Qualitative Analysis : Dry Tests

 

Dry Tests

1. Physical Examination

In physical examination of compound colour, smell, density etc, are observed.

Physical Examination

Observation Inference
(A) Colour of Compound
(i) Blue, Whose aqueous solution is also blue. Hydrated salts of Cu2+ (except CO32–and Cl–)
(ii) Blue, Whose aqueous solution is pink Anhydrous salts of cobalt
(iii) Colourless substance Transition metals absent (except some salts of Mn2+)
(iv) Green, Whose aqueous solution is also green and on dilution colour is not changed too much Slats of Ni2+, Cr3+
(v) yellow-green, aqueous solution is light green or almost colourless   Salts if Fe2+
(vi) Yellow-brown, aqueous solution is yellow Salts of Fe3+
(vii) Violet-pink, aqueous solution is pink Salts of cobalt
(viii) Black, Substance. CuO, Nio, SnO, FeS, CuS, HgS, PbS, NiS, CoS, CuBr2, Ag2S, Cu2S, MnO2, Fe3O4, FeO, Co3O4, Ni2O3 etc.
(ix) Brown Substance CdO, PbO2, SnS, Bi2S3 MnCO3 (pale brown), CuCrO4, SnS.
(x) Yellow substance Bi2O3, CdS, SnS2, As2S3, PbI2, Chromates, As2S5, AgI etc.
(xi) Red substance Cu2O, HgO, Pb3O4, HgI2 etc.
(xii) Orange substance Cr2O72– salts, Sb2S3 etc.
(xiii) Green substance K2MnO4, Carbonate or Chloride of Cu2+
(xiv) Light pink Hydrated Mn2+ salts
(xv) Purple. KMnO4 & other permanganates, some Cr3+ salts.
(B) Odour of Compound
(i) Ammonical smell NH4+ salts
(ii) Vinegar like smell Acetates
(iii) Like smell from rotten eggs. S2– salts.
(C) Density
(i) Mixture or salt is heavy Hg and Pb salts
(ii) Mixture or salt is light. Salts of Zn, Al, Bi, Ca, Ba, Sr, Mg etc. (Chiefly carbonates)
(D) Exposure of Air
(i) Colourless, deliquescent substance which is changed to paste. SbCl3, ZnCl2, CaCl2, Zn(NO3)2
(ii) Coloured, deliquescent substance stance which is changed to paste. Cu(NO3)2, FeCl3, Fe(NO3)2, MnCl2 etc.
(iii) Colour of substance is changed from white to yellow Cd salts
(iv) Colour of substance is changed from white to black Pb or Bi salts.

 

NOTE:

  • When mixture contains more than one coloured salts, then original colour of salt may change.
  • Some ammonium salts and sulphide do not give smell.

 

2. Effect of Heating

In a clean, dry test tube, substance is first heated gently and then strongly.

Effect of heating

Observation Inference
1. Substance melts  Slats of alkali metals chloride of Hg, Pb, Ag etc. having water of crystallization
2. Substance cracks KI, NaCl, Pb(NO3)2, Ba(NO3)2 etc.
3. Substance swells Alum, borates, phosphates etc
4. Colour of residue obtained after heating
Hot Cold
(i) Orange yellow White Zn salts  
(ii) Brown Yellow salts of Pb, Bi or Sn
(iii) Red or black Brown Cd salts
(iv) white Blue Cupric salts (CuSO4)
(v) Blue Blue-red COCl2
(vi) Green Dark red CoBr2
(vii) Violet Dark red CoI2
(viii) Yellow Green Ni salts
(ix) Green Violet Cr salts
(x) Black

    (non-fusible)

 Black CuO, MnO2, NiO
(xi) Dark red brown

     (non-fusible)

