Qualitative analysis

Question 1

Describe the test for carbonates. (3)

Answer 1

• Add dilute nitric acid to unknown substance.
• Bubbling/ fizzing occurs which indicates that gas has been produced (CO2)
• To ensure it is CO2, gas is bubbled through lime water and a positive result should go cloudy/milky indicating Calcium carbonate has formed

Question 2

Equations for the test for carbonates. (2)

Answer 2

When carbonate reacts with HNO3:
XCO3 + HNO3 = XNO3 + CO2 + H2O

When CO2 is passed through limewater:
CO2 + Ca(OH)2 = CaCO3 + H2O

Question 3

Describe the test for sulfates (2)

Answer 3

• Barium ions are added to unknown compound

- Forms BaSO4 which is insoluble and is therefore a white ppt

Question 4

Equations for Sulfate test. (2)

Answer 4

eg BaX + YSO4 = BaSO4 + YX

Ionic: Ba2+ + SO4 2- = BaSO4 (s)

Question 5

Describe the test for ammonium ions. (3)

Answer 5

• Add NaOH (aq) to the unknown solution.
• NH3 gas should be produced but since it dissolves in water, it has to be heated to release the gas.
• Test the gas released with pH indicator paper which should turn purple as ammonia is alkali.

Question 6

Ionic equation for ammonium test.

Answer 6

NH4+ + OH- = NH3(g) + H20(l)

Question 7

Explain the sequence of tests for all the qualitative analysis. (3)

Answer 7

1. ) Carbonate Test: other tests do not produce bubbles with dilute acid
2. ) Sulfate test: BaCO3 is also white ppt so CO3- test is before to ensure it is not that.
3. ) Halide test: Silver carbonate and sulfate are also insoluble ppt so the previous tests go first to eliminate those ions.