Chemistry viva questions class 12, with physics and biology practical viva: 50 short spoken answers for CBSE

Why is no indicator added in the oxalic acid versus potassium permanganate titration?

Potassium permanganate is a self-indicator. It is deep purple, and as long as oxalic acid is present it is reduced to colourless manganese two plus ions; the first excess drop of permanganate gives the solution a permanent light pink colour, which is the end-point.

If asked the colour change: colourless to permanent light pink.

Why is the oxalic acid solution warmed to about 60 to 70 degrees before titrating?

At room temperature the reaction between oxalic acid and permanganate is slow, so the pink colour would persist before the end-point and give a wrong reading. Warming speeds up the reaction, and the manganese two plus ions formed then catalyse it further, so the reaction becomes autocatalytic.

Do not boil; oxalic acid decomposes at high temperature.

Why is dilute sulphuric acid added and not hydrochloric or nitric acid?

Permanganate works as an oxidising agent only in acidic medium, and sulphuric acid provides the hydrogen ions without taking part in the reaction. Hydrochloric acid would itself be oxidised to chlorine and consume extra permanganate, and nitric acid is an oxidising agent that would react with the oxalic acid.

This is the most frequently asked titration question in CBSE vivas.

Why is potassium permanganate taken in the burette and not in the pipette?

Permanganate is a strong oxidising agent that attacks rubber and organic matter, so it should not be drawn by mouth or through a pipette with a rubber bulb that it can damage, and it is deeply coloured, so the lower meniscus is hard to read; we read the upper meniscus in the burette instead.

Also: the permanganate solution is a secondary standard, so it is the solution being standardised and goes in the burette.

What is a primary standard, and is oxalic acid one?

A primary standard is a substance pure and stable enough to be weighed directly to make a solution of exactly known concentration. Oxalic acid dihydrate qualifies because it is a stable crystalline solid of definite composition. Potassium permanganate does not, because it is not available pure and decomposes in light, so it is standardised against oxalic acid.

Mohr's salt is the other primary standard in the Class 12 syllabus.

What is Mohr's salt and why is it preferred to ferrous sulphate?

Mohr's salt is ferrous ammonium sulphate, a double salt with the formula FeSO4 dot (NH4)2SO4 dot 6H2O. It is preferred because ferrous sulphate alone is oxidised by air to the ferric form and becomes efflorescent, while in Mohr's salt the ammonium sulphate stabilises the ferrous ion, so its solution keeps a known strength.

Add a little dilute sulphuric acid while preparing the solution to prevent hydrolysis of the ferrous ion.

What does normality mean and how do you convert it to molarity for KMnO4?

Normality is the number of gram equivalents of solute per litre of solution. For potassium permanganate in acidic medium, each mole accepts five electrons, so the equivalent mass is the molar mass divided by five and normality is five times the molarity. Molarity is moles of solute per litre.

Be ready to write the balanced ionic equation for the permanganate and oxalate reaction.

Why should the funnel be removed from the burette before taking the initial reading?

A drop hanging in the funnel can fall into the burette after the initial reading and change the volume, giving a lower titre than the real value. The funnel is also removed so that the burette reading is not disturbed by the funnel stem touching the solution.

Similarly, remove air bubbles from the burette nozzle before starting.

What is the group reagent for group two cations and why is the medium acidic?

The group reagent is hydrogen sulphide in the presence of dilute hydrochloric acid. The acid suppresses the ionisation of hydrogen sulphide by the common ion effect, so the sulphide ion concentration is low and only the sulphides with very low solubility products, like copper and lead, precipitate; zinc and manganese stay in solution for group four.

The examiner will ask the opposite for group four: ammonium hydroxide increases sulphide ion concentration.

Why is ammonium chloride added before ammonium hydroxide in group three?

Ammonium chloride supplies ammonium ions that suppress the ionisation of ammonium hydroxide by the common ion effect, keeping the hydroxide ion concentration low. Only the hydroxides with very low solubility products, aluminium, iron three and chromium three, precipitate, while the group five and six cations stay in solution.

Follow-up: why is the solution boiled before adding ammonium hydroxide? To expel hydrogen sulphide, which would otherwise precipitate group four sulphides.

How do you test for the ammonium ion, and why is it tested first?

A little of the salt is heated with sodium hydroxide solution; ammonia gas is given off, which turns moist red litmus blue and gives dense white fumes with a glass rod dipped in hydrochloric acid. It is tested first, as the zero group, because ammonium salts are added as reagents in later groups and would interfere.

