4.2.2 Identifying Anions & Cations

Question 1

How can you generally test for group 2 metal ions?

Answer 1

• Test Tubes
• Add Group 2 chlorides to 4 separate tubes
• Add (Excess NaOH, Ammonia solution, Excess H2SO4) dropwise

Question 2

Positive tests for Mg 2+ ions?

Answer 2

Ammonia - White precipitate ( Mg(OH)2 )
Excess NaOH - White precipitate ( Mg(OH)2 )
Excess H2SO4 - Colourless solution

Question 3

Positive tests for Ca 2+ ions?

Answer 3

Ammonia - No change
Excess NaOH - White precipitate ( Ca(OH)2 )
Excess H2SO4 - Slight White precipitate (CaSO4)

Question 4

Positive tests for Sr 2+ ions?

Answer 4

Ammonia - No change
Excess NaOH - Slight white precipitate ( Sr(OH)2 )
Excess H2SO4 - White precipitate ( SrSO4)

Question 5

Positive tests for Ba 2+ ions?

Answer 5

Ammonia - No change
Excess NaOH - No change
Excess H2SO4 - White precipitate ( BaSO4 )

Question 6

How can one test for Ammonium ions?

Answer 6

• NaOH added to ammonium (chloride)
• Swirl, mix
• Place test tube in warm water bath
• Fumes produced
• Use tongs to hold damp RED litmus paper to mouth of test tube
• Turns blue if ammonia gas present

Question 7

Testing for Halide Ions?

Answer 7

• Nitric Acid + Silver nitrate
• Silver (Chloride, bromide, iodide) formed
• Cl, Br to I form white, cream to yellow precipitates
  .
  (If necessary, adding ammonium : AgCl and AgBr form soluble complex, colourless, AgI forms INSOLUBLE complex, remains yellow)

Question 8

Testing for Hydroxide Ions (OH)

Answer 8

• Red litmus paper turns blue
• Universal Indicator indicates alkaline solution

Question 9

Testing for Carbonate Ions

Answer 9

• Add dilute HCL
• CO2 gas should be observed
  .
  (Can deliver limewater (Ca(OH2)) solution into the test tube via a tube which would turn the test tube solution cloudy because calcium carbonate forms in the presence of CO2)

Question 10

Testing for Sulphate ions

Answer 10

• Add dilute HCL
• Add BaCl
• BaSO4 forms (white precipitate)