9.1 - 9.10 Qualitative analysis: tests for ions
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Explain why the test for any ion must be unique
So that test only detects that ion so there is no confusion with other ions. Otherwise, if the same result is given by more than one ion you wouldn’t know which ion is present.
Describe how to carry out the flame tests
Hold a sample on a wire in a blue Bunsen burner flame observe the resulting colour. This can be used to identify ions in solids.
State the ions tested for in flame tests and the corresponding flame colour for each
• Red – Lithium, Li⁺ • Yellow – Sodium, Na⁺ • Lilac – Potassium, K⁺ • Orange-red – Calcium, Ca²⁺ • Blue-green – Copper (II), Cu²⁺
Describe how to carry out the tests for cations
Add sodium hydroxide solution to a sample. Observe the precipitate that forms. This can be used to identify ions in solids or solutions.
State the results observed in the tests for cations and their related ion
• Aluminium, Al³⁺ – White precipitate that dissolves when excess sodium hydroxide (NaOH) is added • Calcium, Ca²⁺ – White precipitate that doesn’t dissolve in NaOH • Copper (II), Cu²⁺ – Blue precipitate • Iron (II), Fe²⁺ – Green precipitate • Iron (III), Fe³⁺ – Brown precipitate • Ammonium, NH⁴⁺ – No precipitate. Gas is given off which turns damp red litmus paper blue
Describe the chemical test for ammonia
-Makes damp red litmus paper turn blue -Forms a white smoke of ammonium chloride when hydrogen chloride gas, from concentrated hydrochloric acid, is held near it
Describe the test to identify carbonate ions
Identifies carbonate, CO₃²⎺, ions in solids or solutions. Add dilute acid to a sample. If carbonate ions are present a carbon dioxide gas is released. If bubbled through limewater this turns cloudy.
Describe the test to identify sulfate ions
Identifies sulfate, SO₄²⎺, ions in solids or solutions. Add dilute hydrochloric acid to a sample. Then, add barium chloride solution. If a white precipitate is produced then the sample contains sulphate ions.
Describe the anion test involving silver
Add dilute nitric acid to a sample. Then, add silver nitrate solution. * If a white precipitate forms, the sample contains chloride ions, Cl⎺. * If a cream precipitate forms, the sample contains bromide ions, Br⎺. * If a yellow precipitate forms, the sample contains iodide ions, I⎺.
What alternative methods of analysis are available?
Elements and compounds can be detected and identified using instrumental methods. These include gas chromatography and mass spectrometry. These may improve sensitivity, accuracy and speed of tests.
Describe the use of a flame photometer
It is an example of an instrumental method used to analyse metal ions in solutions. A sample is put into a flame and the light given out is passed through a photometer. The output is a line spectrum that can be analysed to identify the metal ions in a solution and measure their concentrations. This can be drawn as a calibration graph with a calibration curve or line of best fit.