Nessler's reagent giving a brown precipitate is the confirmatory test.

What is the brown ring test and for which ion is it used?

The brown ring test confirms the nitrate ion. Freshly prepared ferrous sulphate solution is added to the salt solution, and concentrated sulphuric acid is poured slowly down the side of the test tube. A brown ring forms at the junction of the two layers because of the formation of the nitroso ferrous sulphate complex.

Nitrite also gives a brown ring with dilute acid, so the salt is tested for nitrite first.

Describe the chromyl chloride test.

The chromyl chloride test confirms chloride. The solid salt is heated with potassium dichromate and concentrated sulphuric acid; orange-red vapours of chromyl chloride are produced. When these are passed into sodium hydroxide solution, a yellow solution of sodium chromate forms, which gives a yellow precipitate of lead chromate with lead acetate.

Bromide and iodide do not give this test; they give coloured vapours of the free halogen instead.

How do you distinguish between carbonate and sulphite with dilute sulphuric acid?

Both give a gas with dilute sulphuric acid, but carbonate gives colourless, odourless carbon dioxide that turns lime water milky, while sulphite gives sulphur dioxide with a burning sulphur smell that turns acidified potassium dichromate paper green. Sulphur dioxide also turns lime water milky, so the smell and the dichromate test are the distinguishing points.

Sulphide gives hydrogen sulphide, which blackens lead acetate paper.

Why is the original solution prepared, and in what solvent?

The original solution is a clear solution of the salt used for all the cation group tests. It is prepared by trying solvents in order: water, dilute hydrochloric acid, concentrated hydrochloric acid, and finally aqua regia, using the first in which the salt dissolves completely, because the choice of solvent itself gives a clue to the cation.

If the salt dissolves only in acid, the examiner expects you to mention that the acid is boiled off before the group two test.

What is a flame test and what colours do calcium, strontium and barium give?

In a flame test a paste of the salt with concentrated hydrochloric acid is heated on a platinum wire in a non-luminous flame; the volatile chloride colours the flame as electrons return from excited states. Calcium gives brick red, strontium crimson red, barium apple green, and copper gives a bluish-green flame.

Concentrated HCl is used because metal chlorides are more volatile than other salts.

Why is the salt heated with concentrated sulphuric acid in the preliminary test?

Concentrated sulphuric acid displaces the volatile acids from the salt, so each anion gives a characteristic gas: chloride gives colourless hydrogen chloride that fumes with ammonia, bromide gives reddish-brown bromine vapours, iodide gives violet iodine vapours, nitrate gives brown nitrogen dioxide, and acetate gives the smell of vinegar.

Always point the test tube away from yourself and the examiner; it is a safety mark as much as a chemistry one.

How do you distinguish an aldehyde from a ketone?

Aldehydes are easily oxidised and ketones are not. Tollens' reagent gives a silver mirror with aldehydes, Fehling's solution gives a red precipitate of cuprous oxide with aliphatic aldehydes, and Schiff's reagent turns pink. Ketones give none of these but do give a precipitate with 2,4-dinitrophenylhydrazine, which both carbonyl compounds share.

Benzaldehyde gives Tollens' but not Fehling's; it is the examiner's favourite exception.

What is the test for a phenolic group?

Phenols give a violet or purple colour with neutral ferric chloride solution because of a coloured iron-phenol complex. A second test is the Liebermann test, or the bromine water test, where phenol gives a white precipitate of 2,4,6-tribromophenol. Alcohols do not give these tests.

If asked to distinguish phenol from a carboxylic acid: the acid gives brisk effervescence with sodium bicarbonate; phenol does not.

What is the Molisch test and the Benedict test?

The Molisch test is a general test for carbohydrates: a few drops of alcoholic alpha-naphthol are added to the sample and concentrated sulphuric acid is poured down the side, giving a violet ring at the junction. Benedict's test is for reducing sugars such as glucose, which give a green, yellow or brick-red precipitate of cuprous oxide on heating depending on the amount of sugar.

Sucrose is a non-reducing sugar and gives a negative Benedict test unless hydrolysed first.

Name two tests for proteins.

The biuret test gives a violet colour with sodium hydroxide and a few drops of copper sulphate, because copper ions complex with peptide bonds. The xanthoproteic test gives a yellow colour on heating with concentrated nitric acid, which turns orange with ammonia, because aromatic amino acids are nitrated. The ninhydrin test gives a blue-violet colour with free amino acids.

