Once an organic compound is extracted from a natural source or synthesised in the laboratory, it is essential to purify it. Various methods used for the purification of organic compounds are based on the nature of the compound and the impurity present in it. The common techniques used for purification are as follows : (i) Sublimation (ii) Crystallisation (iii) Distillation (iv) Differential extraction and (v) Chromatography Finally, the purity of a compound is ascertained by determining its melting or boiling point. Most of the pure compounds have sharp melting points and boiling points. New methods of checking the purity of an organic compound are based on different types of chromatographic and spectroscopic techniques.
-- NCERT Class 11 Chemistry, Ch. 8, p. 278Sublimation
8 MCQs9-step worked example
Source: NCERT Purification and Characterisation of Organic CompoundsPYQ coverage: NEET 2024Official key: NTA-verifiedLast updated: 24 Sep 2026
Sublimation, explained for NEET
The trap in sublimation questions is not the definition — it is the boundary of what the method can do. Sublimation separates a solid that passes directly to vapour from one that does not. Put two sublimable solids together and the technique collapses: both leave the mixture, both condense on the cold surface, and nothing is separated. An option offering sublimation for such a pair is written to be chosen.
The second trap is the phase path. Sublimation is the direct solid-to-vapour change with no intervening liquid, as NCERT Class 11 Chemistry Chapter 8 defines it on page 279. Any option that has the substance melting first and then boiling has described distillation wearing a different name. Watch for "the solid melts and the vapour is collected" — the melting step disqualifies it.
The working principle, from the same chapter on page 279: some solid substances change from solid to vapour on heating without passing through the liquid state, and the technique built on that principle separates sublimable compounds from non-sublimable impurities. The vapour is condensed back to solid on a cooled surface, and the non-sublimable residue stays behind. The chapter states the principle and does not list examples; the ones you will meet in questions — camphor, naphthalene, anthracene, benzoic acid — sublime, while ordinary inorganic salts and most non-volatile organic impurities do not.
For NEET this is a low-weight, elimination-style topic — roughly one question every three years, usually asking whether a named pair can be separated at all. The marks come from recognising the disqualifying condition, not from any calculation. There is no formula here.
Two closing watch-outs. Sublimation is not evaporation: evaporation starts from a liquid surface, sublimation never has a liquid stage. And a pair that fails sublimation is not unseparable — it usually needs crystallisation or chromatography instead.
Can you answer these Sublimation MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Sublimation is the process in which a substance:
Show answer and why every option is right or wrong
Answer: C. C is correct. NCERT Class 11 Chemistry Chapter 8, page 279, defines sublimation as the direct change from solid to vapour with no liquid stage in between.
Why A is wrong: A is wrong because the presence of a melting step disqualifies the process as sublimation; a solid that melts and then vaporises is being distilled, not sublimed.
Why B is wrong: B is wrong because it describes evaporation, which begins from a liquid surface. Confusing sublimation with the evaporation of a liquid is a recorded student error for this topic.
Why D is wrong: D is wrong because vapour to liquid is condensation. In sublimation the vapour deposits directly as a solid on the cold surface.
MCQ 2Easy RecallPractice
Which one of the following solids passes directly from the solid to the vapour state on heating, and can therefore be purified by sublimation?
Show answer and why every option is right or wrong
Answer: B. B is correct. Camphor goes straight from solid to vapour on heating, which is the behaviour sublimation exploits — the principle is stated in NCERT Class 11 Chemistry, Chapter 8, page 279. Of the other three, one melts, one loses water of crystallisation and one decomposes, so none of them sublimes.
Why A is wrong: A is wrong because sodium chloride is a non-volatile ionic solid that melts at high temperature rather than subliming; it is the kind of residue sublimation leaves behind.
Why C is wrong: C is wrong because copper sulphate pentahydrate loses water of crystallisation on heating. Dehydration is not a solid-to-vapour change of the compound itself.
Why D is wrong: D is wrong because calcium carbonate decomposes on strong heating to calcium oxide and carbon dioxide. Decomposition is a chemical change, not sublimation.