 Light brown Fe2O3
(xii) Dark orange-red Light orange-red HgO
5. Substance sublimes on heating and colour of sublimate is
(i) White HgCl2, Hg2Cl2, As2O3, AlCl3, NH4+, halides, Sb2O3 etc.
(ii) Greyish black HgS
(iii) Yellow S, As2S3, HgI2 etc
(iv) Yellow-blue or violet vapour I2
(v) Grey (having garlic odour) As
6. Observation of gases evolved
(A) Colourless and odourless gas:
(i) Which supports the combustion of burning match stick (O2). Nitrates of alkali metals
(ii) Which turns the lime water milky (CO2). CO32– or C2O42– salts
(B) Colourless, odorous gas:
(i) Smell of ammonia (NH3). Ammonium salts.
(ii) Smell of burning sulphur (SO2). Sulphite salts or FeSO4
(iii) Smell of rotten eggs (H2S) Sulphide salts.
(iv) Pungent smelling gas, which gives white fumes with NH4OH. Hydrated chloride salt
(C) Coloured odorous gases:
(i) Brown gas which turns starch-iodide paper blue (NO2) Nitrates or nitrites of heavy metals
(ii) Red-brown gas which turns starch-paper yellow. Bromide salts
(iii) Violet gas which turns starch paper blue Iodide salts
(iv) Yellow-green gas which decolorizes the litmus paper. Chloride salts

 

3. Smell of Compound

Smell of a compound also gives idea about the salt.

 

S. No. Smell Inference
1. Bitter almond type smell Cyanides
2. Ammonical smell Ammonium salts
3. Chlorine like smell Hypochlorites 
4. Vinegar like smell Acetates

 


4. Solubility of Salts

S. No. Observation Solubility in water
1. Nitrates and nitrites All metal salts are water soluble.
2. Acetates All metal acetates are water soluble.
3. Chlorides All are water soluble except AgCl, Hg2Cl and PbCl2.
4. Bromides All are water soluble except AgBr, Hg2Br2, PbBr2 and HgBr2.
5. Iodides All are water soluble except AgI, Hg­2I2, PbI2, HgI2.
6. Sulphates All are water soluble except CaSO4, SrSO4, BaSO4, PbSO4, Hg2SO4, Ag2SO4
7. Sulphides All are water insoluble except gp. 1 & 2 sulphides and (NH4)2S.
8. Carbonates All are water insoluble except gp. 1 carbonates and (NH4)2CO3.
9. Phosphates All are water insoluble except gp. 1 phosphates and (NH4)3PO4.
10. Hydroxides All are water insoluble except gp. 1 hydroxides Ba(OH)2, Sr(OH)2 and Ca(OH)2.

 

NOTE:

Action of Heat on Different Compounds:

(a)    Some oxides liberate O2:

                  \underset { (red) }{ 2HgO } \underrightarrow { \quad Heat\quad } \underset { (Silvery\quad deposit) }{ 2Hg } +{ O }_{ 2 }\uparrow

                  \underset { (red) }{ 2P{ b }_{ 3 }{ O }_{ 4 } } \quad \underrightarrow { \quad Heat\quad } \quad \underset { (Yellow) }{ 6PbO } +{ O }_{ 2 }\uparrow

                  \underset { (Brown) }{ 2P{ b }{ O }_{ 2 } } \quad \underrightarrow { \quad Heat\quad } \quad 2PbO+{ O }_{ 2 }\uparrow

                   2Ag2O  \underrightarrow { \quad Heat\quad }   4Ag + O2 ↑

 

(b)    Some carbonates liberate CO2:

                    \underset { (Green) }{ CaC{ O }_{ 3 } } \quad \underrightarrow { \quad Heat\quad } \quad \underset { (Black) }{ CuO } +CO_{ 2 }\uparrow

                  \underset { (White) }{ ZnS{ O }_{ 3 } } \quad \underrightarrow { \quad Heat\quad } \quad \underset { Yellow(hot)\\ White(cold) }{ ZnO } +CO_{ 2 }\uparrow

                   2Ag2CO3  \underrightarrow { \quad Heat\quad }   4Ag + 2CO2 ↑ + O2 ↑

                   CaCO3  \underrightarrow { \quad Heat\quad }   CaO + CO2 ↑

                   MgCO3  \underrightarrow { \quad Heat\quad }   CaO + CO2 ↑

                   Li2CO3  \underrightarrow { \quad Heat\quad }   Li2O + CO2 ↑

 

(C)    Some bicarbonates liberate CO2:

                   2NaHCO3 → Na2CO3 + CO2 + H2O

                   NH4HCO3 → NH3 + CO2 + H2O

 

(D)    Some Sulphates liberate SO3:

