Topic 25 - Qualitative Analysis : Test for Ions

Question 1

9.1C - Explain why the test for any ion must be unique

Answer 1

So that you can distinguish between two ions producing the same results.
E.g - Calcium and aluminium ions both produce hydroxide precipitates while only aluminium hydroxide solution becomes colourless when excess sodium hydroxide is added.

Question 2

9.2C - How do you set up the apparatus for flame tests?

Answer 2

Light a bunsen burner and open air hole for a hot blue flame. ( So you can see colour change )
Clean the nichrome inoculating loop with hydrochloric acid. ( Nichrome used as cheaper )
Pick up a sample of the substance using a wire loop
Hold the sample in the edge of the flame and observe the flame colour.

Solid substances are easier to see.

Question 3

9.2C - What colours are produced in flame tests?

Answer 3

Lithium - red
Sodium - yellow
Potassium - lilac
Calcium - orange-red
Copper - blue-green

Question 4

9.3C - How do you test for ions using NaOH solution?

Answer 4

Few drops of NaOH to test solution.
Iron (II) - green
Iron (III) - brown
Copper - blue
Calcium - white
Aluminium - white

Question 5

9.4C - Describe the chemical test for ammonia

Answer 5

NaOH is reacted with ammonium ions to form ammonium gas, this has a sharp smell indicating presence of ammonium ions.
A confirmatory test can be made by holding red litmus paper above the solution changing the colour of damp red litmus paper to blue.

Question 6

9.5/7C -Describe the tests to identify the carbonate, sulfate and halide ions

Answer 6

Carbonate - Add HCL to the substance and look for bubbling which is the production of CO2.
The bubbles can be hydrogen so a confirmatory test should be done by bubbling the CO2 through limewater, if it turns milky, CO2 is present.

Sulfate - Add HCL to acid, this acidifies the solution and reacts with any carbonate ions .
Add a few drops of barium chloride solution, a white precipitate of barium sulfate forms.

Halide - Add nitric acid to acidify and remove carbonate ions.
Add silver nitrate solution as it is insoluble and forms a precipitate.
Chloride - white
Bromide - cream
Iodide - yellow

Question 7

9.8C - Describe instrumental methods of analysis

Answer 7

Machines can also analyse substances, they improve sensitivity, accuracy and speed.
A flame photometer measures light intensity of flame colours produced by metal ions.
It determines the concentration of metal ion in a solution.
It can seperate colours from the light in a flame test to produce emission spectra. Each metal ion has a different one.