Quick answer: Choose sublimation for a suitable solid–solid mixture when one component changes directly from solid to gas under the stated conditions while another remains. Explain where the vapour deposits, how the fractions are collected, and why appearance or mass alone does not prove purity.
The 2027 Singapore–Cambridge SEC G3 Chemistry syllabus is K324. It assesses method selection and interpretation, not just definitions. This learner’s guide shows the physical pathway, compares alternatives, and gives worked reasoning exercises that progress from first principles to honest evaluation.
At a glance: identify the two solids and their properties; distinguish sublimation from deposition; follow each fraction; check recovery; separate observation from inference; read the question’s command word; verify any purity claim independently.
Start with the decision, not the apparatus
A mixture contains two solids. One substance changes directly from solid to gas when heated under suitable conditions; the other does not do so under the same conditions. Sublimation may help separate them, but that statement is only a beginning. To solve the examination question, identify which material changes state, what happens to its vapour on a cooler surface, where each fraction ends up, and which observations would actually support the claimed separation.
This guide starts a new problem after the previous crystal-purity sequence. Volume 0853 considered what melting-point evidence can and cannot establish. Volume 0856 considered independent review before publishing a scientific claim. We now examine a different question: can a proposed procedure separate a mixed solid without mistaking a change of appearance for a proof of identity?
Read this page in three passes. First, understand the physical process and draw the substance pathway. Next, compare sublimation with dissolving, filtering, evaporating and crystallising. Finally, attempt the worked evidence cases and answer in the restrained language examiners expect. You are learning to select and defend a method, not memorise an impressive-sounding procedure.
The fast facts for a learner
The core distinction is simple. Sublimation is a change of state from solid directly to gas. Deposition is the reverse transition, from gas directly to solid. Neither necessarily changes the chemical identity of a substance. A suitable solid may be separated from a non-subliming solid when conditions allow the first material to vaporise and be recovered elsewhere.
The method is not a universal cure for solid mixtures. Two solids that behave similarly, substances that decompose on heating, or materials that react with each other demand another approach. The examiner may provide information about their properties; use that information instead of assuming every solid behaves like a familiar example. A substance may also have more than one physical route depending on temperature and pressure. Explain only what the question’s conditions justify.
As of the 2027 G3 SEC syllabus, Chemistry is K324, the successor code to 6092. Paper 1 comprises 40 multiple-choice questions in 1 hour and carries 30%; Paper 2 lasts 1 hour 45 minutes, carries 80 marks and 50%; Paper 3 is the 1 hour 50 minute practical worth 40 marks and 20%. Those figures come from the official syllabus, not from an invented mock examination. Sublimation appears under methods of purification and analysis. Candidates should also be ready to choose methods for a range of mixtures and evaluate observations. Check the official document linked below when preparing for a particular examination year.
A map of where each substance goes
Imagine that a dry mixture is labelled A + B. The question states that A sublimes at a manageable temperature, whereas B remains as a solid over the relevant temperature interval. On applying the appropriate heat in controlled laboratory conditions, some A enters the gas phase. If the apparatus provides a suitable cool collecting region, A vapour may deposit there. B remains in the original region. The fractions are therefore physically separated: the material at the collection surface is enriched in A and the residue is enriched in B.
The words ‘enriched in’ matter. Unless the question establishes complete transfer and absence of contamination, it is too confident to call either fraction absolutely pure. Fine particles can be carried mechanically, material can stick to glass, and vapour may escape a poorly controlled collection arrangement. These are not reasons to abandon the method. They are reasons to distinguish the model from real measurements. A good answer first explains the intended separation and then names a plausible limitation when evaluation is requested.
Draw the pathway with words if the exam question has no diagram: feed mixture; warmed region; A as vapour; cooler region; deposited A; original vessel; remaining B. Label states of matter and locations. Every arrow should have a cause. ‘Heat separates it’ is too vague because heating alone does not tell the reader which material moved or where it was collected.
