Module 3 - Qualitative Analysis

Question 1

which negative ions (anions) can be tested for using qualitative analysis

Answer 1

-carbonate ions (CO32-)
-sulphate ions (SO42-)
-halide ions (Cl-, Br-, I-)

Question 2

describe the method to test for carbonate ions

Answer 2

1. add dilute nitric acid to the solid or solution
2. bubble any gas made through limewater

Question 3

describe the observations for the test for carbonate ions

Answer 3

-fizzing when acid added
-limewater turn cloudy
-white precipitate formed

Question 4

describe the equipment used to test for carbonate ions

Answer 4

-conical flask of nitric acid and carbonate
-bung and delivery tube connected to conical flask
-leading to a test tube with limewater (calcium hydroxide)

Question 5

equations for carbonate ion test

Answer 5

1. when acid is added - any element but for example sodium
   Na2CO3 + 2HNO3 -> 2NaNO3 + CO2 + H2O
2. when gas bubbled through limewater
   Ca(OH)2 + CO2 -> CaCO3 + H2O

Question 6

describe the method to test for sulphate ions ( stand alone test )

Answer 6

1. add dilute hydrochloric acid
2. follow with aqueous barium chloride solution

Question 7

describe the method to test for sulphate ions ( when doing a series of tests )

Answer 7

1. add dilute nitric acid
2. follow with aqueous barium nitrate solution

Question 8

describe the result of testing for sulphate ions

Answer 8

-white precipitate formed = solid barium sulphate

Question 9

equation for sulphate ion test

Answer 9

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

Question 10

describe the method to test for halide ions

Answer 10

1. add dilute nitric acid
2. followed by aqueous silver nitrate solution (AgNO3)

Question 11

describe the results of halide ion tests

Answer 11

Cl- -> white precipitate
Br- -> cream precipitate
I- -> yellow precipitate

Question 12

equations for halide ion tests

Answer 12

Ag+ (aq) + Cl- (aq) -> AgCl (s)
Ag+ (aq) + Br- (aq) -> AgBr (s)
Ag+ (aq) + I- (aq) -> AgI (s)

Question 13

further test 1 for halide ions

Answer 13

-add aqueous dilute ammonia
-only Cl- will redissolve
-Br- and I- don’t react

Question 14

observations for further test 1 for halide ions

Answer 14

-if chlorine ions present, turns colourless

Question 15

further test 2 for halide ions

Answer 15

-then add aqueous concentrated ammonia
-only Br- will redissolve
-I- doesn’t react

Question 16

observations for further test 2 for halide ions

Answer 16

-if bromine ions present, turns colourless

Question 17

state the order of the sequence of tests for negative ions

Answer 17

Ca - Carbonate
S - Sulphate
H - Halide

Question 18

explain the order of the sequence of tests for negative ions

Answer 18

1. carbonate
   -as sulfates and halide ions don’t produce a gas when acid is added
2. sulfate
   -as barium carbonate is also a white precipitate, so prescence of carbonate needs to be ruled out first
3. halide
   -as silver chloride can form a white precipitate

Question 19

which positive ions (cations) can be tested for using qualitative analysis

Answer 19

-ammonium ions (NH4+)

Question 20

describe the method for testing for ammonium ions

Answer 20

1. add sodium hydroxide
2. gently warm with bunsen burner
3. hold damp pH paper above

Question 21

describe the results of testing for ammonium ions

Answer 21

-paper turns blue