                   CuS4.5H2O \xrightarrow [ \quad -5{ H }_{ 2 }O\quad ]{ Heat } CuSO4  \underrightarrow { \quad Heat\quad }   CuO + SO3

                   2FeSO4  \underrightarrow { \quad Heat\quad }   Fe2O3 + SO2 + SO3

                   Al2(SO4)3  \underrightarrow { \quad Red Hot\quad }   Al2O3 + 3SO3

 

(E)    Some Sulphates liberate SO2:

                   2MgSO4  \underrightarrow { \quad Heat\quad }   2MgO + 2SO2 + O2

                          2ZnSO4  \underrightarrow { \quad Heat(hightemp.)\quad } 2ZnO + 2SO2 + O2

                   2BeSO4  \underrightarrow { \quad Heat\quad } 2BeO + 2SO2 + O2

                  

(F)    Some Sulphates lose water of crystallization:

                   2CaSO4.2H2O \underrightarrow { \quad Heat\quad } 2(CaSO4).H2O + 2H­2O

                   ZnSO4.7H2O \xrightarrow [ \quad -{ H }_{ 2 }O\quad ]{ { 70 }^{ 0 }C } ZnSO4.6H2O \xrightarrow [ \quad -5{ H }_{ 2 }O\quad ]{ { 100 }^{ 0 }C }

                   ZnSO4.H2O \xrightarrow [ \quad -{ H }_{ 2 }O\quad ]{ { 450 }^{ 0 }C } ZnSO4

 

(g)    Some nitrates liberate NO2 and O2:

                  \underset { (White) }{ 2Zn(N{ O }_{ 3 })_{ 2 } } \quad \underrightarrow { \quad Heat\quad } \quad 2ZnO+\underset { Brown }{ 4NO_{ 2 } } +O_{ 2 }

                   2Cu(NO3)2  \underrightarrow { \quad Heat\quad } 2CuO + 4NO2 + O2

                   2PB(NO3)2  \underrightarrow { \quad Heat\quad } 2PbO + 4NO2 + O2

                   2Mg(NO3)2  \underrightarrow { \quad Heat\quad } 2MgO + 4NO2 + O2

                   2Ca(NO3)2  \underrightarrow { \quad Heat\quad } 2CaO + 4NO2 + O2

                   2LiNO3  \underrightarrow { \quad Heat\quad } LiO2 + 2NO2 + ½ O2

                   Hg(NO3)2  \underrightarrow { \quad Heat\quad } Hg + 2NO2 + O2

                   2AgNO3  \underrightarrow { \quad Heat\quad } 2Ag + 2NO2 + O2

                   2Co(NO3)2  \underrightarrow { \quad Heat\quad } 2CaO + 4NO2 + O2

     

(H)   Some nitrates liberate O2:

                   2NaNO3  \underrightarrow { \quad Heat\quad } 2NaNO2 + O2

                   2AgNO3  \underrightarrow { \quad Heat\quad } 2AgNO2 + O2

                  

(I)     Some nitrates liberates N2O:

                   NH4NO3  \underrightarrow { \quad Heat\quad } N2O + 2H2O

 

(J)    Hydrated chlorides liberate HCl:

                   2[AlCl3.6H2O] \underrightarrow { \quad Heat\quad } Al2O3 + 6HCl + 9H2O

                   MgCl.6H2O \underrightarrow { \quad Heat\quad } MgO + 2HCl + 5H2O

                   ZnCl2.2H2O \underrightarrow { \quad Heat\quad } Zn(OH)Cl + HCl + H2O

                   2(ZnCl2.6H2O) \underrightarrow { \quad Heat\quad } Zn2OCl2 + 2HCl + H2O

                   2[FeCl3.6H2O] \underrightarrow { \quad Heat\quad } Fe2O3 + 6HCl + 9H2O

                   SnCl2.2H2O \underrightarrow { \quad Heat\quad } Sn(OH)Cl + HCl + H2O

                  

(K)    Some chlorides decompose as:

                   2FeCl3  \underrightarrow { \quad Heat\quad } 2FeCl2 + Cl2

                   2CuCl2  \underrightarrow { \quad Heat\quad } Cu2Cl2 + Cl2

                   NH4Cl \underrightarrow { \quad Heat\quad } NH3 + HCl

                   Hg2Cl2  \underrightarrow { \quad Heat\quad } HgCl2 + Hg

         

