8.3 qualitative analysis module 3

Question 1

define qualitative analysis

Answer 1

analysis that relies on observations and can be carried out quickly on test-tube scale

Question 2

test for carbonates

Answer 2

1) add dilute nitric acid
2) if bubbles are seen, could be a carbonate
3) bubble gas through limewater- saturated solution of calcium hydroxide
carbon dioxide reacts to form a white precipitate of calcium carbonate which makes the limewater cloudy

Question 3

test for sulfates

Answer 3

1) aqueous barium ions are added as barium chloride or barium nitrate to a solution of an unknown compound
2) if halide test is carried out after, barium nitrate must be used

Question 4

test for halide ions

Answer 4

1) add aqueous silver nitrate to an aqueous solution of a halide
2) silver halide precipitate forms different colours
silver chloride- white
silver bromide- cream
silver iodide- yellow
3) add aqueous ammonia to test solubility of the precipitate as it is difficult to tell between the colours
chloride- soluble in dilute NH3
bromide- soluble in conc NH3
iodide- insoluble in conc NH3

Question 5

what is the sequence of tests for anions?

Answer 5

1) carbonates
2) sulfates
3) halides

Question 6

why is the carbonate test done first?

Answer 6

neither sulfate nor halide ions produce bubbles
test can be carried out without the possibility of an incorrect conclusion

Question 7

why is the sulfate test done second?

Answer 7

barium carbonate is white and insoluble in water
carbonate test must be carried out first to insure no carbonate is present

Question 8

why is the halide test done last?

Answer 8

silver carbonate and silver sulfate are insoluble in water and will form precipitates in this test
halide test must be carried out last to rule out possibility of it being a sulfate or a carbonate

Question 9

what happens if there is a mixture of ions in the solution?

Answer 9

1) carbonate test first, if there are bubbles keep adding nitric acid until bubbling stops so all carbonate ions are removed
2) sulfate test next, add excess of barium nitrate, any sulphates will precipitate out as barium sulfate
3) halide test, add silver nitrate

Question 10

what is the test for ammonium ions?

Answer 10

1) add aqueous sodium hydroxide to a solution of an ammonium ion
2) ammonia gas is released, ammonia is very soluble in water so unlikely to see bubbles
3) mixture is warmed and ammonia gas is released
4) may be able to tell by smell
use moist pH indicator paper to test gas
ammonia is alkaline so paper will turn blue