Viva Voce – Questions on Protein, Fat, Carbohydrate Test

  • What are proteins? Name their basic unit.

Proteins are complex nitrogenous organic compounds of high molecular masses. Amino acids are the basic unit of proteins.

  • What is the ring due to in Molisch’s test?

Concentrated sulphuric acid (H2SO4) produces furfural or its derivative on reacting with carbohydrates.  This furfural forms a violet coloured compound on coupling with naphthol.

  • What is the chemistry of Acrolein Test?

Oil or fat gets hydrolyzed to free glycol which forms pungent smelling acrolein on further reaction.

Chemical Equation of Acrolein Test
Chemical Equation of Acrolein Test

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Viva Voce – Questions on Preparation of Sols

  • What is a sol?

Sol is a colloidal suspension of a solid in a liquid.

  • What is the criteria for classifying sols? What type of sols are known?

Sols are classified based on their affinity for the dispersed phase for the dispersion medium. There are types of sols – lyophobic and lyophilic.

  • What is the difference between lyophilic and lyophobic sol?

A sol in which colloidal particles have an affinity for the dispersion medium is called lyophilic sol whereas, in a lyophobic sol, colloidal particles have no or very little affinity for the dispersion medium.

  • What is meant by ‘dialysis’?

The phenomena of purification of sols by passing through a semipermeable membrane is known as dialysis.

  • What is the special characteristic of the paper used in dialysis?

It only permits the electrolyte to pass through it and doesn’t allow the passage of colloidal particles.

  • What is the nature of charge on (i) ferric hydroxide and (ii) arsenious sulphide sols?

Ferric hydroxide sol is positively charged whereas arsenious sulphide is negatively charged.

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Viva Voce – Questions on Chromatography

  • What is chromatography?

It is a rapid and efficient technique for separating various components of a mixture.

  • What is a chromatogram?

The paper strip after running of the solvent and drying is called chromatogram.

  • What are the moving and stationary phases in paper chromatography?

Both phases are liquid. The solvent is the mobile on moving phase while the water absorbed in the pores of the paper acts as a stationary phase.

  • Why should the strip not touch the walls of the jar?

That is necessary to avoid the uneven flow of the developing solvent.

  • What do you understand by ‘Rf’?

Rf (Retention Factor) is the ratio of the distance travelled by the component from the origin line and distance travelled by solvent from origin line.

  • Why is the rate of movement of mobile phase less in ascending paper chromatography?

This is because the mobile phase moves against gravity.

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Viva Voce – Questions on Salt Analysis Practicals

  • What is qualitative analysis?

The type of analysis that deals with the methods which are used to determine the constituents of a compound.

  • What is a radical?

A radical may be defined as an atom or group of atoms which carry charge & behaves like a single unit in chemical reactions.

  • What are preliminary tests?

The tests which are done for getting an indication of the radicals are called preliminary tests.

  • Name a few preliminary tests.

Physical examination of salt, dry heating, the action of diluted and concentrated sulphuric acid (H2SO4).

  • Which radicals are absent in a white salt?

Copper – Cu2+, Iron(II)-  Fe2+,  Iron(III) – Fe3+,  Cobalt – Co2+, and Nickel – Ni2+

  • Can sulphuric acid (H2SO4) be used for preparing the original solution?

No, because sulphuric acid (H2SO4) will produce sulphates of the cations and some sulphates like lead sulphate (PbSO4) are insoluble. So, we may not get the required clear original solution.

  • Explain how Aluminium (III) [Al3+] can be confirmed using cobalt nitrate.

Take a salt solution and add cobalt nitrate to it. Burn a filter paper which has been dipped into this solution. If you detect blue colour on black ash of filter paper, Aluminium (III) [Al3+] confirmed.

  • Why is ammonium chloride [NH4Cl] in III group? Can we add ammonium hydroxide (NH4OH) first?

NH4Cl is added to prevent precipitation of radicals other than III group as hydroxides by common ion effect. If we add NH4OH first, radical of group IV, V & VI will also precipitate and accordingly we cannot do it.

  • Why is it essential to boil off hydrogen sulphide [H2S] gas before precipitating III group?

If H2S isn’t boiled off group IV cations will also get precipitated.

  • How does the addition of ammonium hydroxide (NH4OH) help in precipitation of cations as sulphides?

Solubility product of Group IV sulphides is greater than those of group IV radicals. Thus, a higher concentration of S2- ions are required to precipitate Group IV radicals as sulphides. OH ions of ammonium hydroxide (NH4OH) with H+ ions of H2S, shift the equilibrium towards right or increasing the concentration of S2- ions.

  • Black precipitates in Group IV indicate which cations?

Presence of either Ni2+ or Co2+.

  • Why can sodium hydroxide [NaOH] not be used instead of ammonium chloride [NH4Cl] and ammonium hydroxide (NH4OH)?

NaOH is a very strong electrolyte and its ionization cannot be supressed. Moreover, it dissolves precipitates of Al3+, and Cr3+ as a result of formation of AlO2-, CrO42- complexes.

  • Can Barium Nitrate [Ba(NO3)2]solution be used instead of Barium Chloride [BaCl2]for testing sulphate ions [SO42-]?

Yes, since it is the Ba2+ ions which interact with SO42- and form the white precipitate of Barium sulphate [Ba SO4].

  • Sometimes a white precipitate is obtained in Group VI even in the absence of magnesium cation [Mg2+]. How?

This happens if the fifth group cations have not been precipitated there completely.

  • In the test for bromide ion [Br], why does only organic layer acquire brown colour?

Because Br2 formed is soluble in organic solvents.

  • Ammonium molybdate test applied for phosphate ions [PO43-] on a solution is positive but it doesn’t contain phosphate ions [PO43-] . How?

If the solution contains arsenate salt of arsenic, then this happens.

  • Why does Bromide not give chromyl chloride test?

Because chromyl chloride isn’t stable.

  • What is the chemical name of the yellow precipitate formed in Ammonium molybdate test of phosphate ion [PO43-] ?

Ammonium phosphomolybdate

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Viva Voce – Questions on Titration Practicals

  • Why is diluted sulphuric acid [H2SO4] added while preparing standard Mohr’s solution?

It is added to prevent hydrolysis of ferrous salt.

  • Can you use hydrochloric acid [HCl] or nitric acid [HNO3] in potassium permanganate [KMNO4] titrations? Why?

No. HNO3 is itself an oxidising agent whereas HCl being oxidisable to chlorine and hydrogen consumes permanganate.

  • What is the oxidation number of manganese in potassium permanganate [KMNO4] ?

Oxidation number of manganese in KMNO4 is 7.

  • Why are hot solutions not poured into graduated apparatus like durette, measuring flask etc.?

Hot solutions may expand glass and thereby introduce errors in volume measurement.

  • Why do we heat the oxalic acid to 60o– 70o C?

Heating is done to enhance the rate of reaction and increase activation energy.

  • What happens if we heat oxalic acid above 100o C?

It may decompose oxalic acid into carbon dioxide [CO2] and carbon monoxide [CO].

  • What is autocatalysis?

The reaction in which product acts as a catalyst to the reaction.

  • Should a titration flask also be rinsed?

No, rinsing of the flask will increase the volume more than pipetted one.

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