Sublimation has a way of sneaking into your memory at the worst time.
You’re revising late, the house is quiet, and then you picture dry ice “smoking” like a science fair magic trick. Your brain files it under cool, not useful. But in IB Chemistry, sublimation isn’t trivia. It’s one of those small phase-change ideas that quietly connects particle theory, intermolecular forces, enthalpy changes, and phase diagrams--the exact mix exam questions love.
Sublimation in IB Chemistry (the simple definition)
In IB Chemistry, sublimation is the phase change where a solid turns directly into a gas without becoming a liquid.
That “skipping the liquid” detail is the whole point. If a question describes a solid producing vapor straight away, you’re in sublimation territory.
Quick checklist: how to recognize sublimation fast
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The change is solid --> gas (no liquid phase mentioned).
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The solid is volatile (it produces vapor easily).
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The process is endothermic (energy absorbed by the system).
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The explanation should mention particles overcoming intermolecular forces (IMFs).
If you want to drill the language that earns marks, keep a glossary tab open while you revise: IB Chemistry Key Definitions.
Why sublimation happens (the particle story)
A solid looks still, but particles are always vibrating. In IB Chemistry, the key is what happens when some particles gain enough energy to escape the solid structure entirely.
Sublimation becomes more likely when:
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Intermolecular forces are relatively weak (often London dispersion forces in molecular solids).
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The solid has a noticeable vapor pressure at that temperature.
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The conditions make the liquid phase less stable (pressure matters here).
An exam-ready way to phrase it: As temperature increases, particles gain kinetic energy. When enough particles can overcome the intermolecular attractions holding the lattice/structure together, they leave directly as gas particles.
To tighten this skill, pair theory with practice: IB Chemistry Resources and then immediately apply it in the Questionbank (more on that below).
Sublimation is endothermic (and ΔHsub is your friend)
In IB Chemistry, sublimation is endothermic because energy must be absorbed to separate particles and overcome attractions.
That energy change is the enthalpy of sublimation, written as ΔHsub.
What the energy does (in marking language):
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Overcomes intermolecular forces between particles in the solid.
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Increases potential energy as particles move further apart.
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Allows particles to become a gas with much greater freedom of movement.
This is also why dry ice feels painfully cold--it absorbs heat quickly from your skin and surroundings as it sublimates.
Common IB Chemistry examples of sublimation
Not every solid sublimes easily. The classic IB Chemistry examples are memorable because they show a clear solid --> gas transition:
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Dry ice (solid CO₂): sublimes at about -78 °C at 1 atm, creating dense fog-like CO₂ gas.
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Iodine: gently heated crystals form a purple vapor.
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Ammonium chloride: on heating, forms gases that can recombine on cooler surfaces.
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Naphthalene (mothballs): slowly sublimes at room temperature.
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Solid air fresheners: release fragrance by slow sublimation.
When you revise these, don’t just memorize the list. Ask: What does this imply about IMFs and vapor pressure? That’s where the marks live.
Sublimation on phase diagrams (where students lose easy marks)
Phase diagrams often look intimidating, but sublimation is one of the simplest parts: it’s the boundary line between solid and gas.
The most testable idea is the triple point. Below the triple point pressure, the liquid phase cannot exist, so heating a solid leads to sublimation instead of melting.
CO₂ is the usual example: at atmospheric pressure it goes solid --> gas rather than forming a stable liquid.
If phase diagrams feel foggy, this is worth a focused read: Phase Diagrams Explained Simply.
How exam questions test sublimation in IB Chemistry
Most IB Chemistry sublimation questions are really testing whether you can connect the story (particles and energy) to the data (diagrams, curves, wording).
You might be asked to:
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Identify a change as sublimation from a description or equation.
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State that sublimation is endothermic and explain why.
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Use ΔHsub language appropriately.
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Interpret a phase diagram and explain the role of the triple point.
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Compare sublimation with vaporization (liquid --> gas) or deposition (gas --> solid).
For quick reinforcement, read the “neighbor” concepts too: Vaporization Explained Simply and IB Chemistry: Deposition Explained Simply.
A RevisionDojo mini-plan for sublimation (15 minutes that actually sticks)
Here’s a calm, realistic routine built for IB Chemistry students who want results without cramming.
Build the concept
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Skim the definition and key language in IB Chemistry Key Definitions.
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Review the particle model in the relevant syllabus area: Models of the Particulate Nature of Matter.
Practice like the exam
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Use RevisionDojo’s Questionbank to do a short set on phase changes and classification language: Stoichiometric Relationships Questionbank.
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Then build a tiny mixed quiz (sublimation vs vaporization vs deposition) using the Questionbank’s AI feedback to correct your wording.
Lock it in
- Convert the tricky parts into Flashcards (ΔHsub definition, triple point explanation, “overcome IMFs” phrasing). Start here: IB Flashcards with Spaced Repetition (SRS).
RevisionDojo’s Study Notes, Flashcards, AI Chat, and Grading tools work best as a loop: learn, test, correct, repeat. When you’re closer to exams, add Predicted Papers and Mock Exams to simulate timing and stress, and use the Tutors option when a topic keeps slipping.
Closing: make sublimation an easy mark in IB Chemistry
Sublimation in IB Chemistry is simple in definition but powerful in questions: solid --> gas, endothermic, explained by particle energy and intermolecular forces, and made crystal clear on phase diagrams below the triple point.
If you want this to feel effortless under time pressure, use RevisionDojo as your loop: learn the wording in Study Notes, challenge yourself in the Questionbank, cement it with Flashcards, and then pressure-test with Mock Exams and Predicted Papers. Sublimation stops being a “cool dry ice fact” and becomes what it’s meant to be--a reliable, repeatable exam mark in IB Chemistry.