Do not confuse sublimation with evaporation
Evaporation usually refers to liquid becoming gas. Sublimation bypasses the liquid state in the change being described. When you evaporate water from a salt solution, water moves into the gas phase and dissolved salt may remain; you are not subliming the salt. When a sublimable solid passes directly into the gas phase, it is the solid itself that has undergone the state change.
Some diagrams are intentionally misleading. A student sees white deposits on a cool cover and writes, ‘Water condensed.’ That may be an unsupported guess. First ask what substance was in the original solid mixture and what the problem says about vapour formation. Likewise, a student may write ‘The vapour solidifies’ without clarifying that deposition is direct gas-to-solid, which loses a useful distinction in a definition question.
An examiner might ask whether mass is lost. Mass is not destroyed by a change of state. Yet a particular open vessel can lose measured mass because matter leaves its boundary. Track the system boundary. A closed, well-defined system and a partially open collection device are not interchangeable when discussing mass. This connects to the primary-school habit of asking where matter went before saying it disappeared; see the PSLE learner’s guide on matter disappearing from view.
Deciding among five separation methods
A sensible method is chosen from the physical properties of the mixture. Filtration separates an insoluble solid from a liquid by particle size and passage through a filter. Crystallisation can obtain crystals from a suitable solution by changing concentration and conditions. Simple distillation separates a volatile liquid from a non-volatile dissolved solid, or liquids in some suitable contexts. A separating funnel works for liquids that form distinct immiscible layers. Paper chromatography allows components to move differently with a solvent and permits comparison of spots. Sublimation offers a route for certain solid–solid mixtures when one constituent has the required volatility under the stated conditions.
The exam skill is comparative rather than encyclopaedic. If one solid dissolves in a solvent and the other does not, dissolve–filter–recover may be sensible. If neither dissolves readily but only one sublimes without decomposition, sublimation may be preferable. If both solids sublime under similar conditions, that fact weakens the proposal. If heating changes the substance chemically, the method could destroy what you are meant to recover.
The separation-techniques science owner gives the wider survey. Volume 0589 covers a water-recovery sequence. This article owns only the more precise decision about subliming solid mixtures and the quality of their evidence.
What ‘suitable conditions’ really means
Sublimation depends on the behaviour of the actual substance and the conditions. Heating does not create a universal temperature that separates all mixtures. Questions may tell you which solid sublimes, what temperature range is safe, whether another material decomposes, and whether a cool collecting surface is available. These details are constraints, not background decoration.
For an unfamiliar substance, never assert a sublimation point or safety procedure from memory unless it is taught or supplied. Work from the question’s data. A method may fail if the collection region is too warm, if vapour can escape, if the solids react, or if both components transfer at once. One useful sentence is: ‘This method is suitable if A can sublime under the stated conditions while B remains in the original region without reacting or decomposing.’ The conditional structure prevents overclaiming.
Laboratory safety is part of scientific judgement. Some substances traditionally shown in sublimation demonstrations have hazardous vapours or require specialist equipment. This page is a written examination guide, not permission to improvise heating or chemical demonstrations at home. Practical work belongs in a supervised laboratory with the school’s risk assessment, protective measures and approved materials. The examination answer should focus on the scientific principle and the information in the stem rather than inventing a hazardous procedure.
How to read a two-vessel recovery record
Consider a hypothetical dataset. The original dry sample has a mass of 12.0 g. After a controlled separation, the student records 4.3 g of deposited material and 7.4 g of residue. The reported recovered mass is 11.7 g, leaving an unaccounted difference of 0.3 g. This difference is not automatically ‘experimental error’ with no further explanation. It is a specific discrepancy between the feed and the measured products.
Several possibilities could produce it. Some vapour might not have been collected. Material might remain attached to apparatus surfaces outside the weighed collecting region. Transfer may be incomplete. One of the masses might include an incorrectly tared container. A good evaluation identifies how to test the explanation: clean and dry the relevant weighing vessels, weigh consistently, check where deposits formed and repeat the measurement where safe and appropriate.
