3.1.4 Qualitative Analysis

Question 1

what is the basis of qualitative analysis

Answer 1

• relies on simple observations rather than measurements
• quick and test tube scale

Question 2

what are the 3 anions you test for in order

Answer 2

• carbonate ions
• sulfate ions
• halide ions

Question 3

how do you test for carbonate ions

Answer 3

• add DILUTE NITRIC ACID (HCl or H2SO4 will interfere with other tests), to solid or solution
• should see EFFERVESCENCE (bubbling/fizzing) due to CO2 produced
• to make sure it is CO2, bubble the gas through LIME WATER (saturated, aqueous Ca(OH)2)
• should turn CLOUDY/MILKY, as CO2 produces a a fine, white precipitate of CaCO3 is produced

Question 4

what is the equation for carbonate ion test

Answer 4

• acid + carbonate ===> salt + water + carbon dioxide
• you are testing the production of CO2, which occurs when a carbonate reacts with acid

Question 5

how do you test for presence of sulfate ion

Answer 5

1) add BARIUM NITRATE solution (could use barium chloride, but this would interfere with halide test later on)
2) should see formation of WHITE PRECIPITATE, due to formation of barium sulfate, which unlike most sulfates, is very insoluble in water

Question 6

what is the equation for the sulfate ion test

Answer 6

Ba2+(aq) + SO42-(aq) ===> BaSO4(s)

Question 7

how do you test for presence of halide ion (Cl, Br, I)

Answer 7

1) add aqueous SILVER NITRATE SOLUTION (AgNO3) to aqueous halide solution
2) should see the formation of coloured precipitates, as silver halides produces, which unlike most are insoluble in water
- AgCl = WHITE
- AgBr = CREAM
- AgI = YELLOW
3) to tell apart the halides, as colours may be close together, test their solubility in AQUEOUS AMMONIA
- AgCl = SOLUBLE IN DILUTE
- AgBr = SOLUBLE IN CONCENTRATED
- AgI = SOLUBLE IN NEITHER, so doesn’t dissolve

Question 8

show the equation for testing for halide ions

Answer 8

Ag+(aq) + X-(aq) ===> AgX(s)

• where X is a halide

Question 9

explain which order the carbonate test is carried out

Answer 9

FIRST:
- neither sulfate or halide ions produce any bubbles when reacting with dilute acid, so would only go positive for carbonate

Question 10

explain which order the sulfate test is carried out

Answer 10

SECOND:
- must be AFTER carbonate test, as if Ba2+ is added to carbonate, it will produce BaCO3, which is also white and insoluble in water, so would show a false positive
- must complete carbonate test first
- must continue adding nitric acid in test until all bubbling stops, to confirm that all the carbonate has been used up and none left to interfere with sulfate test

Question 11

explain the order of the halide test

Answer 11

LAST:
- if silver reacts with carbonate or sulfate, it will produce Ag2CO3/Ag2SO4, which are both insoluble and form precipitates in water
- must be after carbonate test, which has completely finished and got rid of all CO3
- must be after sulfate test, so that all the sulfate ions present will have precipitated out as BaSO4, which can then be filtered out of mixture

Question 12

what is the cation test

Answer 12

NH4+ - ammonium ions

Question 13

how do you test for the presence of ammonium ions

Answer 13

1) add AQUEOUS SODIUM HYDROXIDE (NaOH)
2) WARM MIXTURE, e.g. in a water bath
3) ammonia gas should be released (won’t see as gas bubbles though as ammonia is very soluble in water)
4) may be able to SMELL AMMONIA
5) BUT, test with damp pH indicator litmus paper, originally red
6) will TURN BLUE, as ammonia is alkaline

Question 14

what is the equation for the presence of ammonium ions

Answer 14

NH4+(aq) + OH-(aq) ===> NH3(g) + H2O(l)