Why can dark iodine crystals produce purple vapour, and what does that have to do with sublimation? Iodine is a molecular solid containing I₂ molecules that can enter the gas phase directly from the solid surface under suitable conditions. That direct solid-to-gas process is sublimation; the purple vapour is molecular iodine in the gas phase. On a cooler suitable surface, iodine vapour can become solid again through deposition. The I₂ molecules remain iodine molecules during the physical state change: ordinary sublimation does not turn them into iodine ions or break them into isolated iodine atoms.
For Hougang parents searching for Secondary 4 Chemistry tuition, an SEC G3 Chemistry K324 tutor, a guide to sublimation, solid-solid mixture separation or the difference between physical and chemical changes, start with the exact question: What property allows one substance to enter the vapour phase while another solid stays behind? The 2027 SEAB G3 Chemistry syllabus explicitly includes sublimation as a method of purification and analysis. Iodine is a memorable example, but it can also melt at sufficiently high temperature under ordinary pressure, so do not teach that iodine is physically incapable of becoming a liquid. Iodine vapour is hazardous and must not be generated in home demonstrations.
A little Chemistry can be hidden in a colour change. Solid iodine crystals are dark, yet iodine vapour is violet or purple. When a vapour later forms crystals on a cooler surface, the story seems to run backwards. The interesting lesson is not the colour by itself; it is how the same chemical substance can travel between physical states.
The three questions that make sublimation understandable
First: what is the substance? Molecular iodine, I₂. Second: what physical change occurs? Some I₂ molecules go from the solid into the gas without passing through a bulk liquid stage in that particular transition. Third: does a chemical reaction have to occur? Not for ordinary sublimation.
The student’s answer should identify the molecules and the state change before using the word ‘purple’. Colour is an observation; the moving molecular species is the explanation.
This distinction becomes especially useful in unfamiliar mixture questions where the examiner supplies one material that sublimes and another that does not under the chosen conditions.
What exactly is sublimation?
Sublimation is the direct physical change of a solid to a gas. The reverse direct gas-to-solid change is deposition.
The ordinary solid-to-liquid transition is melting, and liquid-to-gas transition is vaporisation. Sublimation differs because the directly observed route bypasses the bulk liquid phase.
A clear state diagram should show arrows between solid and gas in both directions, separately from the arrows between solid and liquid and between liquid and gas.
Why can iodine molecules leave the solid?
Iodine crystals contain I₂ molecules attracted to neighbouring iodine molecules through intermolecular forces. At a given temperature, molecules at the solid surface have a distribution of energies, and some can escape to the gas phase.
The solid’s vapour pressure and the surrounding gas conditions influence how much iodine vapour is present. Heating generally raises the tendency for more material to enter the vapour phase, but phase behaviour depends on temperature and pressure.
The I–I bond inside each I₂ molecule is not the same interaction as the attraction between separate I₂ molecules in the crystal.
Why the purple vapour does not mean iodine has reacted with oxygen
A dark solid transforming into purple vapour is a physical state change of iodine under the normal example. The vapour may have a different visible colour and optical behaviour from the solid because the particles and their interactions are distributed differently.
A student should not infer that purple vapour is a new compound merely because it looks different from the dark starting crystals.
The essential chemical formula remains I₂. Iodide ions, I⁻, would be a different chemical species and cannot simply be assumed from the vapour colour.
Important correction: iodine can melt
Iodine is often introduced as a substance that sublimes, but that does not mean it has no melting point. At about one atmosphere, iodine has a melting temperature near 114°C and a boiling temperature around 184°C.
Under suitable conditions iodine can change from solid to liquid, and a heating demonstration may show melting as well as purple vapour. The visible presence of purple vapour alone does not prove that no liquid was present anywhere in the material.
The scientifically careful statement is that iodine can sublime and is a familiar example of a solid with appreciable vapour pressure. The exact route depends on temperature and pressure.
Why do school questions use iodine anyway?
The combination of dark solid crystals and distinctive purple vapour makes iodine a visually memorable illustration of vapour formation. A cooler receiving surface may then collect darker iodine crystals through deposition.
The same identity of the chemical is maintained through these physical changes. That helps learners understand why a separation method can move one substance to a new location without chemically converting it.
The school question should still specify relevant information about the substances rather than depend on children knowing every detail of iodine’s phase diagram.
What happens during deposition?
Deposition is the transition from gas directly to solid under suitable conditions. When iodine vapour encounters conditions where the solid phase is favoured, molecular iodine can form solid material again.
The gas molecules do not first have to become iodide ions or chemically react with a metal. They can form the same molecular substance in a different phase.
