practical viva

practical viva , lassaigne test

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Question1.What is Radical ?

Ans.    It is a group of atoms of one or more elements which behaves as a single entity & maintains its identity in a chemical reaction

Ex. NH4+, Cl,  SO4– – etc.

Question2. What is Acidic Radical?

Ans.    The part of a salt which comes from acid is called acidic radical.

Ex. Cl, NO3, SO4– –

Question3.What is Basic Radical?

Ans.    The part of a salt which comes from base is called basic radical.

Ex. NH4+, Ca++, Ba++, Pb++

Question4. What do you mean by qualitative analysis?

Ans     Qualitative analysis deals with the detection & identification of constituent elements or ions present in a chemical substance.

Question5. What do you mean by quantitative analysis?

Ans.    Quantitative analysis deals with the determination of the amount of constituents present in a substance.

Question6. Why ‘Soda Extract’ is prepared in the identification of acidic radicals.

Ans.    To avoid the interference of basic radicals, soda extract is prepared for identification of acidic radicals.

Question7. Which burner is commonly used in laboratory?

Ans.    Bunsen Burner

Question8. What is lime water?

Ans. It is a solution obtained by dissolving lime in water & then filtering the solution.

Question9.Which gas turns lime water milky?

Ans.      CO2 & SO2

Question10. What is the compound formed when CO2 or SO2 is passed in lime water & the solution becomes milky?

Ans.      CaCO3 (Calcium Carbonate)


CaSO3 (Calcium Sulphite)

Question11. Why the milky colour disappears on passing excess of CO2 gas in lime water?

Ans.      Due to formation of soluble colourless calcium bicarbonate.


Question12.Name some interfering radicals?

Ans.     Oxalate (C2O4– –), Borate (BO3– – –), F (Fluoride), Phosphate (PO4– – –)

These are called interfering radicals because they interfere in the systematic analysis of basic radicals after second group.

Question13. What is Lassaigne’s test?

Ans.      Fusion of small amount of organic compound with sodium metal in an ignition tube at red hot and extraction of the fused mass with distilled water to get lassaigne’s solution or Sodium extract is called Lassaigne’s Test.

Question14. Why organic compound is fused with sodium metal? Can we use potassium in place of sodium?

Ans.      To detect N, S & halogens present in organic compound, sodium extract is prepared when organic compound is fused with sodium metal, it is converted into water soluble ionic salts of sodium.

Na+C+N –>NaCN

NaCN –>Na+ + CN

(in aq. solution)

2Na+ S –> Na2S

Na2S –>2Na++S– –

(in aq. solution)

Na + X–> NaX


NaX –>Na+ +X

(in aq. solution)

Na+C +N +S –> NaCNS

NaCNS  –>Na+ +CNS

(in aq. solution)

Potassium metal is very reactive & it is very difficult to handle potassium metal. So we can not use potassium metal in place of sodium metal.

Question 15. Why Sodium metal is kept is Kerosine Oil?

Ans       Sodium metal is very reactive metal, even it reacts with water vigrously. So it is kept in kerosine oil. It protects sodium from the action of air & moisture

2Na +1/2 O2 –> Na2O


2Na + 2H2O –> 2NaOH +H2

Question 16. Can we use ordinary tap water in place of distilled water while preparing sodium extract?

Ans.      Ordinary water contains chloride ions. So sodium extract will give the test of Chlorine even it is not present in organic compound.

Question 17.Why freshly prepared Ferrous Sulphate Solution is used?

Ans.      Because it gets oxidised to Basic ferric sulphate

4FeSO4 +2H2O+O2 –> 4Fe(OH)SO4

Question18.How can you identify Aliphatic & Aromatic Compound?

Ans.      Aliphatic compound burns with nonsmoky flame while aromatic compound burns with black & smoky flame.

Question19.Why aromatic compounds burn with black & smoky flame?

Ans.      Because percentage of carbon is more in aromatic compounds.

Question20.How can you test nitrogen in sodium extract?

Ans.      Sodium extract + Small amount of freshly prepared Ferrous sulphate solution then dull green ppt of Ferrous hydroxide is obtained. Boil it & add dil H2SO4 then prussian blue or green colour confirms the presence of nitrogen. Prussian blue colour is due to formation of Ferro Ferricyanide Fe4[Fe(CN)6]3.

Question 21. How can you test sulphur in sodium extract?

Ans.      Sodium extract + Sodium nitroprusside gives violet or purple colour due to formation of sodium thio nitroprusside.

Question 22. What is the formula of sodium nitroprusside & Sodium thio nitroprusside?

Ans.      Na2[Fe(CN)5NO] Sodium Nitroprusside

Na4 [Fe(CN)5NOS]. Sodium Thionitroprusside

Question 23. How can you test halogens in Sodium Extract?

Ans.      Take sodium extract & add dil HNO3 & Silver nitrate solution then observe the colour of ppt & its behaviour towards aq. NH3 solution (NH4OH).