You cannot conclude from this record alone that the 4.3 g is pure A. Its mass does not reveal its chemical identity. Nor can you conclude that 0.3 g was chemically destroyed. The account covers what the student measured, not every molecule that existed. In the next volume, Mathematics will make the mass ledger and its plausible bounds explicit.
A complete written answer to an unfamiliar mixture question
Suppose a question supplies these facts: two dry solids form a mixture; substance P sublimes under the specified conditions; substance Q does not sublime or decompose there; the vapour can be collected at a cooler surface. The task is to name an appropriate separation method and explain why it works. A strong answer is short but causal.
‘Sublimation is suitable. Warm the mixture under the specified controlled conditions so that P changes directly from solid to gas while Q remains solid. Allow P vapour to reach a cooler collecting surface where it deposits as a solid. Collect P from that surface and Q from the original region.’ That explanation earns its strength by keeping substance, state and location aligned.
A weak answer says: ‘Boil both solids and filter out Q.’ Neither boiling nor filtering was supported by the facts. An answer can also be technically true but incomplete: ‘P sublimes.’ This does not explain how the resulting vapour is recovered or separated from Q. The difference between an adequate and an excellent explanation is often the missing physical step, not a more complicated word.
Data question: partial rather than complete recovery
Now change one detail. The mass of P in the original mixture is known from an independent reference preparation to be 5.0 g. The apparatus collects 4.4 g of solid attributed to P. The student claims the recovery was 100%. Can you accept that? No. Even if all collected matter is P, 4.4 out of 5.0 is 88% recovery by mass. The remaining 12% has not been accounted for within the measured collection fraction.
Do not confuse percentage recovery with percentage purity. Recovery compares the amount obtained with the amount that could have been obtained under the stated reference. Purity concerns what fraction of the recovered product is the desired substance. An 88% recovery could coexist with a fairly pure sample, an impure sample, or insufficient identity evidence. No number here settles purity without another measurement.
If an answer asks for an improvement, match it to the limitation. Improving the collecting region may address escaped vapour. Better tare practice may address mass differences. Repeated trials help judge consistency. Do not respond with the stock sentence ‘make the experiment more accurate’ unless you explain which step is improved and what kind of error it reduces.
Evidence level: observation, inference and verification
Observation: ‘A deposit appeared on the upper cool region, and solid remained below.’ This is a record of what was seen. Inference: ‘A component may have vaporised and deposited elsewhere.’ This interpretation depends on the stated conditions and the behaviour of the substances. Verification: ‘A separate, appropriate test supports the identity of the collected fraction.’ These are three different levels of claim.
The temptation to turn every observation into a final identity declaration is strong. A white deposit does not uniquely identify a particular material. Many materials can be white. A smaller sample mass does not necessarily mean the desired fraction has been fully recovered. A clean-looking vessel does not prove it is free of contaminants. Science marks reward observations that are accurate, deductions that follow and limitations that are proportionate to the evidence.
When the question explicitly gives chemical identities or unambiguous properties, you may use them. When it supplies only appearance, do not invent a conclusion. A useful examination phrase is: ‘The result is consistent with separation by sublimation, but the observation alone does not establish the purity of the recovered fraction.’ Such sentences are clear rather than defensive.
What examination command words ask you to do
‘State’ requests a fact, often a method or a property. ‘Describe’ usually requires an observable sequence or clear account. ‘Explain’ asks for a cause or reason; include the relevant property and why it produces separation. ‘Suggest’ expects an appropriate choice based on supplied conditions, particularly when the exact mixture is unfamiliar. ‘Evaluate’ calls for strengths, limitations and a justified judgement. ‘Predict’ asks what should happen if the given principles apply.
Do not give a lengthy essay to every short command. A two-mark question might need only ‘sublimation’ plus the distinction that one solid becomes vapour while the other remains. A longer question involving an unfamiliar apparatus may require a pathway, collection step and limitation. Reading the mark allocation can guide detail, but it does not replace reading the command word.
