RP 4 - Identification of Cations and Anions

Question 1

How do you test for Group 2 metal cations?

Answer 1

• Place 10 drops of a Group 2 compound in test tube (0.1 mol dm^-3)
• Add 10 drops of 0.6 mol dm^-3 NaOH to test tube. Record any observations
• Continue to add NaOH so it’s in excess. Record any observations
• Repeat with other Group 2 compounds

Question 2

What are the results from testing for Group 2 metal cations?

Answer 2

• For BaCl2, colourless solution
• For CaCl2, initially colourless solution, slight white precipitate with 10 drops of NaOH or even excess NaOH
• For MgCl2, initially colourless solution, slight white precipitate with 10 drops of NaOH and white precipitate with excess NaOH
• For SrCl2, initially colourless solution, slight white precipitate with 10 drops of NaOH or even excess NaOH

Question 3

How would you distinguish between CaBr2 and SrCl2 and what are the results from these reactions?

Answer 3

• Place 10 drops of BaCl2 (0.1 mol dm^-3) in a test tube
• Add 10 drops of H2SO4 (1 mol dm^-3). Record observations
• Continue to add H2SO4 until in excess. Record observations
  Repeat for the other Group 2 compounds
• Results:
  1. BaCl2 is initially a colourless solution, then a white precipitate when H2SO4 is added as both 10 drops & in excess
  2. CaCl2 is initially a colourless solution, then a slight white precipitate when H2SO4 is added as both 10 drops & in excess
  3. MgCl2 is a colourless solution initially, then a slight white precipitate when 10 drops of H2SO4 is added, then a colourless solution again when H2SO4 is in excess
  4. SrCl2 is initially a colourless solution, then a white precipitate when H2SO4 is added as both 10 drops & in excess

Question 4

What are the overall results for the Group 2 reactions?

Answer 4

• For excess NaOH, there’s no change with Ba^2+, a slight white precipitate for both Ca^2+ and Mg^2+ of Ca(OH)2 and Mg(OH)2 respectfully and a slight white precipitate with Sr^2+
• For excess H2SO4, there’s a white precipitate with both Ba^2+ & Sr^2+, a slight white precipitate with Ca^2+ and a colourless solution with Mg^2+

Question 5

How do you test for NH4^+ ions?

Answer 5

1. Place 10 drops of NH4Cl into a test tube
2. Add 10 drops of NaOH solution and shake
3. Warm the solution in the test tube
4. Test the gas released with damp red litmus paper. If it goes blue, ammonium ions are present

Question 6

How do you test for Group7/Halide ions and what are the results from these reactions?

Answer 6

1. To the compound being tested, add nitric acid and silver nitrate. Record observations
2. To samples of this solution, add dilute and then concentrated ammonia
3. Results:
   - Cl- is a white precipitate from silver nitrate, then the precipitate disappears and the solution is colourless with dilute NH3. It remains colourless with concentrated NH3
   - Br- is a cream precipitate for silver nitrate as well as dilute NH3. The precipitate disappears the the solution is colourless when concentrated NH3 is added
   - I- is a yellow precipitate for silver nitrate, dilute NH3 and concentrated NH3

Question 7

How do you test for OH- ions?

Answer 7

1. Test a 1 cm depth of solution in a test tube with red litmus paper or universal indicator paper
2. Record your observations
3. NaOH till turn damp red litmus paper blue

Question 8

How do you test for carbonate (CO3^2-) ions?

Answer 8

1. Put 2 cm^3 of Ca(OH)2 into a test tube
2. Add 3 cm^3 Na2CO3 (0.5 mol dm^-3) in another test tube then add an equal volume of dilute HCl (1.0 mol dm^-3)
3. Immediately put in delivery tube with open end into the Ca(OH)2 test tube
4. If Ca(OH)2 goes cloudy, carbonate ions were present in the other test tube

Question 9

How do you test for sulphate (SO4^2-) ions?

Answer 9

• Add HCl and BaCl2 to the suspected sulphate solution
• If sulphate ions are present, a white precipitate of BaSO4 will form

Question 10

What is the order of testing ions and why?

Answer 10

• Carbonate -> Sulphate -> Halide
• This prevents false positive results from occurring i.e. Unexpected insoluble precipitates such as Ag2SO4, Ag2CO3 and BaCO3 could form