3.8.3 Qualitative Analysis

Question 1

Carbonate test

Answer 1

• Add a dilute acid and look for effervescence from CO2 gas. Produces bubbles in acid.
• Bubble through limewater = cloudy.
  CO3(2-) + 2H+ –> CO2 + H2O

Question 2

Sulfate test

Answer 2

1. Add a solution containing Ba 2+ ions (e.g Ba(NO3)^2
2. Forms white precipitate of BaSO4
   Ba2+(aq) + SO4^2- –> BaSO4

Question 3

Specific order for tests to avoid false positives

Answer 3

Carbonate then sulfate test then halide test.

Question 4

Why is it important to test false positives?

Answer 4

If sulfate test is carried out first the Ba2+ will react with CO3^2- and form BaCO3 white precipitate (false positive). If halide test is carried out first, carbonate and sulfate will react with Ag+ to make Ag2CO3 and Ag2SO4 precipitates.

Question 5

Testing ammonium compounds

Answer 5

1. Add NaOH
2. Heat gently
3. Positive test= ammonia gas will be produced (rotten egg smell)
4. Use damp red litmus paper and NH3 will dissolve and turn paper blue