Egg white or milk is the usual sample; be ready to say what you used.

How is Mohr's salt prepared in the lab, and why is dilute sulphuric acid added?

Equimolar amounts of ferrous sulphate and ammonium sulphate are dissolved in warm water containing a little dilute sulphuric acid, the solution is filtered and cooled slowly, and light green crystals of ferrous ammonium sulphate separate out. The acid prevents hydrolysis of ferrous sulphate and oxidation of ferrous to ferric ions, which would make the solution brown.

Potash alum is prepared similarly from potassium sulphate and aluminium sulphate and gives octahedral crystals.

What is Rf value in paper chromatography?

Rf, the retention factor, is the distance travelled by the component divided by the distance travelled by the solvent front, measured from the base line. It is always less than one and is characteristic of a substance for a given solvent and paper, so it helps identify the components of a mixture such as the pigments in a leaf extract.

If asked why the base line is drawn in pencil: ink would itself run and separate in the solvent.

How does concentration affect the rate of reaction between sodium thiosulphate and hydrochloric acid?

The reaction produces a colloidal precipitate of sulphur that makes the solution turbid, and we measure the time for a cross marked under the flask to disappear. Doubling the thiosulphate concentration roughly halves the time, showing that the rate is directly proportional to concentration, because more particles collide per second.

The same experiment with temperature shows the rate roughly doubling for every ten degree rise.

What is the difference between a lyophilic and a lyophobic sol?

A lyophilic sol is solvent-loving, formed directly by mixing substances like starch, gum or egg albumin with water; it is stable, reversible and not easily coagulated. A lyophobic sol is solvent-hating, like ferric hydroxide or arsenious sulphide sol, prepared by special methods, unstable, irreversible and easily coagulated by small amounts of electrolyte.

Ferric hydroxide sol is prepared by hydrolysis, adding ferric chloride to boiling water; the examiner may ask why the water is boiling.

Tell me about your investigatory project in two sentences.

My project studied the effect of potassium bisulphite as a food preservative on fresh apple juice stored at room temperature. I compared the day of visible spoilage and the pH of samples with 0, 50 and 100 ppm of the preservative, and found that the treated samples stayed clear and acidic for about twice as long, which supports its use in commercial juices.

The examiner will pick one detail from your two sentences, so only mention what you actually measured. For project vivas in college, see final-year project viva questions.

What is the principle of the metre bridge?

The metre bridge works on the principle of the Wheatstone bridge: when the bridge is balanced, no current flows through the galvanometer, and the ratio of the two resistances equals the ratio of the two lengths of the wire on either side of the null point, so an unknown resistance can be found from a known one.

Follow-up: 'what is the condition for balance?' P by Q equals R by S.

Why should the null point in a metre bridge be near the middle of the wire?

Near the middle, the two arms of the bridge are nearly equal and the bridge is most sensitive, so a small shift produces a clear galvanometer deflection. It also minimises the effect of end resistances at the copper strips and the percentage error in measuring the lengths, because both lengths are reasonably large.

If asked how to bring the null point to the middle: change the known resistance in the resistance box.

Why is the metre bridge wire made of constantan or manganin?

Constantan and manganin have a high resistivity, so a one-metre wire has a usable resistance, and a very low temperature coefficient of resistance, so the resistance does not change when current heats the wire during the experiment. Copper would have too low a resistance and would change with temperature.

The jockey should be tapped, not slid, along the wire to avoid changing its cross-section.

Why is a potentiometer better than a voltmeter for measuring emf?

At the null point a potentiometer draws no current from the cell, so it measures the true emf, while a voltmeter draws a small current and measures the terminal voltage, which is less than the emf by the drop across the internal resistance. A potentiometer behaves like a voltmeter of infinite resistance.

Follow-up: how to increase sensitivity? Increase the length of the wire or reduce the current with a rheostat so the potential gradient falls.

How do you find the internal resistance of a cell with a potentiometer?

First I find the balancing length with the cell on open circuit, which corresponds to its emf. Then I connect a known resistance across the cell and find the new balancing length, which corresponds to the terminal voltage. The internal resistance equals the known resistance multiplied by the difference of the two lengths divided by the second length.

The cell's emf must be less than the driver cell's potential drop across the wire, otherwise no null point is found.

What is the figure of merit of a galvanometer and how is it found by the half-deflection method?

