8.3 Qualitative analysis of ions (tests)

Question 1

What is the test for carbonates?

Answer 1

Carbonates react with acids to form carbon dioxide gas
Na2CO3 (aq) + 2HNO3 (aq) –> 2NaNO3 (aq) + CO2 (g) + H2O (l)
After this bubble the gas through lime water - Ca(OH)2 (aq) which should form white ppt of calcium carbonate

Question 2

What is the test for sulfates?

Answer 2

Aqueous barium ions (usually barium chloride or barium nitrate - use nitrate if halide test after) are added and a white precipitate of barium sulphate forms
Ba2+ (aq) + SO42- (aq) –> BaSO4 (s)

Question 3

What is the test for halides?

Answer 3

Add aqueous silver nitrate, AgNO3, to an aqueous solution of a halide
Ag+ (aq) + X-(aq) –> AgX(s)
Chloride - white (soluble in dilute NH3)
Bromide - Cream (soluble in conc NH3)
Iodide - Yellow (insoluble in conc NH3)
Add aqueous ammonia to test the solubility of the precipitate to tell the 3 colours apart

Question 4

What are the sequence of the tests?

Answer 4

Carbonate, CO3 2-
Sulfate, SO4 2-
Halides CL-, Br- and I-

Question 5

Explain the sequence of the tests

Answer 5

• Sulfate nor halide ions produced bubbles with dilute acid so carbonate test will not have an incorrect conclusion
• Barium carbonate is white and insoluble in water, if you carry out a sulfate test on a carbonate you will get a white ppt so need to know no carbonate is present
  -Silver carbonate and silver sulfate are both insoluble in water and will form as precipitates in this test

Question 6

How would you test a mixture of ions?

Answer 6

Carry out the same sequence of tests on the same solution
1. Carbonate test (use nitric acid) and continue adding until bubbles stops so none left over
2. Sulfate test Add excess of barium nitrate and filter to remove barium sulfate
3. Halide test, add silver nitrate