Revision Notes on Principles Related to Practical Chemistry

• Qualitative Analysis of Organic Compounds: 

This is used to detect nitrogen, halogen and   sulphur present in organic compound. 

a) Sodium Extract: Aqueous solution containing soluble sodium salt of the elements i.e NaCl, Na₂S and NaCNS formed by fusion of compound with sodium metal.

b) Formation of Sodium Extract: It is a two step process 

Step 1: Organic compounds are fused with dry sodium in a fusion-tube

Step 2: Fused mass after extraction with water is boiled and filtered.

c) Use of Sodium Extract: Sodium extract (S.E.) is used to detect elements (other than C and H) and the tests are given in the table.

• Qualitative Analysis of Inorganic Salts:

1.  Physical Examination of Salts/Mixture

2. Effect of Heating:

3.  Flame test:

• Test for Anions:

1. Carbonate (CO₃^2- ) 

i) Dilute HCl : gives effervescence, due to the evolution of carbon dioxide.

CO₃^2-   +  2H⁺ →   CO₂­  +  H₂O

The gas gives turbidity with lime water and baryta water.

CO₂ + Ca²⁺ + 2OH^- → CaCO₃ ¯ + H₂O

CO₂ + Ba²⁺ + 2OH^-→ BaCO₃ ¯ + H₂O

On prolonged passage of carbon dioxide in lime water, the turbidity slowly disappears due to the formation of soluble hydrogen carbonate.

CaCO₃  ¯→ + CO₂ + H₂O → Ca(HCO₃)₂

ii)    Barium chloride or Calcium chloride solution: White ppt of  barium  or calcium carbonate is obtained, which is soluble in mineral acid.

CO₃^2- + Ba²⁺ → BaCO₃ ¯

CO₃^2- + Ca²⁺ → CaCO₃ ¯

iii)   Silver nitrate solution: White ppt of silver carbonate is obtained.

CO₃^2- + 2Ag⁺ → Ag₂CO₃¯

The ppt so obtained is soluble in nitric acid and in ammonia. The ppt becomes yellow or brown on addition of excess reagent and same may also happen if the mix is boiled, due to the formation of silver oxide

Ag₂CO₃¯ → Ag₂O ¯ + CO₂ ­

2.   Sulphites (SO₃^2-)

i) Dilute HCl or Dilute H₂SO₄ : decomposes with the evolution of sulphur dioxide

SO₃^2- + 2H⁺ → SO₂ + H₂O

The gas has a suffocating odour of burning sulphur.

ii) Acidified potassium dichromate solution:  Turns filter paper moistened with acidified potassium dichromate solution,  green due to the formation of  Cr³⁺ions.

iii) Lime  water : On passing the gas through lime water, a milky ppt is formed.

Precipitate dissolves on prolonged passage of the gas, due to the formation of hydrogen sulphite ions.

iv) Barium chloride or Strontium chloride solution: Gives white ppt. of barium or strontium sulphite.

3.   Sulphide (S^-2)

i) Dil. HCl or Dil. H₂SO₄: A colourless gas smelling of rotten eggs (H₂S) is evolved.

S^2- + 2H⁺ →  H₂S­

ii) The gas turns lead acetate paper black

iii) Gives yellow ppt. with CdCO₃

Na₂S + CdCO₃ → CdS¯ + Na­₂CO₃

iv) Silver nitrate solution: black ppt. of silver sulphide insoluble in cold but soluble in hot dil nitric acid.

S^2- + 2Ag⁺ → Ag₂S ¯

v) Sodium nitroprusside solution : Turns sodium nitroprusside solution purple

Na₂S + Na₂[Fe(CN)₅NO] → Na₄[Fe(CN)₅NOS]

4.   Nitrites (NO₂^-)

i)     Dil HCl and Dil. H₂SO₄ : Adding to solid nitrite in cold yields pale  blue liquid (due to the presence of free nitrous acid HNO₂ or its anhydride N­₂O₃) & the evolution of brown fumes of nitrogen dioxide, the latter being largely produced by combination of nitric oxide with the oxygen of the air

ii)    Silver nitrate solution : White crystalline ppt. is obtained

NO₂^- + Ag⁺ → AgNO₂¯

iii) Turns acidified KI - starch paper blue

2KI + 2NO₂ → 2KNO₂ + I₂ ­

Starch + I₂  → Blue colour

iv)   Brown ring test: When the nitrite solution is added carefully to a conc. solution of Iron(II) sulphate acidified with dil acetic acid or with dilute sulphuric acid, a brown ring is formed, due to the formation of [FeNO]SO₄  at the junction of the two liquids.

