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       Qualitative Analysis of Salts

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                                                                                           What is qualitative analysis?

        (a) Check the physical characteristic of the salt

        (b) Test for certain gases

        (b) The effect of the heat on the salt

        (c) Preparation aqueous salt solution to test for the cation and anion in it

        (d) Confirmatory tests for ions.         

 

 

 

 

 

                                                                                        Physical properties

        (a) physical state

        (b) colour of the salt

        (c) solubility of the salt in water

 

1. Physical state of the salt

2. Colour of the salt

Colour Solid state Aqueous state
Sodium, Potassium, Calsium, Magnesium, Aluminium, Zinc, Lead (II), Ammonium (If anion is colourless) White Colourless
Nitrates, Chlorides, Carbonates, sulphates (If cation is colourless) White Colourless
Copper (II) salts:

Copper (II) carbonate

Copper (II) sulphate, Copper (II) nitrate, Copper (II) chloride

Copper (II) oxide

 

Green

Blue

Black

 

Insoluble

Blue

Insoluble

Iron (II) salts:

Iron (II) sulphate, Iron (II) nitrate, Iron (II) chloride

 

Light green

 

Light green

Iron (III) salts:

Iron (III) sulphate, Iron (III) nitrate, Iron (III) chloride

Brown Brown/Brownish yellow
Triiron tetraoxide, Fe3O4 Black Insoluble
Zinc oxide Yellow when hot

White when cold

Insoluble
Lead (II) oxide Brown when hot

Yellow when cold

Insoluble
Magnesium oxide, Aluminium oxide White Insoluble
Sodium oxide, Potassium oxide, Calsium oxide White Colourless

 

 

 

        

       

                                                                                                           

 

 

        

 

 

 

 

 

 

 

 

 

 

 

 

3. Solubility of salt in water

Compounds Solubility of salt in water
Sodium, potassium and ammonium salts All are soluble
Nitrate salts All are soluble
Chlorides salts All are soluble except silver chloride, mercury (I) chloride, lead (II) chloride (soluble in hot water)
Sulphates All are soluble except barium sulphate, lead (II) sulphate, calsium sulphate
Carbonates All are insoluble  except sodium carbonate, potassium carbonate and ammonium carbonate
Oxides All are insoluble except sodium oxide, potassium oxide and ammonium oxide
Hydroxides All are insoluble except sodium hydroxide, potassium hydroxide, calsium hydroxide and barium hydroxide

 

 

 

 

 

 

 

 

 

 

 

 

 

 

        (a) Salt X insoluble in water and contains sulphate ion.

             => Salt X might be lead (II) sulphate, barium sulphate, or calsium sulphate

        (b) Salt Y soluble in water and contains carbonate ion.

              => Salt Y might be sodium carbonate, potassium carbonate, or ammonium carbonate

 

 

               

 

                                                                                               Tests for gases

        (a) is heated

        (b) reacts with dilute or concentrated acid

        (c) is heated with an alkali

Gas Colour Smell Effect on damp litmus paper Confirmatory test
Hydrogen Colourless Odourless - Gives 'pop' sound when burning splint is introduced
Oxygen Colourless Odourless - Rekindless glowing splint
Water vapour Colourless Odourless - White anhydrous copper(II) sulphate turns blue
Carbon dioxide Colourless Odourless Blue to red Turns limewater chalky
Ammonia Colourless Pungent Red to blue White fumes with glass rod dipped in concentrated HCl
Chlorine Greenish yellow Pungent Blue to red, then bleached -
Nitrogen dioxide Brown Pungent Blue to red -
Sulphur dioxide Colourless Pungent Blue to red Turns acidified KMnO4 solution from purple to colourless
Hydrogen chloride Colourless Pungent Blue to red Fumes with glass rod dipped in concentrated NH3 solution

 

 

 

 

 

 

 

                                                                                                                                

 

                                                                                    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                                                            Effect of  heat on salts

        (a) Gas released gives information about type of anion (nonmetal ion) contains (except for ammonium salts)

        (b) Change in colour (residue left) gives information about type of cation (metal ion) contains

 

1. Effect of heat on carbonates (released carbon dioxide).

 

 