The figure of merit is the current required to produce a deflection of one division on the galvanometer scale. A high resistance from a resistance box is put in series with the cell and galvanometer to get a full deflection, then a shunt is adjusted across the galvanometer until the deflection halves; the shunt value then equals the galvanometer resistance, and the figure of merit is calculated from the emf, series resistance and deflection.

The half-deflection method assumes the series resistance is much larger than the galvanometer resistance.

How do you convert a galvanometer into an ammeter and into a voltmeter?

To make an ammeter, a low resistance called a shunt is connected in parallel with the galvanometer so most of the current bypasses the coil and the combination has a very low resistance. To make a voltmeter, a high resistance is connected in series so that only a small current flows and the combination has a very high resistance.

An ammeter is always connected in series, a voltmeter in parallel; the examiner may hand you a circuit to check.

Why is the focal length of a convex lens found using a distant object first?

For a very distant object the rays are nearly parallel, so the image forms almost exactly at the focus; this gives a rough value of the focal length, which I then use to set the object distance between f and 2f for the accurate u-v method, so the image falls on the screen within the length of the optical bench.

In the u-v method the graph of 1 by v against 1 by u is a straight line with equal intercepts of 1 by f.

What is the angle of minimum deviation and how is it related to the refractive index of a prism?

As the angle of incidence increases, the deviation produced by a prism first decreases, reaches a minimum and then increases. At minimum deviation the ray passes symmetrically through the prism, and the refractive index equals the sine of half of the sum of the prism angle and minimum deviation, divided by the sine of half the prism angle.

The i-D graph is drawn by pinning at several angles; D is read from the lowest point of the smooth curve, not from a single reading.

How do you find the refractive index of a glass slab with a travelling microscope?

I focus the microscope on an ink mark on the base and note the reading, place the slab on the mark and refocus on the raised image of the mark, then sprinkle lycopodium powder on the top surface and focus on it. The refractive index is the real thickness, the third reading minus the first, divided by the apparent thickness, the third minus the second.

The least count of the travelling microscope vernier is usually 0.001 cm; be ready to show how you got it.

What is the knee voltage of a p-n junction diode?

The knee or cut-in voltage is the forward voltage beyond which the current through the diode rises sharply, because the applied voltage overcomes the potential barrier of the depletion region. It is about 0.7 volt for a silicon diode and about 0.3 volt for germanium. In reverse bias only a tiny leakage current flows until breakdown.

Follow-up: what is the use of a diode? Rectification, converting alternating current to direct current.

What is a Zener diode and why is it used in reverse bias?

A Zener diode is a heavily doped p-n junction designed to operate in reverse breakdown without damage. Beyond the Zener voltage the current rises steeply while the voltage across it stays nearly constant, so it is connected in reverse bias across a load to act as a voltage regulator.

The examiner may ask you to identify a Zener, an LED and an ordinary diode from the activity on component identification.

How do you identify a diode, an LED, a resistor and a capacitor using a multimeter?

A resistor shows the same resistance in both directions. A diode shows low resistance in one direction and very high in the other; an LED does the same and glows faintly in the forward direction on the diode range. A capacitor shows a momentary deflection that falls to infinite resistance as it charges, in both directions.

This is one of the activities; the examiner likes it because it takes thirty seconds to test.

Why should the key be pressed only briefly in the metre bridge and potentiometer circuits?

A continuous current heats the bridge or potentiometer wire and the resistance coils, changing their resistance and shifting the null point; it also runs down the cell. Pressing the key briefly while tapping the jockey keeps the readings consistent.

Also check that the jockey is pressed gently so that the wire is not scratched.

How do you prepare a temporary mount of an onion root tip to see mitosis?

I cut the tip of a freshly grown onion root, about one centimetre, fix it, then warm it in one normal hydrochloric acid at about 60 degrees for a few minutes to soften the tissue and separate the cells. I stain the tip with acetocarmine, squash it gently under a cover slip with the thumb, and observe under the microscope for prophase, metaphase, anaphase and telophase.

The root tip is used because the meristem just behind the root cap is actively dividing; the examiner will ask which stage you saw most, usually prophase.

How do you observe pollen germination on a slide?

I prepare a solution of about 10 per cent sucrose with a little boric acid, dust pollen from a fresh flower such as balsam or Vinca onto a drop of it on a slide, and leave it in a moist chamber for 15 to 30 minutes. Under the microscope the pollen grains put out pollen tubes. Sucrose provides the osmotic medium and energy, and boron is needed for tube growth.