5. Acetate (CH₃COO^-)

i) Dilute Sulphuric Acid : Smell of vinegar is observed.

CH₃COO^- + H⁺ → CH₃COOH ­

The following test is performed with the aqueous salt solution.

ii) Iron (III) Chloride Solution: Gives  deep - red colouration

CH₃COONa + FeCl₃  → (CH₃COO)₃Fe + 3NaCl

Brown colour

6. Thiosulphates 

i) Dil Hydrochloric acid: Gives sulphur & sulphur di oxide

ii) Silver nitrate solution: Gives white ppt. of silver thiosulphate.

S₂O₃^2-+ 2Ag⁺ → Ag₂S₂O₃ ¯

The ppt. is unstable, turning dark on standing, when silver sulphide is formed.

Ag₂S₂O₃¯ + H₂O → Ag₂S + H₂SO₄

iii)   Lead acetate or Lead nitrate solution: Gives white ppt.

S₂O₃^2-+ Pb²⁺ → PbS₂O₃ ¯

On boiling it turns black due to the formation of PbS.

PbS₂O₃ ¯ + H₂O →PbS ¯ + 2H⁺ + SO₄² 

7. Chloride (Cl^-)

i) Conc. H₂SO₄ : decomposes with the evolution of HCl.

Cl^- + H₂SO₄ → HCl  + HSO

Gas so produced

(1) Turns blue litmus paper red

(2) Gives white fumes of NH₄Cl when a glass rod moistened with ammonia solution is brought near the mouth of test tube.

 ii)  Silver nitrate solution: White, curdy ppt. of AgCl insoluble in water & in dil .nitric acid, but soluble in dilute ammonia solution.

v)  Chromyl chloride test: When a salt containing chloride ion is heated with K₂Cr₂O₇ and conc. H₂SO₄ orange red fumes of chromyl chloride (CrO₂Cl₂) are formed.

K₂Cr₂O₇ + 4NaCl +  6H₂SO₄ → 2KHSO₄ + 4NaHSO₄ + 2CrO₂Cl₂ ­ + 3H₂O

                                                                               orange – red fumes

Chlorides of mercury, owing to their slight ionization, do not respond to this test and only partial conversion to CrO₂Cl₂ occurs with the chlorides of lead, silver, antimony and tin.

When chromyl chloride vapours are passed into sodium hydroxide a yellow solution of sodium chromate is formed which when treated with lead acetate gives yellow ppt. of lead chromate.

                CrO₂Cl₂ + 2NaOH → Na₂CrO₄ + 2HCl

                                               Yellow solution

                Na₂CrO₄ + (CH₃COO)₂ Pb → 2CH₃COONa + PbCrO₄ ¯   (yellow ppt.)

8.   Bromide (Br^-)

i)  Conc. H₂SO₄ : Gives reddish brown vapours of bromine accompanying the hydrogen bromide.

ii)    Manganese dioxide and conc. sulphuric acid : When a mix of solid bromide, MnO₂ and conc. H₂SO₄ is heated reddish brown vapours of bromine are evolved.

                2KBr + MnO₂ + 2H₂SO₄ → Br₂ ­ + K₂SO₄ + MnSO₄ + 2H₂O

The following tests are performed with the aqueous salt solution.

iii)   Silver nitrate solution: Pale yellow ppt. of silver bromide is obtained. This ppt. is sparingly soluble in dil but readily soluble in conc. ammonia solution and insoluble in dil. HNO₃.

iv) Lead acetate solution: White crystalline ppt. of lead bromide which is soluble in
boiling water.                   

9.   Iodide (I^-)

i)     Conc. H₂SO₄ : Gives violet vapours of iodine

ii)    Silver nitrate solution: Yellow  ppt. of silver iodide AgI, very slightly soluble in conc. ammonia solution and insoluble in dil nitric acid.

10.  Nitrate (NO₃^- )

i)     Conc H₂SO₄ : Gives reddish - brown vapours of nitrogen dioxide

                4NO₃^- + 2H₂SO₄ →  4NO₂­ + 2SO₄^2- + 2H₂O + O₂­

 ii)    Brown ring test: When freshly saturated solution of iron (II) sulphate is added to nitrate solution and conc. H₂SO₄ is poured slowly down the side of the test - tube, a brown ring is obtained.

