module 3: 8.3 qualitative analysis

Question 1

describe the carbonate test

Answer 1

1. add dilute nitric acid
2. if bubbles are present a carbonate could be present
3. to prove the presence of CO2, bubble gas through limewater and the limewater should turn cloudy if present

Question 2

describe the sulfate test

Answer 2

barium sulfate (BaSO4) is very insoluble. 
if sulfate is thought to be present add barium chloride. If a white precipitate forms, then sulphate is present. If you want to do a halide test afterwards, use barium nitrate.`

Question 3

describe halide test

Answer 3

1. add silver nitrate (AgNO3) to aq solution of a halide
2. silver halide precipitates are different colours (Cl = white, Br = cream, I = yellow)
3. add aq ammonia to test the solubility of the precipitate

Question 4

Cl- halide colour and solubility

Answer 4

white

soluble in dilute aq NH3 (ammonia)

Question 5

Br- halide colour and solubility

Answer 5

cream

soluble in conc aq NH3 (ammonia)

Question 6

I- halide colour and solubility

Answer 6

yellow

insoluble in conc aq NH3 (ammonia)

Question 7

What are the sequences of the tests and why?

Answer 7

Carbonate —> sulfate —> halide
Neither sulfate or halide produce bubbles with dilute acid meaning they are not affected by carbonate test.
barium carbonate is white and insoluble in water, meaning if a sulfate test is done on it a white precipitate will form
Silver carbonate and silver sulfate are both insoluble in water and form precipitates.

Question 8

Mixture of ions: carbonate test

Answer 8

If you see bubbles, keep adding dilute nitric acid until the bubbling stops
This means all the CO3 ions will have been removed, leaving none to be left to react with other tests.

Question 9

Mixture of ions: sulfate test

Answer 9

To the solution left from CO2 test, add an excess of Ba(NO3)2. And sulfate ions present will precipitate out as barium sulfate
Filter the solution to remove barium sulfate

Question 10

Mixture of ions: halide test

Answer 10

To the solution left from the sulfate test add AgNO3.
Any carbonate or sulfate ions initially present have already been removed; therefore any precipitate formed must involve halide ions.
Add NH3 to confirm which halide you have