(l)     Some other salts decomposes as: 

                  \underset { (Orange) }{ (NH_{ 4 })_{ 2 }Cr_{ 2 }O_{ 7 } } \quad \underrightarrow { \quad Heat\quad } \quad N\underset { Green }{ _{ 2 }+CrO_{ 3 } } +4H_{ 2 }O

                   4K2Cr2O7  \underrightarrow { \quad Heat\quad } 4K2CrO4 + 2Cr2O3 + 3O2

                   NH4NO3  \underrightarrow { \quad Heat\quad } N2 + 2H2O

                   NH4NO3  \underrightarrow { \quad Heat\quad } N2O + 2H2O

                   2Mg(NH4)PO4  \underrightarrow { \quad Heat\quad } Mg2P2O + H2O + 2NH3

                   2Zn(NH4)PO4  \underrightarrow { \quad Heat\quad } Zn2P2O7 + H2O + 2NH3

                   (CH3COO)2Pb \underrightarrow { \quad Heat\quad } PbCO3 + CH3COCH3

                   FeC2O4  \underrightarrow { \quad Heat\quad } FeO + CO2 + CO

                   SnC2O4  \underrightarrow { \quad Heat\quad } SnO + CO2 + CO

                   CAaC2O4  \underrightarrow { \quad Heat\quad } CaCO3 + CO

                   K2[Fe(CN)6] \underrightarrow { \quad Heat\quad } 4KCN + Fe + 2C + N2

                   H3BO3  \underrightarrow { \quad 10{ 0 }^{ 0 }C\quad } HBO2  \underrightarrow { \quad 16{ 0 }^{ 0 }C\quad } H2B4O7  \underrightarrow { \quad Red\quad Hot\quad } B2O3

                   2KCIO3  \underrightarrow { \quad Heat\quad } 2KCl + 3O2

                   2KMnO4  \underrightarrow { \quad Heat\quad } K2MnO4 + MnO2 + O2

                   Na2B4O7.10H2  \xrightarrow [ \quad -10{ H }_{ 2 }O\quad ]{ Heat }  Na2B4O7 \underrightarrow { \quad Heat\quad } \underbrace { 2NaB{ O }_{ 2 }+B_{ 2 }{ O }_{ 3 } }_{ Glassy\quad bead }

                    Na(NH4)HPO4  \underrightarrow { \quad \Delta \quad } NH3 + NaPO3 + H2O

                  

5. Flame Test

Some volatile substances (especially chlorides) provide colour to oxidizing Bunsen flame. Colour of flame depends upon the metal ion present in the substance. When a slat is brought in contact of oxidizing flame, it dissociates into ion. The electrons of ions are excited to higher energy level and when they de-excites to ground state, they emit in visible region. Because chlorides are more volatile then other salts, so other salts are converted into chlorides.

 

S. No. Colour of Flame Inference
  With naked eye Cobalt glass  
1. Golden yellow Invisible Na
2. Pink violet (Lilac) Crimson red K
3. Brick red Light green Ca
4. Dark red Violet Sr
5. Apple green Blue green Ba
6. Green Blue green Cu, BO33–
7. Blue Visible Pb, Cu, As, Sb, Bi
8. Crimson red   Li

 

NOTE:     

  1. Wire must be cleaned with conc. HCl and HCl must be pure.
  2. If Pb, Sb, Bi, Sn or As may be present in the mixture then flame test must not be performed because they form alloy with Pt and spoil the wire.
  3. Golden yellow colour of Na exists for long time and so other flame tests must be performed after removing Na.
  4. Nichrome wire may also be used in place of Pt but results are not good.
  5. Ca-salts give colour immediately while Ba or Sr salts, gives colour after sometime.
  6. Be and Mg do not impart colour to flame due to high I.E.