2NO₃^- + 4H₂SO₄ + 6Fe²⁺ → 6Fe³⁺ +2NO + 4SO₄^–2 + 4H₂O

Fe²⁺ + NO­ → [Fe(NO)]²⁺

On shaking and warming the mix, the brown colour disappears, nitric oxide is evolved and a yellow solution of Iron(III) ions remains.

11. Sulphate (SO₄^2-)

 i)     Barium chloride solution: White ppt. of barium sulphate BaSO₄ insoluble in warm dil. hydrochloric acid and in dilute nitric acid, but moderately soluble in boiling, conc. hydrochloric acid.

ii)    Mercury (II) nitrate solution: Gives yellow ppt. of basic mercury (II) sulphate

12. Chromate CrO₄^{2 -}and Dichromate (Cr₂O)

 i)     Barium chloride solution: Pale - yellow ppt. of barium chromate soluble in dilute mineral  acids but insoluble in water and acetic acid.

CrO + Ba²⁺    → BaCrO₄ ¯

Dichromate ion also gives the same ppt. but due to the formation of strong acid precipitation is partial.

Cr₂O+ 2Ba²⁺ + H₂O  2 BaCrO₄ ¯ + 2H⁺

If sodium hydroxide or sodium acetate is added, precipitation becomes quantitative.

 ii)    Silver Nitrate Solution: Brownish - red ppt. of silver chromate Ag₂CrO₄ which is soluble in dil. nitric acid & in ammonia solution, but insoluble in acetic acid.

A reddish brown ppt. of silver dichromate Ag₂Cr₂O₇   is formed with a conc. solution of a dichromate.             

13. Permanganate MnO 

i) Hydrogen peroxide : It decolourises acidified potassium permanganate solution

                2MnO₄^- + 5H₂O₂ + 6H⁺ ¾¾® 5O₂ ­ + 2Mn²⁺ + 8H₂O.             

ii) Iron (II) sulphate, in the presence of sulphuric acid, reduces permanganate to manganese (II). The solution becomes yellow  because of the formation of iron (III) ions

                MnO₄^- + 5Fe²⁺ + 8H⁺ →  5Fe³⁺ + Mn²⁺  + 4H₂O

• Test For Cations:

Group I (Pb²⁺, Ag⁺, Hg⁺)

(A)  PbCl₂ gives a yellow ppt. with K₂CrO₄. The ppt. is insoluble in acetic acid but soluble in NaO

(B)  PbCl₂ + 2KI → PbI₂  + 2KCl

                            (Yellow)

PbCl₂ + 2KI (excess) →  K₂[PbI₄]

AgCl is soluble in NH₄OH forming a complex while Hg₂Cl₂ forms a black ppt. with NH₄OH.

AgCl + 2NH₄­OH → Ag(NH₃)₂Cl + 2H₂O

Hg₂Cl₂ + 2NH₄OH → H₂NHgCl + Hg + NH₄Cl + 2H₂O

                           Amino mercuric Chloride

2.    Group II A  (Hg²⁺, Cu²⁺, Bi³⁺, Cd²⁺)

i)     Hg⁺²ions in solution, on addition of SnCl₂, give a white precipitate turning black.

2Hg⁺² + SnCl₂ → Sn⁺⁴ + Hg₂Cl₂ 

                                       White

Hg₂Cl₂ + SnCl₂ → SnCl₄ + 2Hg 

                             Black

ii)    Cu⁺² ions in solution give deep blue colour with excess of NH₄OH

Cu⁺² + 4NH₄OH → [Cu(NH₃)₄ ]⁺² + 4H₂O

                             Deep blue in colour

Cu⁺² ions give chocolate precipitate with K₄Fe(CN)₆.

2Cu⁺² + K₄Fe(CN)₆ → Cu₂[Fe(CN)₆] + 4K⁺

 iii)   Bi⁺³ ions in solution of HCl on addition of water give white cloudy precipitate.

BiCl₃ + H₂O → BiOCl  + 2HCl

                     White ppt.

When treated with sodium stannite a black ppt. is obtained.

2BiCl₃ + 3Na₂SnO₂  → 2Bi ¯ +  3Na₂SnO₃ + 6NaCl + 3H₂O

                                                black

iv)   Cd⁺² ions in solution, with NaOH give a white precipitate.