Carbonate salts Heat effect
Potassium carbonate

Sodium carbonate

Not decomposed by heat
Calsium carbonates

Magnesium  carbonate

Aluminium carbonate

Zinc carbonate

Iron (III) carbonate

Lead (II) carbonate

Copper (II) carbonate

Decomposed by heat to released carbon dioxide and produce oxide metal. For example:

CaCO3 (s)          CaO (s)   +   CO2 (g)

MgCO3 (s)          MgO (s)   +   CO2 (g)

ZnCO3 (s)          ZnO (s)   +   CO2 (g)

CuCO3 (s)          CuO (s)   +   CO2 (g)

Mercury (II) carbonate

Silver carbonate

Aurum (II) carbonate

Decomposed by heat to released oxygen and carbon dioxide and produce metal. For example:

2HgCO3 (s)          2Hg (s)   +   2CO2 (g)   +   O2 (g)

2AgCO3 (s)          4Hg (s)   +   2CO2 (g)   +   O2 (g)

 

Ammonium carbonate Decomposed by heat to release ammonia, carbon dioxide and water

2(NH4)2CO3 (s)          2NH3 (s)   +   CO2 (g)   +   H2O(g)

 

 

 

                                                                                            

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2. Effect of heat on nitrates salt

Nitrate salts Heat effect
Ammonium nitrate Not decomposed by heat
Sodium nitrate

Potassium nitrate

Decomposed by heat to released oxygen and produce nitrite metal. For example:

2NaNO3 (s)          2NaNO2 (s)   +   O2 (g)

2KNO3 (s)          2KNO2 (s)   +   O2 (g)

Calsium nitrate

Magnesium  nitrate

Aluminium nitrate

Zinc nitrate

Iron (III) nitrate

Lead (II) nitrate

Copper (II) nitrate

 

Decomposed by heat to released oxygen, nitrogen dioxide and metal oxide. For example:

2Ca(NO3)2(s)          2CaO (s)   +   4NO2 (g)   +   O2 (g)

2Mg(NO3)2(s)          2MgO (s)   +   4NO2 (g)   +   O2 (g)

4Al(NO3)3(s)          2Al2O3 (s)   +  12NO2 (g)   +   3O2 (g)

2Zn(NO3)2(s)          2ZnO (s)   +  4NO2 (g)   +   3O2 (g)

2Cu(NO3)2(s)          2CuO (s)   +  4NO2 (g)   +   3O2 (g)

 

Mercury (II) nitrate

Silver nitrate

Aurum (II) nitrate

Decomposed by heat to release oxygen, nitrogen dioxide and produce metal

Hg(NO3)2(s)          Hg (l)   +  2NO2 (g)   +   3O2 (g)

2AgNO3(s)          2Ag (s)   +  2NO2 (g)   +   3O2 (g)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3. Effect oh heat on sulphate salts

        (a) 2FeSO4.7H2O (s)         Fe2O3 (s)   +  SO2 (g)   +   SO3 (g)   +   14H2O (g)

             When iron (II) sulphate is heated strongly, it will released steam fume, slphur dioxide, sulphur trioxide and brown solid of

             iron (III) oxide is left. 

       (b) ZnSO4 (s)          ZnO (s)   +  SO3 (g)  

            CuSO4 (s)          CuO (s)   +  SO3 (g)  

            Fe2(SO4)3 (s)          Fe2O3 (s)   +  3SO3 (g)  

            When zinc sulphate, copper sulphate and iron (III) sulphate heated strongly, oxide metal ang gas sulphur trioxide (acidic) are

            produced.

       (c) When ammonium sulphate is strongly heated, ammonia (alkalic) and acid sulphuric fume are released as below:

               (NH4)2SO4 (s)          2NH3 (g)   +   H2SO4 (g)

 

4. Effect oh heat on chloride salts

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

             When ammonium chloride heated strongly, ammonia (turns damp red litmus paper into blue) and hidrogen chloride fume are

             released. When the white fume cold, white

             solid of ammonium chloride reproduced. The reaction is reversible. Hidrogen chloride is a thick white fume when touched with

             a drop of concentrated liquid ammonia.

