Qualitative analysis

Question 1

general info- Quant analysis

Answer 1

these tests indicate whether a particular species is present or not

doesn’t tell us conc present

allow us to identify cations and anion in salt - most only have 2 ions some have 3

for salt to be tested it must be dissolved In water + be in aqueous solution

negative results as important as positive

involves formation of colored precipitate- an insoluble solid formed when 2 solutions are mixed

Question 2

test for carbonate ions - CO3 2-

Answer 2

add dilute aqueous acid to suspected carbonate solid/solution

usually HNO3 as it doesn’t interfere with other tests

obsevations if positive- effervescent due to CO2 produced, can identify Co2 by bubbling through lime water to produce white precipitate

ionic equation-CO3 2- +2H+–>H2O+CO2

Question 3

Test for sulfate (Vl) ions, SO4 2-

Answer 3

Add dilute nitric adic followed by barium nitrate solution to suspected sulfate solution

can use chloride but don’t often as it interacts with later tests

positive result- white precipitate forms- insoluble barium sulfate

ionic equation- Ba 2+ + SO4 2- –> BaSO4

Question 4

Test for Halide ions

Answer 4

Add dilute nitric acid followed by aqueous sliver nitrate solution, AgNO3
to suspected halide solution

positive result- white(Cl) cream(Br) or yellow (I) precipitate forming- insoluble silver halide

ionic equation- Ag+ + X- —–> AgX

can further distinguish between the 3 precipitates by solubility in Ammonia solution

chlorine- completely soluble (dilute+conc)
bromine- partially soluble (only conc)
Iodine- insoluble (doesn’t dissolve in either)

Question 5

Sequence for tests

Answer 5

CASH

Carbonate- add dilute nitric acid till bubbling stops to remove all CO3 ion

sulphate- add excess Ba(NO)3 to remove all SO4 ions

halide

why?
carbonates will produce CO2 with an acid, no effervescence no carbonate

sulphates produce white precipitate od BaSO4 when Ba ions added, carbonates will also form a white precipitate BaCO3 so if u do sulphate test in carbonate u get false positive.Have to do Carbonate test first to insure no carbonates present

Halides produce precipitate of AgX when Ag ions added. carbonates and sulphates will also form precipitates Ag2CO3 and Ag2SO4 so a false positive. So you must insure neither is present first

Question 6

test for cations

Answer 6

Test for Ammonium ions NH4+

Add warm dilute aqueous sodium hydroxide to solution

positive result- ammonia gas NH3 produced when solution warmed, this can be detected as it turns moist red litmus paper blue (also has a distinctive smell)

ionic equation- NH4+(aq) + OH- (aq) —–> NH3 (g) +H2O (l)