2.10   Identification Tests

 

Observations and chemistry involved in the following tests (mixtures excluded).

Flame test colours  Li, Na, K, Ca, Sr, Ba ions.

 

Flame test colours

 

Metal          

Colour

lithium           

magenta

sodium

yellow

potassium      

lilac

calcium         

brick red

strontium       

blood red

barium        

apple green

 

 

Gases- any one chemical test          

 H2,  O2,  Cl2,  CO2,  SO2,  HCl,  NH3

 

GASES

 

(i)           Hydrogen - H2

              Burns with a yellow flame - pops in air

              2H2 (g)  +   O2 (g)                              2H2O (l)

 

 

(ii)          Oxygen - O2

 

              Rekindles a glowing splint

 

 

(iii)          Chlorine - Cl2

             

(a)  bleaches damp litmus paper

 

              Cl2 (g) +  H2O (l)                            HCl (aq) +  HOCl (aq)

                                                                                        (bleach)

              (b)  bubble through potassium bromide solution colour change: colourless to brown

 

              Cl2 (g)  + 2KBr (aq)                           Br2 (aq)  + 2KCI (aq)

                             colourless                            brown

 

(iv)         Carbon dioxide - CO2

 

              Bubble gas through lime water - turns milky

 

              CO2 (g)  +  Ca(OH )2 (aq)                              CaCO3 (s)  +  H2O (1)

 

 

(v)          Sulphur dioxide - SO2

 

               (a)  turns damp blue litmus red

               (b)  bubble through potassium dichromate(VI) solution: colour change from orange to green.

 

               2H+ (aq) + 3SO2 (g) + Cr2O72- (aq)                     2Cr3+ (aq) + 3 SO42- (aq) + H2O (l)                

                                                  (orange)                           (green)

 

 

 (vi)         Hydrogen chloride  - HCl

 

              (a)  turns damp blue litmus red

              (b)  forms white fumes with conc. ammonia

 

              HCl (g)  +  NH3 (g)                            NH4Cl (s)

                                                                         white

 

   (vii)       Ammonia  - NH3                                                          

 

              (a) turns damp red litmus blue

              (b) forms white fumes with conc. hydrochloric acid

 

              HCl (g) + NH3 (g)                                  NH4Cl (s)

 

 

Anions

 

Cl-, Br- and I- using Ag+ followed by NH3 (aq)

CO32- and HCO3 using dilute acid and Mg2+

SO42- using dilute acid and Ba2+

SO32- using dilute acid

 

 

(a)               Chloride Cl-, Bromide  Br-, and Iodide I-

                

                 To a solution of the ions add a solution of silver nitrate.

 

 

Ag+ (aq)

Chloride

White precipitate insoluble in dilute HNO3.

Soluble in dilute NH3 (aq)

Soluble in conc. NH3

Bromide

Cream precipitate insoluble in dilute HNO3.

Insoluble in dilute NH3 (aq).

Soluble in conc. NH3

Iodide

Yellow precipitate insoluble in dilute HNO3.

Insoluble in dilute NH3 (aq).

Insoluble in conc. NH3.

 

                                                                                                                            

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

              where x is Cl-, Br-, I-.

 

 

(b) Carbonate, CO32- and hydrogen carbonate, HCO3-

 

(i)  Add dilute acid - in each case carbon dioxide gas is evolved.

(ii)  To a solution add a few drops of magnesium sulphate solution.

 

 

CO32- (aq)

HCO3- (aq)

 

          Mg2+(aq)

White precipitate of MgCO3

No precipitate.

Mg(HCO3)2 , is soluble.

On heating it decomposes forming a precipitate of MgCO3

 

 

   (c)        Sulphate (V1), SO42-, and sulphate(IV), (sulphite) SO32-

 

(i)           Addition of dilute acid

 

               Na2SO3 + 2 HCl                       2 NaCl + SO2 + H2O then test for SO2.

 

(ii)          To a solution add dilute HCl followed by barium chloride solution.

 

Sulphate (VI)  SO42-  -  white precipitate.

 

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

 

Sulphate (IV)  SO32-  -  no precipitate.

 

 

SO42- (aq)

SO32- (aq)

(i) dilute acid

no reaction

effervescence of SO2

(ii) dilute HCl  +  BaCl2 (aq)

white precipitate of BaSO4

no precipitate.

(iii) BaCl2 (aq)  +  HCl

white precipitate. 

Insoluble in HCl

white precipitate.

Soluble in HCl

 

                                                                                                                            

                                                                                                                       

(d)          Nitrate (V) ion - NO3-

 

              Dissolve 2 or 3 small crystals in one quarter of a test tube of sodium hydroxide solution.  Add a small quantity of aluminium.  Warm the mixture.  Ammonia gas is evolved which can be tested in the usual way.

 

               5OH-  + 18H2O + 3NO3- + 8A1                             3NH3   +  8Al(OH)4-

 

 

(e)        Nitrate (III) ion - NO2-

 

              To a solution add dilute hydrochloric acid.  Immediate effervescence of a brown gas NO2,  is observed and a pale blue solution.

 

                        3NO2- (aq) + 2H+                     2NO (g) + NO3- + H2O

 

               NO2 gas is formed when NO reacts with air

 

                        2NO (aq) + O2                        2NO2 (g)

                                                                                  (brown gas)

 

 

 

 

 

Cations in aqueous solution

 

Ag+ using halide ions

Cu2+, Fe2+, Al3+, Zn2+, Ni2+ using NaOH (aq) and NH3 (aq)

Mn2+ using NaOH followed by H2O2

Fe3+ using SCN-

Fe2+ using [Fe(CN)6]3-

Co2+ using conc. HCl

 

Identification of cations in solution

 

(i)         Silver (Ag+) and lead (Pb2+)

Similar test to that for halide ions

 

To a solution of the ions add a solution of sodium halide.

 

 

Chloride

Bromide

Iodide

 

 

Ag+ (aq)

White precipitate insoluble in dilute HNO3.

Soluble in dilute NH3 (aq)

Soluble in conc. NH3

Cream precipitate insoluble in dilute HNO3.

Insoluble in dilute NH3 (aq).

Soluble in conc. NH3

Yellow precipitate insoluble in dilute HNO3.

Insoluble in dilute NH3 (aq).

Insoluble in conc. NH3.

 

Pb2+ (aq)

 

White precipitate. Soluble in hot water.

 

Cream precipitate

 

Yellow precipitate

 

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

              Pb2+ (aq) + 2X- (aq)                           PbX2 (s)

where x is Cl-, Br-, I-.

 

 

(ii)                    Cu2+, Fe2+, Al3+, Zn2+, Ni2+

 

 

Cu2+

Fe2+

Al3+

Zn2+