The official G3 Chemistry syllabus distinguishes knowledge with understanding, handling information and solving problems, and practical skills. This guide is designed to practise all three modes without falsely claiming that the same exact model question will appear in the examination.
Common error: writing a process without a destination
Imagine the question asks, ‘How will you obtain the sublimable component?’ Some learners say, ‘Heat the mixture until the substance evaporates.’ Even setting aside the incorrect name of the state change, the answer ends just where recovery should begin. What does the gas do? Where is it held? How can the desired substance be collected?
Improvement: state the pathway through the cooler region and deposition. Do not say ‘everything condenses’ when the intended process is gas-to-solid deposition. If the apparatus includes a funnel-like collector in an illustration, identify its role from the actual diagram rather than assuming the exact apparatus is always required. Draw a line to separate ‘material remaining in the original vessel’ from ‘material deposited elsewhere.’
This habit transfers to many science questions. Students describe a pump moving fluid but fail to name the destination; they describe a current flowing but omit the closed route; they describe a chemical product without naming the evidence by which it was identified. Following matter all the way through the procedure is a universal repair.
Common error: treating a clean result as proof
A collected solid looks uniform. Does that prove it is pure? No. Uniform appearance is one observation. Depending on the question, a measurement of melting behaviour, another appropriate physical test, or chemical analysis might help. The chosen test must be relevant and the inference must respect its limitations. If the sample decomposes or has no suitable reference behaviour, do not promise a conclusive answer from an unsuitable test.
The relation to Volume 0853 is deliberate. That volume dealt with identity and melting-point evidence after crystallisation. This volume does not repeat that owner; it places the correct separation decision before the identity question. Good laboratory reasoning has an order: choose, separate, record, then interpret and verify.
A careful learner is also willing to say, ‘Not enough information is provided.’ That may be the scientifically strongest judgement when evidence is missing. The sentence should specify what additional information is required, such as different behaviours of the constituent solids, the collection conditions or results of an independent check.
Common error: assuming all heating is the same
Heating can melt, evaporate, sublime, decompose or provoke chemical reaction depending on a substance and its surroundings. The word ‘heat’ in a question is therefore insufficient by itself to infer a particular change. The question may supply a table describing a solid’s properties. Read it before selecting the technique.
If one substance decomposes at the proposed temperature, recovery by sublimation might be unsuitable even if it would otherwise volatilise. If both solids can enter the gas phase, the simple one-step model may not isolate the required component. If the residue contains solvent, another preliminary step may be needed; a solid–solid method cannot be applied blindly to a wet mixture. An excellent candidate checks these constraints before explaining a process.
You can rehearse this as a sentence frame: ‘Because [component] has [given property] and [other component] does not under [specified conditions], [method] separates them by [physical pathway].’ Replace the brackets with the actual facts. Do not memorise the example’s materials as if every question were identical.
Worked case A: the student chose filtration
A student is given two dry solids, R and S. R is stated to sublime under the allowed conditions, while S stays solid. The student proposes mixing them with water and filtering, but the question gives no solubility information. Judge the proposal. Filtration could be useful in some solid–liquid separations, but nothing in the stem shows that one solid dissolves and the other does not. The student’s method rests on an assumption.
The appropriate response is not to say that filtration is always wrong. Say instead that its suitability cannot be established from the data. Sublimation is supported because the stem explicitly distinguishes the solids by their state-change behaviour. If a later part provides solubility data, reconsider the methods comparatively. A method can be appropriate for one set of facts and unjustified for another.
An examiner rewards the reasoning that uses the information. This is why practising new contexts matters. Memorising ‘mixture means filter’ or ‘solid means heat’ is weaker than reading the physical properties and choosing from them.
Worked case B: a misleading mass balance
Another student starts with 8.50 g of mixed solids. The residue weighs 5.10 g, and the deposit weighs 3.25 g. The two products total 8.35 g, which is 0.15 g less than the feed. The student writes, ‘The reaction used up 0.15 g.’ This is a category mistake. Sublimation is a physical state change, and the data do not show a chemical reaction that consumed matter.