  • If white ppt is obtained which is readily soluble in NH4OH due to formation of diamine silver(i) Chloride, indicates presence of chlorine.
  • If light yellow ppt is obtained which is soluble in excess of NH4OH due to formationof diamine Silver (I) bromide, indicates presence of bromine
  • If dark yellow ppt is obtained which is insoluble in NH4OH, indicates the presence of iodine.



Question 24.How can you test bromine & iodine with chloroform layer test?

Ans.      Take sodium extract, acidity it with dil H2SO4 & add chloroform (or carbon tetrachloride) & Chlorine water with shaking the contents vigorously & observe the colour of chloroform layer.

  1. If layer becomes yellow or brown, then bromine is present.
  2. If layer becomes purple or violet, then iodine is present.


Question 25.How can you test alcoholic group in organic compound?


Ans.      Take some organic compound & add cerric ammonium nitrate solution & shake. If a change in colour from yellow to red occurs then it indicates the presence of alcoholic group.

Question 26. What is the formula of cerric ammonium nitrate?

Ans.      [(NH4)Ce(NO3)6]

Question 27.How can you test phenolic gr. in organic compound?

Ans.      (i) Aq. solution of phenol turns  blue litmus solution or paper red.

(ii) Take aq. or Alc. solution of phenol & add few drops of neutral FeCl3, then violet blue or green or red colour indicates the presence of phenolic gr.

Question 28. How can you test phenol with phthallin test?

Ans.      Take small amount of phenol & phthalic anhydride in a test tube & add one or two drops of cons. H2SO4 & heat the contents. After cooling add dil NaOH solution then red, blue or fluorescent green colour indicates phenolic group.

Question 29.How can you test carboxyl gr. or carboxylic acid?

Ans.      a) Aq. solution of compound turns blue litmus red.

  1. b) Take aq. soln of compd & add solid NaHCO3, then effervescence due to evolution of CO2 gas indicates the presence of carboxyl gr.
  2. c) Take organic compd & add ethyl alcohol and small amount of Conc. H2SO4, Heat the contents & cool then fruit like smell (ester formation) confirms the presence of carboxyl gr.)


Question 30.What is the test of carbonyl gr. (>c=0)?


Ans.      Aq. Soln of compd or Alc. Soln of compd & add small amount of 2,4-dinitrophenyl hydrazene solution (Brady’s reagent), shake & warm gently & cool, then yellow orange ppt indicates the presence of carbonyl gr. [Carbonyl gr. is present in both aldehyde & ketone)

Question31.What is the test of Aldehyde group?

Ans.      i) Take organic compound & add Tollen’s reagent & heat, then silver mirror on the walls of test tube or black ppt. indicates the presence of aldehyde gr.

  1. ii) Take organic compd & add fehling soln A & Fehling Soln B, Then boil the contents, a red ppt of cuprous oxide indicates the presence of aldehyde gr.

Question 32.What is Tollen’s reagent?

Ans.      Tollen’s reagent is ammonical solution of silver nitrate (AgNO3+NH4OH)

Question 33.What is Fehling’s solution?

Ans.      Fehling’s solution is a mixture of fehling’s A & B solutions in equal volume.

Fehling A soln is aq. copper sulphate solution while fehling B soln is a mixure of NaOH & sodium potassium tartarate (Rochelle Salt)

 Question34.How can you test aldehyde gr. with Schiff’s reagent?

Ans.      Organic compd & add schiff’s reagent, then pink colour indicates the presence of aldehyde gr.

Question35. What is schiff’s reagent?

Ans.      It is an aq. Soln of p-rosaniline hydrochloride dye (Pink in Colour) which is made colourless by passing SO2 gas

Question 36. Which class of compds can be detected by Tollen’s reagent & Fehling’s solution?

Ans.      Aldehydes & reducing carbohydrates

Question37. What is the test of ketonic group?

Ans.      Organic compd & add freshly prepared sodium nitro prusside soln & excess of NaOH solution, then a wine red colour indicates the presence of Ketonic gr.

Question 38. How can you test Ketonic gr. with iodoform test?

Ans.      Take some organic compd, add NaOH solution & excess of Iodine solution, heat & cool the soln, a yellow ppt of iodoform conferms the presence of Ketonic gr.

Question 39. How can you test carbohydrate?

Ans       Take aq. Soln of compd, add molisch reagent & shake the mixture, Add 1-2ml of conc. H2SO4 with the side of test tube then a red-violet ring is formed at the junction of the two liquids

Question 40. What is Molisch reamolischgent?

Ans       10% alcoholic solution of a-naphthol

Question 41. How can you test primary amine (-NH2)?

Ans.      a) Carbylamine test of Isocyanide Test– Take organic compd, add chloroform & Alc. KOH solution Shake & heat gently then a very bad smell of isocyanide is produced.

  1. b) Dye test (only for aromatic primary amine)-Take organic compd dissolve it in dil HCl cool the soln 0-50C, add aq. NaNO2 Soln Prepare a dil. Soln of b-naphthal in NaOH in a separate test tube & add into cold soln, then orange-red dye in formed.