Module 3 - Qualitative Analysis
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which negative ions (anions) can be tested for using qualitative analysis
-carbonate ions (CO32-) -sulphate ions (SO42-) -halide ions (Cl-, Br-, I-)
describe the method to test for carbonate ions
• add dilute nitric acid to the solid or solution • bubble any gas made through limewater
describe the observations for the test for carbonate ions
-fizzing when acid added -limewater turn cloudy -white precipitate formed
describe the equipment used to test for carbonate ions
-conical flask of nitric acid and carbonate -bung and delivery tube connected to conical flask -leading to a test tube with limewater (calcium hydroxide)
equations for carbonate ion test
• when acid is added - any element but for example sodium Na2CO3 + 2HNO3 -> 2NaNO3 + CO2 + H2O • when gas bubbled through limewater Ca(OH)2 + CO2 -> CaCO3 + H2O
describe the method to test for sulphate ions ( stand alone test )
• add dilute hydrochloric acid • follow with aqueous barium chloride solution
describe the method to test for sulphate ions ( when doing a series of tests )
• add dilute nitric acid • follow with aqueous barium nitrate solution
describe the result of testing for sulphate ions
-white precipitate formed = solid barium sulphate
equation for sulphate ion test
Ba2+ (aq) + SO42- (aq) -> BaSO4 (s)
describe the method to test for halide ions
• add dilute nitric acid • followed by aqueous silver nitrate solution (AgNO3)
describe the results of halide ion tests
Cl- -> white precipitate Br- -> cream precipitate I- -> yellow precipitate
equations for halide ion tests
Ag+ (aq) + Cl- (aq) -> AgCl (s) Ag+ (aq) + Br- (aq) -> AgBr (s) Ag+ (aq) + I- (aq) -> AgI (s)
further test 1 for halide ions
-add aqueous dilute ammonia -only Cl- will redissolve -Br- and I- don’t react
observations for further test 1 for halide ions
-if chlorine ions present, turns colourless
further test 2 for halide ions
-then add aqueous concentrated ammonia -only Br- will redissolve -I- doesn’t react
observations for further test 2 for halide ions
-if bromine ions present, turns colourless
state the order of the sequence of tests for negative ions
Ca - Carbonate S - Sulphate H - Halide
explain the order of the sequence of tests for negative ions
• carbonate -as sulfates and halide ions don’t produce a gas when acid is added • sulfate -as barium carbonate is also a white precipitate, so prescence of carbonate needs to be ruled out first • halide -as silver chloride can form a white precipitate
which positive ions (cations) can be tested for using qualitative analysis
-ammonium ions (NH4+)
describe the method for testing for ammonium ions
• add sodium hydroxide • gently warm with bunsen burner • hold damp pH paper above
describe the results of testing for ammonium ions
-paper turns blue