A more careful conclusion is: ‘The measured fractions total 0.15 g less than the original sample; some material may be uncollected or the measurement process may contain an error.’ A suitably designed additional measurement may distinguish the possibilities. Do not automatically blame a balance, because no evidence shows that the balance was faulty. Nor should you automatically blame lost vapour. Name both as hypotheses that must be checked.
The calculation is simple, but its interpretation is not trivial. Good science uses arithmetic to test a story about matter, not to invent a story to fit a discrepancy.
Worked case C: the collector is too warm
A class compares two hypothetical set-ups using the same sample composition. In the first, the collection surface is cooler than the source region. In the second, that surface is not sufficiently cool for the target vapour to deposit under the stated conditions. Predict which design should recover more of the target at that location, all else equal. The first has a better physical route for deposition and collection.
The phrase ‘all else equal’ protects the explanation. If the actual apparatus differs in several other ways, a real result cannot be attributed to temperature alone. This is the language of a controlled comparison. The target result is not merely to maximise a number; it is to make the proposed cause testable. Observe deposits, measure collected mass if appropriate, and avoid handling unfamiliar heated substances outside supervised practice.
In Paper 3, candidates may be required to follow instructions, present observations and evaluate methods. The official practical scheme addresses planning, manipulation, measurement, presentation and analysis. This written example demonstrates a reasoning skill; it is not a prediction of a specific practical station.
Worked case D: the identities are unknown
The problem describes an unknown dry mixture. After a proposed sublimation trial, the operator reports a residue and a new solid deposit. They conclude, ‘The original mixture definitely contained iodine.’ This is not justified simply by seeing a deposited solid. A named identity needs chemical or other specific evidence. The given process and appearance can establish that a component appears to have behaved in a way consistent with sublimation, not which compound it was.
Write the more defensible inference: ‘At least one component may be capable of sublimation under these conditions, provided the deposited material originated from the starting mixture rather than contamination.’ That qualification can feel cautious, but it demonstrates an understanding of alternative explanations. In an examination, avoid unnecessary qualifiers if the task explicitly supplies the identities and assumptions; use them only when evidence truly is incomplete.
The lesson is to avoid treating a familiar textbook photograph as a substitute for evidence. A known classroom example helps learning, but the examination question may deliberately alter the substance or the set-up.
A small method-selection clinic
Case 1: A soluble salt is mixed with insoluble sand, and a suitable solvent is available. A dissolve–filter–recover route is better supported than sublimation. Case 2: Two immiscible liquids are present in separate layers. A separating funnel is more suitable. Case 3: A suitable liquid containing a non-volatile solute must be recovered as liquid. Distillation is a natural candidate when the necessary boiling behaviour and conditions are supplied. Case 4: Two coloured substances migrate differently on paper with a suitable solvent. Chromatography is supported.
Case 5: A dry solid mixture has one component that sublimes and deposits on a cool surface, while the other remains unchanged. Sublimation is supported. Case 6: The two solids both readily sublime under the allowed conditions. A simple single-stage sublimation is not automatically selective. Case 7: One solid decomposes before it can be recovered. Reject a proposed procedure that destroys it. Case 8: The sample has a damp residue, and the question requires a dry mass. A drying step under appropriate conditions and a reliable mass measurement become relevant before you trust the ledger.
These examples are not separate lists of tricks. They illustrate one question asked repeatedly: what difference in physical behaviour can this method exploit? If you cannot name the property, your choice of apparatus is probably premature.
A five-minute retrieval exercise
Close the notes and answer four questions aloud. What is the difference between sublimation and deposition? Why does a cooler collecting surface matter? Which fraction remains at the source? What evidence would you need before calling a collected fraction pure? Do not stop at an isolated definition. Give a cause and destination in ordinary language.
Then draw the pathway on paper and label the states: original mixture, solid component in place, sublimed component as gas, deposited component at collector. Mark a possible loss route and write the sentence ‘uncollected material is not destroyed material’. Next compare the same dry-mixture problem with a water-based filtration problem. The aim is to practise selecting the method from properties rather than memorising an apparatus name.
