Paper 3: Required practical 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 a test tube
• Add 10 drops of 0.6 mol dm^-3 NaOH to test tube - record obs
• Continue to add NaOH so it is in excess - record obs
• Repeat with other group 2 compounds

Question 2

What are the results from testing for group 2 cations?

Answer 2

Barium chloride - colourless soltution, colourless solution, colourless solution
Calcium bromide - colourless soltution, slight white precipitate, slight white precipitate
Magnesium chloride - colourless soltution, slight white precipitate, white precipitate
Strontium chloride - colourless soltution, slight white precipitate, slight white precipitate

Question 3

How would you distinguish between calcium bromide and strontium chloride?

Answer 3

• Add 10 drops of BaCl2 in a test tube
• Add 10 drops of sulfuric acid - record obs
• Continue to add H2SO4 until in excess - record obs
• Repeat for the other group 2 compound

Question 4

What will the results be?

Answer 4

Barium chloride - colourless soltution, white precipitate, white precipitate
Calcium bromide - colourless soltution, slight white precipitate, slight white precipitate
Magnesium chloride - colourless soltution, slight white precipitate, colourless solution
Strontium chloride - colourless solution, white precipitate, white precipitate

Question 5

How do you test for ammonium ions?

Answer 5

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

Question 6

How do you test for group 7/halide ions?

Answer 6

• To the compound being tested, add nitric acid and silver nitrate - record obs
• To samples of this solution, add dilute and then conc ammonia

Question 7

What are the results?

Answer 7

• Cl- -> white precipitate, colourless solution, remains colourless
• Br- -> cream precipitate, cream precipitate, colourless solution
• I- -> yellow precipitate, yellow precipitate, yellow precipitate

Question 8

How do you test for hydroxide ions?

Answer 8

1. Test a 1cm depth of solution in a test tube with red litmus paper or universal indicator paper
2. Record obs
3. Sodium hydroxide will turn damp red litmus paper blue

Question 9

How do you test for carbonate ions?

Answer 9

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

Question 10

How do you test for sulfate ions?

Answer 10

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

Question 11

What is the order of testing ions?

Answer 11

Carbonate -> Sulfate -> Halide
prevents false positives