Qualitative Analysis

Question 1

What is a test for cations (ammonium test)?

Answer 1

1. Ammonium ions and hydroxide ions react to form ammonia gas
2. Add aqueous sodium hydroxide to the unknown solution and if ammonium is present ammonia gas will be produced
3. Ammonia gas is very soluble in water so it is helpful to warm the solution and test any gas evolved with damp indicator paper, and ammonia will turn the indicator paper blue

Question 2

What is a test for cations (precipitation reactions of transition metal ions)?

Answer 2

1. Transition metal ions react with NaOH to make coloured precipitates
2. Add aqueous sodium hydroxide to the unknown solution and observe colour of precipitate

Question 3

What colour is Fe2+?

Answer 3

• Colour of starting solution: pale green
• Colour of precipitate: green
• Formula of precipitate: Fe(OH)2

Question 4

What colour is Fe3+?

Answer 4

• Colour of starting solution: pale yellow
• Colour of precipitate: orange-brown
• Formula of precipitate: Fe(OH)3

Question 5

What colour is Cu2+?

Answer 5

• Colour of starting solution: light blue
• Colour of precipitate: blue
• Formula of precipitate: Cu(OH)2

Question 6

What colour is Mn2+?

Answer 6

• Colour of starting solution: pale pink (looks colourless tho)
• Colour of precipitate: light brown
• Formula of precipitate: Mn(OH)2

Question 7

What colour is Cr3+?

Answer 7

• Colour of starting solution: green
• Colour of precipitate: grey-green
• Formula of precipitate: Cr(OH)2

Question 8

What is a test for anions (carbonate test)?

Answer 8

1. Carbonates react with acids to form carbon dioxide gas
2. Add dilute nitric acid to the unknown solution and see if you observe bubbling
3. To prove that the gas is carbon dioxide, bubble through limewater

Question 9

What is a test for anions (sulphate test)?

Answer 9

1. Barium sulfate is very insoluble in water
2. Add an aqueous solution of barium ions (usually barium chloride or barium nitrate) to your unknown solution. Any sulphate ions present will form a very dense, white barium sulphate precipitate

Question 10

What is a test for anions (halide test)?

Answer 10

1. Silver halides are insoluble in water
2. Add aqueous silver nitrate to your unknown solution, Any halides will form with either a white (Cl-), cream (Br-) or yellow (I-) precipitate
3. Confirm with solubility in ammonia

Question 11

What is the test for hydrogen, oxygen and nitrogen and chlorine?

Answer 11

1. H2 - squeaky pop
2. O2 - relights a glowing splint
3. N2 - extinguishes a lit splint
4. Cl2 - bleaches damp litmus paper

Question 12

What sequence of tests must be taken out for testing ions? Why is this important?

Answer 12

1. Carbonate test (CO32-)
2. Sulfate test (SO42-)
   3 Halide test (X-)
   -Avoids false positives

Question 13

What false positive could you get?

Answer 13

1. Neither the sulphate or the halide test form bubbles with acid
2. BaCO3 is white and insoluble in water so could give a false positive in the sulphate test
3. Ag2CO3 and Ag2SO4 are both insoluble in water and so could give a false positive in the halide test

Question 14

Why do you use nitric acid?

Answer 14

All nitrates are soluble and so no false positives