MCQ 3Direct ApplicationPractice
A solid organic compound is mixed with a non-volatile inorganic salt. On heating the mixture in a china dish covered with an inverted funnel, a crystalline solid collects on the inner surface of the funnel. That collected solid is:
Show answer and why every option is right or wrong
Answer: D. D is correct. Applying the principle stated in NCERT Class 11 Chemistry Chapter 8, page 279, only the sublimable component leaves the dish as vapour and deposits as solid on the cooled funnel, while the non-sublimable salt remains behind.
Why A is wrong: A is wrong because the salt is stated to be non-volatile; a non-volatile component is precisely the one that stays in the dish as residue.
Why B is wrong: B is wrong because sublimation is a physical separation. No chemical reaction between the components is involved.
Why C is wrong: C is wrong because a melting step would mean the compound is not subliming. Sublimation involves no liquid stage, and molten liquid does not travel up to the funnel surface.
MCQ 4Concept TrapPractice
A mixture contains two solids, both of which sublime readily on heating. Attempting to separate them by sublimation will:
Show answer and why every option is right or wrong
Answer: A. A is correct. Sublimation works only when one component passes to vapour and the other does not, as set out in NCERT Class 11 Chemistry Chapter 8, page 279. If both sublime, both reach the cold surface and the mixture is reconstituted there.
Why B is wrong: B is wrong because a difference in rate is not a separation criterion for this technique; the method depends on one component not subliming at all, not on subliming more slowly. Treating sublimation as a way to separate two sublimable solids is a recorded student error.
Why C is wrong: C is wrong on two counts: it introduces a melting step, which sublimation does not have, and it wrongly makes the more volatile component the residue.
Why D is wrong: D is wrong because the premise states both solids sublime readily, so both certainly do leave the mixture. The failure is that both arrive at the same place, not that neither moves.
MCQ 5Direct ApplicationPractice
Four impure solids are offered for purification by sublimation. In which case is the technique appropriate?
Show answer and why every option is right or wrong
Answer: C. C is correct. Benzoic acid sublimes while the inorganic residue does not, which is exactly the one-sublimable-one-not condition NCERT Class 11 Chemistry Chapter 8, page 279, requires.
Why A is wrong: A is wrong because naphthalene and anthracene both sublime, so both would deposit on the cold surface and no separation would be achieved.
Why B is wrong: B is wrong because neither chloride sublimes; both are non-volatile ionic solids and nothing would leave the dish.
Why D is wrong: D is wrong because camphor and naphthalene both sublime. An option pairing two sublimable solids is the standard distractor for this topic.
MCQ 6Concept TrapPractice
A student states that sublimation and evaporation are the same process occurring at different temperatures. The correct assessment of this statement is that it is:
Show answer and why every option is right or wrong
Answer: D. D is correct. The defining feature of sublimation in NCERT Class 11 Chemistry Chapter 8, page 279, is the absence of a liquid state; evaporation by definition starts from a liquid, so the two processes are distinct in kind, not merely in temperature.
Why A is wrong: A is wrong because sharing an end product does not make two processes identical. The starting phase and the phase path differ, and that difference is what defines sublimation.
Why B is wrong: B is wrong because it reverses both products: evaporation gives vapour from a liquid, and sublimation gives vapour from a solid.
Why C is wrong: C is wrong because it smuggles in a melting step. Any description in which the solid first becomes liquid has stopped describing sublimation.
MCQ 7CalculationPractice
A solid X is contaminated with a solid Y. Heating the mixture under an inverted funnel deposits material on the funnel and leaves a residue in the dish. Analysis shows the deposit and the residue have the same composition as each other. The correct inference and the appropriate next technique are:
Show answer and why every option is right or wrong
Answer: A. A is correct. Identical composition on both sides means both components travelled and both stayed, so both are sublimable and the one-sublimable-one-not condition of NCERT Class 11 Chemistry Chapter 8, page 279, is not met; a different purification technique is required.
Why B is wrong: B is wrong because if only X sublimed, the deposit would be X alone and the residue Y alone, giving two different compositions rather than the same one. Repeating the run would reproduce the same failure.