Follow-up: what is the stigma's role in nature? It provides the same sugar and boron environment.

What is the quadrat method and how are density and frequency calculated?

A quadrat is a square frame of known area, usually one metre square, placed at random in a field to sample plants. Density is the total number of individuals of a species counted divided by the total number of quadrats laid, expressed per unit area. Frequency is the number of quadrats in which the species occurred divided by the total number of quadrats, as a percentage.

Be ready to name the species you counted and your actual values.

How do you isolate DNA from a plant material like spinach or papaya?

I grind the material with a little water, add detergent to dissolve the cell and nuclear membranes and some salt to help the DNA aggregate, warm it briefly, filter, and then pour chilled ethanol slowly down the side of the tube. The DNA precipitates as fine white threads at the junction, which can be spooled out on a glass rod.

Follow-up: why chilled ethanol? DNA is insoluble in cold ethanol and precipitates out of the aqueous layer.

How is dissolved oxygen in water samples compared, and why does it matter?

Water samples from different sources are tested by the Winkler method or a dissolved oxygen kit, and a polluted sample with more organic matter has less dissolved oxygen because decomposing bacteria consume it. A low dissolved oxygen value is a sign of water pollution and harms fish and other aquatic life.

Suspended particulate matter and soil pH are the other ecology experiments; have one number from your own record ready for each.

What is emasculation and why is bagging done?

Emasculation is the removal of the anthers from a flower bud before they shed pollen, so that the flower cannot self-pollinate. Bagging is covering the emasculated flower with a butter-paper bag to prevent contamination by unwanted pollen; the desired pollen is then dusted on the stigma and the flower is rebagged and tagged. It is the first step in artificial hybridisation.

The spotter shows the bag and tag; the examiner asks why the bud stage is chosen.

How would you identify a wind-pollinated flower in the spotting?

A wind-pollinated flower is small, dull, without scent or nectar, with light, dry pollen in large amounts, well-exposed stamens that hang out, and a large feathery stigma to trap pollen, as in grasses or maize. An insect-pollinated flower is large, coloured, scented, has nectar and sticky pollen.

Vallisneria is the water-pollinated example the examiner may add.

What are homologous and analogous organs? Give the examples from your spotting.

Homologous organs have the same basic structure and origin but different functions, like the forelimbs of a human, a whale and a bat, or the thorn of Bougainvillea and the tendril of Cucurbita; they show divergent evolution. Analogous organs have different origins but the same function, like the wings of a bird and an insect, or the eye of an octopus and a mammal; they show convergent evolution.

Say 'divergent' and 'convergent' in the same breath as the examples.

What is the difference between Cuscuta, lichens and root nodules as shown in the spotting?

Cuscuta is a parasitic plant that lacks chlorophyll and sends haustoria into its host to draw food. A lichen is a mutualistic association between a fungus, which gives shelter and absorbs water, and an alga, which makes food by photosynthesis. Root nodules of legumes are a symbiosis between the plant and Rhizobium bacteria that fix atmospheric nitrogen.

Three spotters, one answer each; the examiner is checking you know which interaction is which.

Name the stages of meiosis you identified in the grasshopper testis or onion bud slide.

In prophase one I identified the substages where the chromosomes pair and chiasmata appear, then metaphase one with bivalents at the equator, anaphase one with homologous chromosomes moving apart, and the second division resembling mitosis that produces four haploid cells. The key difference from mitosis is the pairing of homologous chromosomes and crossing over in prophase one.

If asked why meiosis matters: it halves the chromosome number and creates variation.

What causes malaria, amoebiasis and ringworm, and which specimens did you see?

Malaria is caused by Plasmodium, a protozoan transmitted by the female Anopheles mosquito; amoebiasis by Entamoeba histolytica, spread through contaminated food and water; and ringworm by fungi such as Microsporum, Trichophyton and Epidermophyton, spread by contact. Ascaris, the roundworm, is the fourth common specimen and causes ascariasis through contaminated food.

For each organism, give the disease, the mode of transmission and one symptom; that is the full-marks pattern.

What did your pedigree chart show?

The pedigree I analysed showed a trait appearing in every generation and in both sexes, with affected children always having an affected parent, so I concluded it was autosomal dominant. A trait that skips generations and appears in children of unaffected parents would be autosomal recessive, and one appearing mostly in males through carrier mothers would be X-linked recessive.

Know the symbols: square for male, circle for female, shaded for affected, a horizontal line for mating.