Identification of Ions by Chemical and Spectroscopic Means

1. What is the name of the ion that causes a crimson flame?

Answer: lithium


2. What is the formula of the ion that causes a yellow flame?

Answer: Na+ (natrium)


3. What is the colour of the flame caused by a potassium ion?

Answer: lilac


4. What is the colour of the flame caused by Ca2+ ?

Answer: orange-red


5. Which element would be present if a flame test showed a green colour?

Answer: copper


6. What colour precipitate is made when sodium hydroxide is added to a solution containing calcium ions?

Answer: white


7. What ion causes a blue precipitate with sodium hydroxide?

Answer: Cu2+ (copper II)


8. What gas is made when carbonates react with acids?

Answer: CO2 (carbon dioxide)


9. Name the halide that causes a yellow precipitate with silver nitrate.

Answer: iodide


10. Give the formula of the halide that causes a cream precipitate with silver nitrate.

Answer: Br (bromine)


11. What is the name of the reagent used to show the presence of sulfate ions?

Answer: barium chloride


12. What are the advantages of using instrumental methods of analysis?

Answer: They are accurate, sensitive and rapid.


13. What is flame emission spectroscopy used for?

Answer: To analyse metal ions in solution.


14. When sodium hydroxide is added to some solutions a coloured precipitate is formed. The colour indicates the cation present.

a) Define the term "precipitate".

Answer: A solid formed when two solutions mix.


b) Write the equation that shows copper(II) sulfate reacting with sodium hydroxide.

Answer: CuSO4 + 2NaOH  →  Cu(OH)2 + Na2SO4


c) Give the name of the blue precipitate formed in this reaction.

Answer: copper (II) hydroxide or copper hydroxide


d) Outline an experiment to determine whether a solution contained iron(II) or iron(III) ions.

Answer: Put a sample of each solution to be tested in separate test tubes. Add sodium hydroxide solution and mix the solutions. Make a note of the colour of the precipitate. If the precipitate is green then iron(II) is present. If the precipitate is brown then iron(III) is present.


15. A solution of calcium chloride was analysed using different chemical analysis.

a) State the cation in calcium chloride solution.

Answer: Ca2+ (calcium)


b) State the anion in calcium chloride solution.

Answer: Cl (chloride)


c) State the colour of the flame in a calcium chloride flame test.

Answer: red-orange


d) Describe the observations when calcium chloride solution was mixed with sodium hydroxide solution.

Answer: White precipitate formed.


e) Describe the observations when calcium chloride solution was mixed with acidified silver nitrate.

Answer: White precipitate formed.


16. A solution of magnesium sulfate (MgSO4 ) was analysed using different chemical analysis.

a) Give the formula of the cation in magnesium sulfate solution.

Answer: Mg2+ (magnesium)


b) Give the formula of the anion in magnesium sulfate solution.

Answer: SO42− (sulfate)


c) Magnesium sulfate can react with sodium hydroxide to make a white precipitate. Write a balanced symbol equation for this reaction.

Answer: MgSO4(aq) + 2NaOH(aq)  →  Mg(OH)2(s) + Na2SO4(aq)


d) Outline a chemical test to show that sulfate ions are present.

Answer: Put a sample of the sample solution in a test-tube. Add dilute hydrochloric acid solution. Add dilute barium chloride solution and mix the solutions. Make a note of the colour of the precipitate. If the precipitate is white then sulfate ions are present.


17. A sample of a colourless liquid thought to be a solution of potassium carbonate (K2CO3 ) was found in a chemical store.

a) Justify the use of flame emission spectroscopy rather than a flame test to determine the cation present.

Answer: Flame emission spectroscopy is more accurate than a flame test as it can be difficult to judge the colour of a flame in a flame test. Flame tests can only identify some metal ions whereas the flame emission spectroscopy can identify the metal ion and give the concentration of the ion.


b) Outline a chemical test that can be used to show the identity of the anion.

Answer: Put a sample of the sample solution in a test tube. Add dilute acid solution and then observe. If bubbles (or fizzing, effervesces) is observed then a gas is made. Collect and test the gas with limewater. If the limewater goes cloudy then the gas was carbon dioxide and a carbonate was present.


c) Suggest and explain the observations made if this solution was mixed with acidified silver nitrate solution.

Answer: No observable change (solution remains colourless). Acidified silver nitrate solution tests for halide ions, but no halide ions are present. So, no chemical reaction will occur.


18. Flame emission spectroscopy can be used to analyse the composition of solutions. The figure below shows the emission spectrums for lithium ions and sodium ions.


a) Calculate the approximate wavelength of the two emission lines for the sodium ion.

Answer: 590 nm


b) Sketch the emissions spectrum for a solution that contains a mixture of sodium iodide and lithium iodide.

Answer: A line at 460, 495, 610 and 670 nm. Two lines close together at 590 nm


c) When acidified silver nitrate is added to the solution, a yellow precipitate is formed. Explain why hydrochloric acid cannot be used to acidify the solution.

Answer: The chloride ions from the acid would cause a white precipitate and mask the accurate result.


19. A selection of chemicals has had their labels fall off in the chemical store. It is known that the solutions are: sodium carbonate, sodium chloride, magnesium chloride and aluminium iodide. Use the information below to determine the name of each solution.

a) Solution A has two characteristic lines on the flame emission spectrum at about 590 nm and forms a white precipitate on the addition of acidified silver nitrate solution.

Answer: Solution A is sodium chloride.


b) Solution B has a yellow flame test and fizzes on the addition of an acid.

Answer: Solution B is sodium carbonate.


c) Solutions C and D produce a white precipitate with addition of sodium hydroxide. On addition of acidified silver nitrate solution both solutions make a precipitate. Solution C makes a yellow precipitate.

Answer: Solution C is aluminium iodide and solution D is magnesium chloride.