Chemistry Practical Exam — The Viva Questions That Repeat
The practical viva is the most predictable part of the whole chemistry course, and the part students prepare last. An external examiner has a few minutes per student and a table full of apparatus in front of them. So the questions come from a small pool: the glassware on the bench, the experiment you just performed, and the reason behind one step of it.
Below are the questions that genuinely repeat, with the short answers that satisfy an examiner. Learn the reasoning, not the sentence — a follow-up "why?" is guaranteed if you sound rehearsed. And check with your own school which experiments are on your list, since the practical syllabus and mark distribution are set by your board's current circular.
Apparatus and technique — asked of everybody
| Question | Answer |
|---|---|
| Why is the burette rinsed with the solution it will hold? | Water left inside would dilute the solution and change its concentration, so every reading would be wrong. |
| Why is the conical flask not rinsed with the solution? | Only the number of moles taken by pipette matters. Extra water changes volume but not moles, so the titre is unaffected. |
| Why must a volumetric flask never be heated? | It is calibrated for one temperature. Heating changes the glass and the liquid volume, so the calibration is lost. |
| Why is the air bubble removed from the burette tip? | The bubble later escapes and is recorded as delivered liquid, giving a falsely high titre. |
| Which meniscus reading do you take? | Lower meniscus for colourless solutions; upper meniscus for dark solutions such as KMnO₄, where the lower level cannot be seen. |
| What is parallax error and how do you avoid it? | Apparent shift in reading when the eye is not level with the meniscus. Keep the eye horizontally in line with the meniscus. |
| Why are concordant readings taken? | Two or more readings agreeing within a small range show the technique was consistent, so the mean is reliable. |
Acid–base titration
- Why is oxalic acid used as a primary standard? It is available in a high state of purity, is stable in air, is not hygroscopic, and its composition (H₂C₂O₄·2H₂O) is definite — so a weighed sample gives a known concentration directly.
- Why is NaOH not a primary standard? It is hygroscopic and absorbs CO₂ from the air, so its mass on the balance does not correspond to a known amount of NaOH.
- Which indicator, and why? Choose an indicator whose colour change lies inside the steep part of the titration curve: phenolphthalein for weak acid–strong base, methyl orange for weak base–strong acid. For strong acid–strong base the jump is wide, so either works.
- What is the end point versus the equivalence point? Equivalence point is where stoichiometrically equal amounts have reacted; end point is where the indicator changes colour. A good indicator makes them practically coincide.
Permanganate titration — the favourite trap
Q. Why is dilute H₂SO₄ used and not HCl or HNO₃?
HCl would be oxidised by KMnO₄ to chlorine, consuming permanganate and giving a falsely
high titre. HNO₃ is itself an oxidising agent and would interfere. H₂SO₄ is neither
oxidised nor reducing under these conditions.
Q. Why is KMnO₄ called a self-indicator?
Its own deep purple colour disappears as it is reduced to colourless Mn²⁺; one drop in
excess gives a permanent light pink. No separate indicator is needed.
Q. Why is the oxalic acid solution warmed to about 60 °C?
The reaction is slow at room temperature. Warming speeds it up. It should not be boiled,
because oxalic acid decomposes.
Q. Why is KMnO₄ filled in the burette and not the pipette?
It is coloured and corrosive to rubber; it also attacks rubber tubing, so a glass-stopcock
burette is used and it is never pipetted by mouth.
Salt analysis
- Why is dilute HCl added before testing for carbonate? Carbonates give brisk effervescence of CO₂ with dilute acid, which turns lime water milky — a confirmatory step done early because it is quick and decisive.
- Describe the brown ring test. For nitrate: to the salt solution add freshly prepared FeSO₄ solution, then pour concentrated H₂SO₄ down the side of the tube. A brown ring forms at the junction of the two layers.
- Why are group reagents added in a fixed order? Because each group's reagent precipitates only its own cations under those conditions. Changing the order would precipitate later-group ions early and give a wrong identification.
- Why must the original solution be freshly prepared? Some ions hydrolyse or oxidise on standing, which changes the results of later tests.
- Why is the flame test wire cleaned with concentrated HCl? To remove residues from a previous salt and to convert the salt to its volatile chloride, which gives the characteristic colour.
The calculation they will ask you to explain
Examiners often point at your record and ask how you got the concentration. Be able to say the relation and what each symbol means, in one breath:
If you are asked why the molarity form needs the coefficients and the normality form does not: normality already builds the number of reacting units into the definition, so it hides the stoichiometry. That is exactly why examiners like the question.
What goes wrong in the practical exam
- A file copied the night before. Examiners ask about a reading in your record. If you cannot explain your own observation table, the marks go whatever your writing looks like.
- Perfect readings. Three identical titre values to two decimal places look invented. Real concordant readings differ slightly.
- Reciting a memorised paragraph. The follow-up question is always "why?", and a memorised answer has no second layer.
- Not knowing your own apparatus. Pipette versus burette, volumetric versus conical flask, the least count of the burette — these are free marks, and they are lost more often than the hard questions.
- Guessing when unsure. "Sir, I am not certain — may I reason it out?" followed by honest reasoning scores better than a confident wrong statement.
A three-day preparation plan
Day 1 — Complete and correct the record: every experiment, aim, apparatus, procedure, observation table, calculation, result and precautions. Fix anything you cannot explain.
Day 2 — One pass over the apparatus and technique table above, plus the reactions of every experiment you performed, written with conditions and colour changes.
Day 3 — Mock viva. Someone reads the questions in random order; you answer aloud in one or two sentences. Anything you stumble on goes on a single revision card for the morning of the exam.
Check every titration calculation in your record before submission. The concentration tool converts between molarity, mass and volume so you can confirm the figure you wrote next to your observation table.
Open the Molarity / Concentration Calculator →Practicals count towards your final chemistry result, and they are the easiest marks to secure with guided preparation. ABC Chemistry runs Class 11–12 chemistry coaching from its Gurugram centre and online across India — abcchemistry.in.