 

 

 

 

                                                                                 Tests for anions

 

        (a) the gas released when

             (i) the salt is heated

             (ii) a dilute acid is added to the salt

        (b) the confirmatory test for specific anions that is carbonates, nitrates, sulphates and chlorides

 

1. Tests for carbonates

        MgCO3 (aq)  +   2HCl (aq)               MgCl2 (aq)   +   H2O (l)   +   CO2

        Na2CO3 (aq)  +   2HCl (aq)               2NaCl (aq)   +   H2O (l)   +   CO2

 

2. Tests for nitrates

     About 2cm3 nitrate solution is mixed with dilute sulphuric acid and iron (II) sulphate solution in a test tube. Then a few drops 

        of concentrated sulphuric acid are added slowly and carefully down rhe side of the test tube. (Do not shake the mixture!)

        brown ring is formed between the layers of concentrated acid and nitrate mixture. The brown ring composed of a complex ion,

        confirm the presence of nitrate.

 

3. Tests for chlorides

 

4. Tests for sulphates

    

 

                                                                                          Tests for cations

        (a) Test with aqueous sodium hydroxide, NaOH (aq)

        (b) Test with aqueous ammonia, NH3 (aq)

        (c) Confirmatory tests with specific reagents

        (a) Sodium ion, Na+

        (b) Calcium ion, Ca+

        (c) Iron (II) ion, Fe2+

        (d) Iron (III) ion, Fe3+

        (e) Copper (II) ion, Cu2+

        (f) Lead (II) ion, Pb2+

        (g) Zinc (II) ion, Zn2+

        (c) Ammonium ion, NH4+

Reagent/ Cation NaOH (aq) NH3 (aq)  Na2CO3  (aq)  H2SO4 (aq) or Na2SO4  (aq)  HCl or NaCl (aq) Confirmatory test
Na+ - - - - - Flame test-yellow flame
Al3+ White precipitate, soluble in excess NaOH White precipitate, insoluble in excess NH3 White precipitate, soluble in HNO3  - - -
Pb2+ White precipitate, soluble in excess NaOH White precipitate, insoluble in excess NH3 White precipitate, soluble in HNO3 White precipitate White precipitate, soluble in hot water > KI - yellow precipitate

>KCl - white precipitate

Both soluble in hot water

Zn2+ White precipitate, insoluble in excess NaOH White precipitate, soluble in excess NH3 White precipitate, soluble in HNO3 - - White precipitate with potassium hexacyanoferrate (II) [K4Fe(CN)6]
Mg2+ White precipitate, insoluble in excess NaOH White precipitate, insoluble in excess NH3 White precipitate, soluble in HNO3 - - -
Ca2+ White precipitate, insoluble in excess NaOH No precipitate White precipitate, soluble in HNO3 White precipitate - -
Cu2+ Blue precipitate, insoluble in excess NaOH Blue precipitate, dissolve in in excess NH3 to form deep blue solution Blue precipitate Blue solution Blue solution Dark blue precipitate with potassium hexacyanoferrate (II) [K4Fe(CN)6]
Fe2+ Dirty green precipitate, insoluble in excess NaOH Dirty green precipitate, insoluble in excess NH3 Green precipitate Green solution Green solution >Blue precipitate with potassium hexacyanoferrate (II) [K4Fe(CN)6]

>Dark blue precipitate with potassium hexacyanoferrate (II) [K4Fe(CN)6]

Fe3+ Brown precipitate, insoluble in excess NaOH Brown precipitate, insoluble in excess NH3 Brown precipitate Brown solution Brown solution >Dark blue precipitate with potassium hexacyanoferrate (II) [K4Fe(CN)6]

>Brown solution with potassium hexacyanoferrate (II) [K4Fe(CN)6]

>Blood red solution with potassium hexacyanoferrate (II) [K4Fe(CN)6]

NH4+ NH3 released upon heating - - - - Brown precipitate with Nessler's reagent

 

            

    

 

 

 

 

 

 

 

 

 

 

 

           

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Confirmatory tests for Fe2+, Fe3+, Pb2+ and NH4+

 

 

 

 

 

 

 

 

 

 

        Pb2+   +   2I-             PbI2 (s) (yellow precipitate)

        Pb2+   +   2Cl-             PbCl2 (s) (white precipitate)

        Pb2+   +   SO42-             PbSO4 (s) (white precipitate)

        (b) When heated, PbI2 and PbCl2 dissolves in hot water. Upon cooling PbI2 forms golden-yellow needle-like crystals,        

             while PbI2 forms white crystals.