Why C is wrong: C is wrong because if neither component sublimed there would be no solid deposit of the mixture's composition on the funnel at all.
Why D is wrong: D is wrong for the same reason as A, with the roles reversed: a single subliming component cannot produce a deposit and a residue of identical composition.
MCQ 8CalculationPractice
An impure solid is heated in a china dish under an inverted funnel. The chemist observes the solid liquefy in the dish, and a distillate later collects. The chemist records the run as a sublimation. The record is:
Show answer and why every option is right or wrong
Answer: B. B is correct. Sublimation is defined in NCERT Class 11 Chemistry Chapter 8, page 279, by the direct solid-to-vapour change; an observed liquid stage rules the process out regardless of what was collected.
Why A is wrong: A is wrong because producing and condensing a vapour is common to several techniques. It is the phase path, not the appearance of vapour, that identifies sublimation.
Why C is wrong: C is wrong because obtaining a pure product is the aim of every purification technique and so cannot define any one of them.
Why D is wrong: D is wrong because sublimation places no such requirement on which component vaporises; either the compound or the impurity may be the sublimable one, provided only one of them is.
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How do you solve a Sublimation question? A worked example
- 1
Given.
A mixture of two solids: benzoic acid and sodium chloride. Benzoic acid passes from solid to vapour on heating without melting. Sodium chloride is non-volatile at the temperature used and does not vaporise.
- 2
Required.
State whether the pair can be separated by sublimation, and identify what is collected on the funnel and what remains in the dish.
- 3
Concept.
Sublimation separates a mixture when exactly one component makes the direct solid-to-vapour transition under the conditions used (NCERT Class 11 Chemistry Chapter 8, page 279). The sublimable component leaves as vapour and deposits as solid on a cooled surface; the non-sublimable component stays behind as residue. The separation criterion is therefore a two-part test on the pair, not a property of either solid alone.
- 4
Formula.
None. This topic has no formula in the dossier; the decision is made by applying the criterion above, not by calculation.
- 5
Substitution.
Apply the two-part test to the pair. Component 1, benzoic acid: sublimes — yes. Component 2, sodium chloride: sublimes — no. Exactly one component sublimes, so the criterion is satisfied.
- 6
Calculation.
No arithmetic is required. The reasoning is a single logical step: (one sublimes) AND (the other does not) ⇒ separable by sublimation. Note that the two counts here, "one" and "the other," are counting integers and carry no significant-figure implication; no measured quantity enters this problem at all.
- 7
Final answer.
The pair can be separated by sublimation. Benzoic acid is collected as a crystalline deposit on the cooled inner surface of the inverted funnel. Sodium chloride remains in the china dish as residue.
- 8
Common trap.
The trap is to apply the criterion to only one component. A student who reasons "benzoic acid sublimes, therefore sublimation works" will give the same answer here and the wrong answer when the second solid is naphthalene, which also sublimes. The dossier records treating sublimation as a way to separate two sublimable solids as a student error, and lists an option offering sublimation where both components sublime as a negative-marking trap. Always test both components before answering.
- 9
Similar NEET-style question.
A mixture of camphor and potassium nitrate is heated in a covered china dish. Which of the following is obtained on the cooled surface of the cover: camphor, potassium nitrate, both, or neither? *(Answer: camphor alone — it is the only sublimable component of the pair, and potassium nitrate remains in the dish.)*
What to remember before solving Sublimation questions
Definition
Sublimation
You have learnt earlier that on heating, some solid substances change from solid to vapour state without passing through liquid state. The purification technique based on the above principle is known as sublimation and is used to separate sublimable compounds from non-sublimable impurities.
-- NCERT Class 11 Chemistry, Ch. 8, p. 279Sublimation questions from past NEET papers
1 question from NEET 2024. Answers verified against NTA official keys.
NEET 2024Revised key
1Crystallization
2Sublimation
3Distillation
4Chromatography
NTA Answer: Option 2(revised_final)
All 7 past-paper questions from Purification and Characterisation of Organic Compounds →
Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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