This explains the general logic of sublimation purification: one component can enter the vapour phase, move, and then be collected as solid material under appropriate controlled laboratory conditions.
Worked comparison: two solids with different volatility
Imagine a theoretical mixture of Solid A and Solid B. The supplied data state that A can sublime appreciably under selected conditions, while B is effectively nonvolatile there and remains unchanged.
An appropriate controlled sublimation process could transfer A into the vapour phase while B remains as a residue. A receiving region maintained under suitable conditions could then collect A after deposition.
The examiner should be looking for the differing physical property. There is no need to invent a chemical reaction between the two solids to explain their separation.
Why not simply use filtration?
Filtration is generally appropriate when an insoluble solid is being separated from a fluid with suitably different particle and medium properties. A dry mixture of two solids does not become selectively separated by pouring it onto filter paper.
If one solid is able to sublime while another is not under suitable conditions, phase behaviour may provide a relevant separation route. In a different problem, selective solubility in a solvent might be more appropriate.
Use the Hougang Filtration, Evaporation and Distillation guide for the other common method choices.
How sublimation differs from evaporation
Evaporation is a liquid-to-gas process occurring at the liquid surface. Sublimation is solid-to-gas. Both can involve molecules entering the gas phase, but they begin from different physical states.
A child who says ‘iodine evaporates from the solid’ may be using informal everyday language; for precise Chemistry, sublimation is the term for direct solid-to-gas change.
If a sample has actually melted first, its subsequent liquid-to-gas process is vaporisation rather than direct sublimation. Correct naming depends on the observed or specified path.
How sublimation differs from distillation
Distillation typically vaporises a liquid component and condenses the vapour as a liquid collected elsewhere. Sublimation purification concerns transferring a solid directly into vapour and then recovering solid under the appropriate conditions.
The two methods share an underlying use of differences in volatility and phase behaviour, but the states being separated and collected differ.
Do not label every piece of apparatus with a condenser ‘distillation’ when the product is deliberately being recovered as deposited solid.
What does the residue contain?
When a suitable mixture is separated by sublimation, the less volatile solid component can remain where the original mixture was held. That remaining material is the residue.
The recovered subliming material is collected separately after deposition. A student’s answer should identify which substance goes to the collection region and which stays behind.
It is not enough to state ‘sublimation separates the mixture’. An examination question often asks what each component does.
Why the method may fail for a different mixture
If both solids readily enter the vapour phase under the selected conditions, sublimation may not provide an effective separation without additional control. If neither has useful vapour pressure, little material will move.
If heating causes a substance to decompose, the process may also fail to preserve its original chemical identity. That would be a chemical change rather than the intended simple physical purification.
Separation method suitability depends on actual material data and safety constraints, not a universal slogan.
The difference between iodine and sodium chloride
Ordinary sodium chloride forms a giant ionic lattice with strong electrostatic attractions. It has a very high melting point and does not exhibit the same convenient low-temperature molecular vapour behaviour as iodine.
Iodine consists of discrete I₂ molecules attracted to one another in the solid state. The distinction between molecular and ionic structure helps explain why they behave so differently when subjected to modest environmental or heating conditions.
The student should be careful not to say that sodium chloride contains weak molecules of NaCl that must be separated before melting. The school model treats it as an ionic lattice.
Why structure controls this property
The ease with which material enters the gas phase is influenced by the energy needed to separate its constituent particles from the solid. Molecular solids can have intermolecular forces weaker than the strong attractions within extended ionic or giant covalent structures.
That does not mean every molecular compound is highly volatile or every ionic compound is completely nonvolatile under all conditions. The actual substances and temperature still matter.
The Hougang Molecular versus Giant Covalent Solids guide explains the earlier structure-to-melting-point connection.
Why iodine vapour must not be used for home demonstrations
Iodine vapour is harmful when inhaled and can irritate eyes and the respiratory system. Solid iodine can also cause injury on inappropriate contact. A household heating experiment cannot be assumed safe simply because the vapour’s colour is interesting.
This article deliberately provides no heating procedure, quantities or apparatus setup for generating iodine vapour. Learners can use teacher-supplied observations, diagrams and data instead.
Any hands-on chemical demonstration belongs in a properly equipped, supervised laboratory with appropriate exposure controls.
What about dry ice?
Solid carbon dioxide, commonly called dry ice, sublimes at ordinary atmospheric pressure instead of forming a stable liquid under those pressure conditions. It is another example of a solid-to-gas change, although its phase diagram is not the same as iodine’s.
Dry ice also presents hazards, including cold burns, displacement of oxygen in poorly ventilated spaces and pressure buildup in sealed containers. It is not needed for an at-home comparison.