Cd⁺² + 2NaOH →  Cd(OH)₂ +  2Na⁺

                             white

With ammonium hydroxide, Cd²⁺ ions give a white precipitate which dissolves in excess.       

Cd²⁺ + 4NH₄OH → [Cd(NH₃)₄](OH)₂ + 2H₂O

3.   Group II B (As³⁺, As⁵⁺, Sb³⁺, Sb⁵⁺, Sn³⁺, Sn⁴⁺)

v)    As⁺³ ions in solution give yellow precipitate with ammonium molybdate and HNO₃ on heating.

H₃AsO₄ +12(NH₄)₂MoO₄ +21HNO₃ → (NH₄)₃ AsMo₁₂O₄₀+ 21NH₄NO₃ + 12H₂O

                                                               Yellow ppt.

vi)   Sn²⁺ ions in solution as SnCl₂ give white ppt. with HgCl₂ ,which turns black on standing.

vii) Sb⁺³ ions in solution as SbCl₃ , on addition of water give white precipitate.

SbCl₃ + H₂O → SbOCl + 2HCI

                       White

4.    Group III A (Al³⁺, Fe³⁺, Cr³⁺)

1. White precipitate of Al(OH)₃ is soluble in NaOH

Al(OH)₃ + NaOH → NaAlO₂ + 2H₂O

2. Precipitate of Cr(OH)₃ is soluble in NaOH + Br₂ water and addition of BaCl₂ to this solution gives  yellow precipitate.

Fe(OH)₃ is insoluble in NaOH

3. Brown precipitate of Fe(OH)₃ is dissolved in HCl and addition of KCNS to this solution gives   blood red colour.

Also on addition of K₄Fe(CN)₆ to this solution, a prussian blue colour is obtained.

FeCl₃ + 3K₄Fe(CN)₆ → Fe₄[Fe(CN)₆]₃ + 12KCl

                                prussian blue colour

5.   Group III B (Ni²⁺, Co²⁺, Mn²⁺, Zn⁺²)

1. Ni⁺² and Co⁺² ions in solution, on addition of KHCO₃ and Br₂ water give apple green colour if Co⁺² is present and black precipitate if Ni⁺² is present.

CoCl₂ + 6KHCO₃ →K₄[Co(CO₃)₃] + 2KCl + 3CO₂­ + 3H₂O

2K₄[Co(CO₃)₃] + 2KHCO₃ + [O] → 2K₃[Co(CO₃)₃] + 2K₂CO₃ + H₂O

                                                     Apple green colour

NiCl₂ + 2KHCO₃ → NiCO₃ + 2KCl + H₂O + CO₂­

2NiCO₃ + 4NaOH + [O] → Ni₂O₃ + 2Na₂CO₃ + 2H₂O

                                    Black ppt.

2. Zn⁺² ions in solution give white precipitate with NaOH, which dissolve in excess of NaOH.

3. Mn⁺² ions in solution give pink precipitate with NaOH turning black or brown on heating.

6. Group IV (Ba²⁺, Sr²⁺, Ca²⁺)

Ba⁺² ions in solution give

(A) Yellow precipitate with K₂CrO₄

Ba⁺² + K₂CrO₄ → BaCrO₄  + 2K⁺

                           Yellow

(B) White precipitate with (NH₄)₂SO₄

Sr⁺² ions give white precipitate with (NH₄)₂SO₄ and  (NH₄)₂C₂O₄

Ca⁺² ions give white precipitate with (NH₄)₂ C₂O₄ only.

7.   Group V (NH₄⁺, Na⁺, K⁺, Mg⁺²)

All ammonium salts on heating with alkali say NaOH give a colourless gas with a pungent smell (NH₃)

(A)  Gas evolved gives white fumes with HCl

(B) Paper soaked in CuSO₄ solution, is turned deep blue by NH₃ due to complex formation

(C) With Hg₂ (NO₃)₂ , a black colour is obtained

Hg₂(NO₃)₂  +  2NH₃ → Hg  + Hg(NH₂)NO₃ ¯ + NH₄NO₃

                                  black

(D) With Nesslers reagent (alkaline solution of potassium tetraiodomercurate(II) ), a brown ppt. is obtained

1. Potassium salts give yellow ppt. with sodium cobalt nitrite

2. Sodium salts give a heavy white ppt. with potassium dihydrogen antimonate

3. Mg²⁺ gives white ppt. of magnesium hydroxide with sodium hydroxide