Identifying Anions & Cations [RPA 4]

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)