 

6. Charcoal Cavity Test

In this test, salt is fused with anhydrous Na2CO3 or oxidizing fusion mixture in a cavity on charcoal block in reducing flame. Reaction yield metal oxides. Some metal oxides are reduced to metal by action of charcoal while some form incrustation on cold parts of charcoal block. The fusion reactions may be represented as given below:

                   CaCl2 + Na2CO3 → CuCO3 + 2NaCl

                   CuCO3 → CuO + CO2

                   CuO + C → Cu + CO

         

Observation Inference
(A) Metallic bead is formed
(i) Lustrous, white, malleable which leaves no stain on paper and form no incrustation. Ag
(ii) White, malleable, leaves no stain on paper Sn
(iii) Greyish white, soft which marks paper Pb
(iv) Red-white, brittle Bi
(v) White, brittle which fumes when hot Sb
(vi) Red Cu
(B) Incrustation is formed
(i) White, which turns yellow and when not  Sn or Zn
(ii) White Sn
(iii) Red-brown Cd
(iv) White with odour of garlic As
(v) Yellow which turns orange on heating  Bi
(vi) Yellow which turns red-brown or heating  Pb
(C) Residue is left behind
(i) Grey or black residue without any incrustation Fe, Co, Ni or Mn
(ii) Red-brown with incrustation Cd
(iii) White, infusible residue which shines on heating Al, Zn, Ca, Sr, Ba, Mg.

 


7. Cobalt-nitrate Test

This test is performed only when a white, infusible mass is obtained in charcoal cavity. It is based on fact that some metal oxides form coloured double oxides when heated with cobalt oxide in oxidizing flame. To perform this test, white infusible mass obtained in charcoal cavity test is taken in charcoal cavity with cobalt nitrate and mixture is heated in oxidizing flame.

 

S. No. Observation Composition Inference
1. Blue residue CoO.Al2O3 Al, PO43– or BO33–
2. Green-residue CoO. ZnO Zn
3. Pink-dirty residue CoO. MgO Mg
4. Bluish Green CoO. SnO Sn

 

NOTE:

(i)     Cobalt nitrate should not be used in excess otherwise black coloured cobalt oxide may be formed. In presence of this black coloured oxide, it is difficult to identify other colours.

(ii)    This test can also be performed with filter paper ash. For this test solution of substance under testing and cobalt nitrate solution are mixed and a filter paper is dipped in it. It is burnt and colour of ash is observed. 

 

8. Borax-bead Test

Some substance give coloured bead when heated with borax bead on platinum loop. Colour of head gives idea of substance. The reactions taking place may be represented as

                   Na2B4.10H2O \xrightarrow [ \quad -10{ H }_{ 2 }O\quad ]{ \Delta } Na2B4O7 → \underbrace { 2NaB{ O }_{ 2 }+B_{ 2 }{ O }_{ 3 } }_{ Glassy\quad borax\quad bead }     

          (Mixture of sodium metaborate and boric anhydride)

          This borax bead form coloured bead with metal salts. e.g.,

                   CuSO4  \underrightarrow { \quad \Delta \quad } CuO + SO3             

                   CuO + B2O3 \underrightarrow { \quad \Delta \quad } \quad \underset { Copper\quad metaborate\quad (Blue) }{ Cu(B{ O }_{ 2 })_{ 2 } }

The colour of bead is different in oxidizing and reducing flames. e.g., coloured copper

metaborate may be reduced to colourless cuprous metaborate or to metallic copper.

                   2Cu(BO2)2 + C → 2CuBO2 + B2O3 + CO       

                   2Cu(BO2)2 + 2C → 2Cu + 2B2O3 + 2CO

                  

Colour of the bead in Metal
Oxidizing-flame   Reducing-flame
Hot Cold Hot Cold
Green Blue Colourless Brown-red Copper
Brown-yellow Pale-yellow Bottle green Bottle green Iron
Green Green Green Green Chromium
Blue Blue Blue Blue Cobalt
Violet Amethyst red Grey Grey Manganese
Red-brown Brown Colourless Colourless Nickel

 

NOTE:

(i)     Borax-bead must be performed for coloured substances only.

(ii)    Small amount of substance must be used otherwise transparent bead is obtained.

 

9. Microcosmic Salt Bead Test

This test is similar to borax bead test and in it microcosmic salt bead in used in place of borax bead.

                   Na(NH4)HPO4 → \underset { Sodium\quad metaphosphate }{ NaP{ O }_{ 3 } } + NH3 + H2O

Sodium metaphosphate combines with metallic oxides to form orthophosphates which are usually coloured. The conclusions can be made on the basis of table given in borax-bead test. 

 

Qualitative Analysis : Analysis of acid radicals