test solutions (2 cm3), the concentration of reagents, and thorough cleaning of testing wire.
⚠️ Lab Risk Assessment
| Hazard | Associated Risk | Control Measure |
|---|---|---|
| Concentrated hydrochloric acid | Highly corrosive to skin and eyes | Wear safety goggles and protective gloves; handle with caution; wash skin immediately if contact occurs. |
| Barium chloride solution | Toxic if swallowed | Wear safety goggles; wash hands immediately after use; avoid creating aerosols. |
| Silver nitrate solution | Stains skin and clothing black | Avoid contact; wear protective gloves. |
Apparatus & Procedure
Required Apparatus
- Nichrome wire loop mounted in a handle
- Concentrated hydrochloric acid
- Bunsen burner
- Test tubes and test tube rack
- Dropping pipettes
- Sodium hydroxide solution (0.4 mol/dm3)
- Dilute hydrochloric acid (1.0 mol/dm3)
- Limewater
- Dilute nitric acid (1.0 mol/dm3)
- Silver nitrate solution (0.1 mol/dm3)
- Barium chloride solution (0.1 mol/dm3)
- Inorganic salt solutions containing cations (Li+, Na+, K+, Ca2+, Mg2+, Al3+, Cu2+, Fe2+, Fe3+) and anions (CO32-, Cl-, Br-, I-, SO42-)
Step-by-Step Procedure
- Cation Test 1 (Flame Test): Clean the nichrome loop by dipping it in concentrated hydrochloric acid and holding it in a blue Bunsen burner flame until it produces no colour.
- Dip the loop in the concentrated hydrochloric acid again, then dip it into the solid test sample.
- Hold the loop at the edge of the blue Bunsen flame and record the colour: Lithium (crimson), Sodium (yellow), Potassium (lilac), Calcium (orange-red), Copper (green).
- Cation Test 2 (Sodium Hydroxide Precipitates): Add 2 cm3 of the test solution to a test tube, then add a few drops of sodium hydroxide solution. Record the precipitate colour.
- Add excess sodium hydroxide solution and shake: Al3+ forms a white precipitate that dissolves in excess; Ca2+ and Mg2+ form white precipitates that do not dissolve; Cu2+ forms a blue precipitate; Fe2+ forms a green precipitate; Fe3+ forms a brown precipitate.
- Anion Test 1 (Carbonates): Add dilute hydrochloric acid to the solid or solution sample. If bubbling occurs, bubble the gas through limewater. If the limewater turns cloudy, carbon dioxide is present, confirming carbonate ions.
- Anion Test 2 (Halides): Add dilute nitric acid to the sample, then add a few drops of silver nitrate solution: chloride forms a white precipitate; bromide forms a cream precipitate; iodide forms a yellow precipitate.
- Anion Test 3 (Sulfates): Add dilute hydrochloric acid to the sample, then add a few drops of barium chloride solution. A white precipitate confirms sulfate ions.
Fig 1. Laboratory experimental setup for Required Practical 7.
Sample Data & Calculations
This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:
| Chemical test applied | Observations recorded | Ion identified |
|---|---|---|
| Flame test | Lilac flame colour observed | Potassium (K+) |
| Sodium hydroxide test | Brown precipitate formed | Iron(III) (Fe3+) |
| Dilute acid + limewater | Effervescence; gas turns limewater cloudy | Carbonate (CO32-) |
| Nitric acid + silver nitrate | Cream precipitate formed | Bromide (Br-) |
| Hydrochloric acid + barium chloride | White precipitate formed | Sulfate (SO42-) |
Data Processing & Analysis
- Reaction at Cation NaOH precipitation (Iron(III)): Fe3+(aq) + 3OH-(aq) -> Fe(OH)3(s) [Brown precipitate].
- Reaction at Anode Halide precipitation (Bromide): Ag+(aq) + Br-(aq) -> AgBr(s) [Cream precipitate].
- Reaction at Anode Sulfate precipitation: Ba2+(aq) + SO42-(aq) -> BaSO4(s) [White precipitate].
- Reaction of Carbonate with acid: CO32-(aq) + 2H+(aq) -> CO2(g) + H2O(l). Gas test: CO2(g) + Ca(OH)2(aq) -> CaCO3(s) + H2O(l) [Cloudy precipitate].
Conclusion & Evaluation
Chemical Explanation: Evaluating the experimental outcomes against known values ensures validity. Understanding the source of systematic and random deviations allows for better experimental designs in future trials.
Experimental Error Analysis
| Error Type & Source | Effect on Final Result | Mitigation Strategy |
|---|---|---|
| Systematic Error Wire loop contaminated with sodium residue |
The intense yellow flame colour of sodium masks all other cation flame colours, preventing identification. | Clean the wire loop repeatedly in concentrated hydrochloric acid and heat it in a Bunsen burner until no colour is observed before testing another solid. |
| Systematic Error Using hydrochloric acid instead of nitric acid in the silver nitrate halide test |
Hydrochloric acid introduces chloride ions, forming a false white silver chloride precipitate even if no halides were in the original sample. | Always acidify halide test solutions with nitric acid only. Use hydrochloric acid only in the barium chloride sulfate test. |
Exam Practice
An unknown solid compound contains one cation and one anion. Plan a series of chemical tests to identify the ions present in this compound, describing the expected positive results for potassium sulfate.
View Model Answer & Mark Scheme
Model Answer (6/6 Marks):
- Cation Identification:
- Clean a nichrome wire loop by dipping it in concentrated hydrochloric acid and heating it in a blue Bunsen burner flame until it produces no colour.
- Dip the clean loop in concentrated acid, touch it to the solid sample, and place it at the edge of the blue Bunsen flame. A lilac flame colour confirms the presence of potassium (K+) ions.
- Anion Identification (Carbonate check): Add dilute hydrochloric acid to a portion of the dissolved solid. If bubbling occurs, test the gas with limewater. No bubbling indicates that carbonate is absent.
- Anion Identification (Sulfate check): Add dilute hydrochloric acid to another portion of the solution, then add barium chloride solution. If a white precipitate forms, it confirms the presence of sulfate (SO42-) ions.
- Anion Identification (Halide check): If no sulfate precipitate had formed, nitric acid followed by silver nitrate would be added. Since it is sulfate, this test is not needed, but would yield no precipitate.
- Sulfate test equation:
Ba2+(aq) + SO42-(aq) -> BaSO4(s).
Examiner Tip:
Make sure you detail the cleaning process for the flame test wire loop, and justify why nitric acid is used to acidify halide solutions while hydrochloric acid is used for sulfate solutions.