The educational value is recognising that sublimation can occur for more than one substance, while phase behaviour still depends on pressure and temperature.
How to interpret a phase-change graph
If a question supplies a phase diagram, temperature and pressure together determine which phase is stable. A student should not assume the same path applies across all pressures.
For iodine, the gas, liquid and solid regions exist under relevant conditions. For carbon dioxide at ordinary atmospheric pressure, the solid can pass into gas without a stable liquid phase along that path.
A tutor should use a simplified diagram only after the learner can first identify solid, liquid, gas and the word sublimation correctly.
A useful five-question comparison
- Observation: what physical state was the dark iodine in initially?
- Change: what state is the purple iodine after it enters the surrounding gas?
- Particle identity: does I₂ become I⁻ solely because the state changed?
- Recovery: how might gas return directly to solid under suitable conditions?
- Limitation: why is ‘iodine never melts’ an inaccurate statement?
A strong learner can answer all five without turning them into unrelated definitions.
A paper-only separation decision
Suppose a worksheet describes two solids and supplies their melting, boiling and sublimation behaviour. The student should identify the property difference and propose a suitable method without inventing apparatus details that are not needed.
They should then check whether the material is recovered unchanged, which part remains as residue and whether the method is appropriate for the stated safety and material conditions.
This reasoning is useful for more than iodine. It trains evidence-led technique selection.
What does the 2027 SEC G3 K324 syllabus actually require?
SEAB’s 2027 G3 Chemistry K324 syllabus explicitly includes sublimation in Section 1.2, Methods of Purification and Analysis. It requires students to describe suitable separation methods and select techniques from information about the mixture’s components.
The syllabus does not require students to carry out a hazardous iodine-vapour home demonstration or to memorise a full university phase diagram. The examinable foundation is the physical separation principle and the correct identification of states.
Refer to the SEAB 2027 G3 subject directory and official K324 Chemistry syllabus, Section 1.2. Students taking Combined Science should check their actual subject route.
Five wrong answers worth repairing
- ‘Iodine vapour is a new purple compound.’ Physical state change does not automatically create a new chemical species.
- ‘Iodine molecules break into iodine atoms whenever it sublimes.’ Ordinary sublimation can retain I₂ molecules.
- ‘Iodine has no liquid phase.’ It can melt under suitable temperature and pressure.
- ‘Sublimation and distillation are identical.’ The starting and collected phase conditions differ.
- ‘Every solid-solid mixture can be sublimed.’ The method requires an exploitable difference in volatility and suitable chemical stability.
A tutor should locate which statement the learner believes before adding more worksheets.
How a three-pax Secondary 4 Chemistry tutorial can help
Imagine three learners receiving the same sublimation diagram. One thinks iodine has chemically reacted with air. Another correctly calls it a physical change but labels purple gas as iodine ions. The third explains both accurately yet selects filtration for a dry mixture of two solids.
Their problems need separate repairs: reaction versus phase, molecule versus ion and method selection. A three-student lesson creates room to hear each explanation and set individual changed mixtures.
The educational outcome is independent technique selection, not just recognising one photograph of purple gas.
A five-minute parent review
- What is the definition of sublimation?
- What is the reverse process called?
- Does I₂ change its chemical identity during ordinary sublimation?
- Why is the phrase ‘iodine never melts’ wrong?
- What must differ between two solids for sublimation to help separate them?
- Why is a vapour-producing iodine demonstration inappropriate at home?
If your child can explain those points for a different hypothetical solid, the concept is becoming transferable.
Questions Hougang parents ask
Does iodine always pass directly from solid to gas?
No. Sublimation is possible, but iodine also has a liquid phase and can melt under appropriate conditions.
Is iodine vapour the same as iodide ions?
No. Molecular I₂ vapour and aqueous I⁻ are different chemical species.
Is sublimation in the SEC G3 K324 syllabus?
Yes. Sublimation is explicitly named as one method of purification and separation.
Does purple vapour prove a reaction has occurred?
Not by itself. The familiar solid iodine to iodine vapour transition is a physical state change.
An unusual colour, an ordinary physical principle
Iodine’s purple vapour gives a vivid illustration of how a substance can change physical state without changing chemical identity. The deeper skill is selecting a separation technique from material properties and explaining where the particles move.
Continue with Filtration, Evaporation or Distillation?, Hougang Secondary 4 Chemistry Tuition, and the Hougang Chemistry Hub. To discuss current diagnosis-led three-pax Chemistry lessons and confirm the actual venue and availability, contact eduKate Singapore.