Tests for ions (Extended response)

Question 1

The labels have fallen off four different containers of white powders:

ammonium chloride NH[4]Cl
sodium chloride NaCl
sodium sulfate Na[2][SO[4]
sodium carbonate Na[2][CO[3]

Explain how, using chemical tests, you could identify which container held which substance. [6]

Answer 1

• flame test
• three sodium compounds will produce a yellow flame due to their sodium ions
• fourth compound should be ammonium chloride
• identity of the ammonium chloride can be confirmed by adding sodium hydroxide solution
• when the mix is warmed, ammonia should be given off, turning damp litmus paper blue
• dissolve the three sodium compounds in water
• add dilute hydrochloric acid
• the sodium carbonate will produce bubbles of CO2 that will turn limewater cloudy
• add BaCl[2] solution to the remaining solutions
• the sodium sulfate solution will produce a white precipitate
• the last solution should be sodium chloride
• to check, dissolve in water
• add a few drops of dilute nitric acid
• add silver nitrate solution
• should produce a white precipitate of silver chloride

Question 2

A technician found some white crystals in an unlabelled beaker in a laboratory. He knew that the substance could be one of the following:

• lithium chloride
• lithium carbonate
• potassium chloride
• potassium iodide

Devise a series of tests that the technician could carry out to identify the substance. [6]

Answer 2

test for carbonate ions:

• add dilute acid/HCl/nitric acid
• if bubbles of gas form then carbonate ions are present
• confirmatory test on gas, limewater turns milky

test for halide ions:

• dissolve in water
• add dilute nitric acid
• add silver nitrate solution
• chloride ions give white precipitate
• iodide ions give yellow precipitate

test for metal ions:

• flame test (on solid or solution)
• lithium ions give red flame
• potassium ions give lilac flame