Finally write a two-sentence evaluation: one claim that the described evidence supports, and one stronger claim it does not yet support. That ability to limit an inference is just as important as remembering a process. It becomes particularly useful when the paper provides unfamiliar details that are meant to be analysed.
Practice tasks with answers and reasons
Task 1. State the change of state in sublimation. Answer: solid directly to gas. Task 2. State the reverse change of state. Answer: deposition, gas directly to solid. Task 3. Explain why one subliming solid may be separated from another solid that stays at the source. Answer: they follow different physical routes under the stated conditions and can be collected from different locations.
Task 4. A 10.0 g feed gives 3.6 g at the cool collector and 6.1 g in the source vessel. What is missing from the measured ledger? Answer: 10.0 − (3.6 + 6.1) = 0.3 g. Task 5. Can the 3.6 g automatically be called pure product? Answer: no; collected mass alone does not establish identity or purity. Task 6. Does the mass difference prove matter has been destroyed? Answer: no; it shows only that the recovered fractions do not sum to the original measurement.
Task 7. Both solids sublime under the allowed conditions. Is a simple one-step collection reliably selective? Answer: not on that fact alone. Task 8. The question provides no solubility data but does provide distinct sublimation behaviour. Should you assert that filtration works? Answer: no; the filtration proposal lacks a stated basis. Task 9. The cooler collector shows no deposit. Is the starting sample proven to contain no sublimable component? Answer: no; the conditions or collection may be unsuitable. Task 10. A process note says ‘evaporated solid’. Improve the wording. Answer: ‘the specified solid sublimed’ if the stated transition was directly solid to gas.
Task 11. Give one evidence-based limitation of mass recovery. Answer: deposits outside the weighed collection zone could leave an unaccounted difference. Task 12. State one way to make the method choice stronger. Answer: obtain or use supplied evidence that the two solids behave differently under the proposed conditions.
How to turn a wrong answer into a useful correction
A wrong answer is useful when you can identify the first unsupported move. Did you overlook that the mixture was dry? Did you assume a solvent would dissolve one component? Did you call the gas-to-solid change condensation? Did you use a clean-looking deposit as proof of purity? Each mistake points to a different repair.
Record a compact error ledger. In the first column write the stem fact you missed; in the second, write the inference you made; in the third, write a corrected sentence that uses the fact properly. Return to that sentence after a day and solve a different mixture problem without seeing your previous answer. This is more reliable than re-reading a perfect model answer until it feels familiar.
For a parent or tutor, an effective conversation asks the student to follow one substance through a process. ‘Where did it start? What state did it enter? What made it move? Where did it end up? How do we know?’ These questions invite an explanation rather than a performance. For subject foundations, see the Secondary 3 Science owner.
Exam readiness without invented shortcuts
The 2027 K324 syllabus puts genuine weight on interpreting information and solving problems, including unfamiliar settings. A learner therefore needs two kinds of fluency: enough conceptual knowledge to identify the relevant process, and enough judgement to decide what an observation permits them to claim. Sublimation is a compact example because the concept is simple but the evidence can be deceptively incomplete.
If you have ten minutes for revision, use two minutes to define and draw sublimation and deposition; three minutes to compare separation techniques; three minutes to attempt one mass-balance and one method-selection question; and two minutes to explain a limitation precisely. If you have more time, make an unfamiliar scenario in which the first proposed method fails and explain how the data changed your choice.
Keep the big picture. This is one method in experimental Chemistry, not a guarantee of a particular exam question. For the official codes, outcomes and schemes, consult SEAB’s 2027 G3 subject list and the K324 Chemistry syllabus. The next volume follows the measured masses into mathematical recovery, bounds and error checking.
Continuity: Read Vol 0856 for the previous evidence-release decision. Read the next Mathematics volume for the measured mass ledger. Return to the PSLE-to-secondary science transition for foundations.
