SC25: Qualitative Analysis: Tests for Ions - Paper 2 Flashcards
Explain how to carry out a flame test for ions: (5)
- The flame test is thus used to identify metal ions by the colour of the flame they produce. (1)
- 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. (1)
- This cleans the wire loop and avoids contamination. (1)
- Dip the loop into the solid sample and place it in the edge of the blue Bunsen flame. (1)
- Avoid letting the wire get so hot that it glows red otherwise this can be confused with a flame colour. (1)
State the colour that will appear in the flame test if the following ions are detected: (5)
- Lithium (Li+) (1)
- Sodium (Na+) (1)
- Potassium (K+) (1)
- Calcium (Ca2+) (1)
- Copper (Cu2+) (1)
- Lithium (Li+): Red (1)
- Sodium (Na+): Yellow (1)
- Potassium (K+): Lilac (1)
- Calcium (Ca2+): Orange-red (1)
- Copper (Cu2+): Blue-green (1)
Explain the benefits of using instrumental methods of analysis: (3)
- Improves accuracy (1)
- Improves sensitivity (1)
- Improves speed of tests (1)
Explain why the test for any ion must be unique: (1)
The results of a test must let you determine which ion is present, rather than being uncertain about which ion it is. (1)
Explain the test for cations using sodium hydroxide solution: (2)
- Metal cations in aqueous solution can be identified by the colour of the precipitate they form on addition of sodium hydroxide. (1)
- A small amount is thus added, very gradually and any colour changes or precipitates formed are noted. (1)
Explain the chemical test for ammonia: (3)
- Ammonia is a gas with a pungent smell that turns damp red litmus paper blue. (1)
- Hold the litmus paper near the mouth of the test tube, but be careful to avoid touch the sides of the test tube. (1)
- If you are testing for ammonia produced from ammonium ions and sodium hydroxide, avoiding touching the sides prevents traces of sodium hydroxide from also turning the red litmus paper blue. (1)
Explain the test to identify a carbonate ion: (2)
- Add dilute acid and test the gas released. (1)
- Effervescence should be seen and the gas produced is CO2 which forms a white precipitate of calcium carbonate when bubbled through limewater. (1)
Explain the test to identify a sulfate ion: (2)
- Acidify with dilute hydrochloric acid and add aqueous barium chloride. (1)
A white precipitate of barium sulfate is formed. (1)
Explain the test to identify chloride, bromide and iodide ions: (4)
- Acidify with dilute nitric acid (HNO3) followed by the addition of silver nitrate solution (AgNO3). (1)
- This forms a silver halide precipitate. (1)
- Depending on the halide present, a different coloured precipitate is formed, allowing for identification of the halide ion. (1)
- Silver chloride is white, silver bromide is cream and silver iodide is yellow. (1)
Define the term flame photometer: (1)
A machine used to identify metal ions in solution and to determine their concentration. (1)
Define the term calibration curve: (1)
A graph used to determine the concentration of a substance in a sample. (1)
Define the term standard solutions (1)
A solution containing a known concentration of a substance. (1)
Define the term spectrum: (1)
Individual components of light arranged in order of wavelength or frequency (1)
Define the term emission spectra: (1)
A set of wavelengths of light or electromagnetic radiation showing which wavelengths have been given out by a substance. (1)
Define the term confirmatory test: (1)
A chemical test carried out to check the conclusion from the results of another test. (1)
