Question
What is sublimation? Explain the conditions required for it to occur, and give three examples from daily life. Also explain how sublimation is used as a separation technique.
Solution — Step by Step
Sublimation is the phase transition of a substance directly from the solid state to the gaseous (vapour) state without passing through the liquid phase. The reverse process — gas directly to solid — is called deposition (or reverse sublimation).
This occurs when a solid’s vapour pressure at a given temperature exceeds atmospheric pressure before the solid melts. At these conditions, the solid is effectively “bypassing” the liquid state because liquid would only be stable at higher pressure.
More precisely, sublimation occurs for any substance below its triple point (the temperature-pressure point where all three phases coexist). At pressures below the triple point, heating the solid directly converts it to vapour.
A substance sublimates readily when:
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Its triple point pressure is above atmospheric pressure: The liquid phase is only stable at pressures higher than normal atmospheric. Example: CO₂ has a triple point at 5.1 atm — at 1 atm, liquid CO₂ simply cannot exist.
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High vapour pressure at room temperature: Even substances with triple points below atmospheric can sublime measurably at room temperature if they have high enough vapour pressure. Iodine, camphor, and naphthalene do this.
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Low intermolecular forces: Substances with weak van der Waals forces (like iodine, which has only dispersion forces between non-polar I₂ molecules) have high vapour pressures — molecules can easily escape the crystal.
Dry ice (solid carbon dioxide, −78.5°C at 1 atm) is the most dramatic everyday example of sublimation. At atmospheric pressure, CO₂ has no stable liquid phase — its triple point is at 5.1 atm and −56.6°C.
When dry ice is taken out of a container, it sublimes (converts directly to CO₂ gas) without ever becoming liquid. The white “fog” visible around dry ice is not CO₂ gas — it is water vapour from the air condensing (the extremely cold CO₂ gas cools the surrounding air below the dew point).
Uses: Food refrigeration, theatrical fog effects, firefighting.
Iodine (purple-black solid at room temperature) sublimes measurably even at room temperature, producing purple vapour. When gently heated (well below its melting point of 113.5°C), iodine produces visibly purple vapours that redeposit as shiny iodine crystals on a cold surface placed above.
This property is what makes iodine useful in forensic fingerprint detection: latent fingerprints on paper or surfaces contain oils and amino acids. Iodine vapour (from a bottle of solid iodine) reacts with these organic components, making the fingerprint temporarily visible as a brown stain.
Also: the purple colour in starch-iodine test (iodine vapour + amylose → deep blue-black complex).
Camphor (used in temples, medicines, and moth repellents) is a white solid that slowly sublimes at room temperature. If you leave a camphor tablet in a closed room for weeks, it gradually disappears — it has converted to vapour that disperses in air.
Naphthalene balls (mothballs) work the same way — the naphthalene sublimes slowly, releasing vapour that repels insects. The gradual disappearance of mothballs over months is visible sublimation.
Why hard difficulty rating? Because the exam expectation is not just to name examples, but to explain the molecular basis: naphthalene is a non-polar aromatic molecule with weak dispersion forces, giving it high vapour pressure relative to its low molecular weight — this makes room-temperature sublimation fast enough to be practically useful.
Why This Works
Sublimation occurs because the energy needed to break the solid’s intermolecular bonds can be supplied directly as heat without the intermediate step of forming a liquid. In substances that sublime readily, the intermolecular forces in the solid are relatively weak — the energy released by vapour-phase molecules condensing to solid is not large enough to sustain a liquid phase at normal pressures.
Phase diagrams make this clearest: if you follow the “solid→gas” boundary in a P-T phase diagram while reducing pressure, you find a sublimation curve. At pressures below the triple point, heating the solid traces a path that crosses the sublimation curve directly — no liquid region is traversed.
Alternative Method — Sublimation as a Separation Technique
Sublimation is used industrially and in laboratory settings to purify substances that can sublime:
Procedure: The impure solid mixture is heated. Only the sublimable component converts to vapour. The vapour is collected on a cold surface (inverted cold flask) above the mixture. The non-sublimable impurities remain in the dish below.
Example: To purify ammonium chloride (NH₄Cl) from a mixture with sand:
- Heat the mixture — ammonium chloride sublimes; sand does not
- NH₄Cl vapour rises and deposits as pure white crystals on the cold surface above
- Sand remains at the bottom
This separation technique is used in laboratories for substances like: ammonium chloride, anthracene (organic chemical), arsenic trioxide, and benzoic acid (which sublimes at elevated temperatures).
Common Mistake
Students often say “sublimation occurs only at very high temperatures.” This is misleading — dry ice sublimes at −78.5°C (very cold!), and camphor sublimes slowly at room temperature. The temperature required for sublimation depends on the substance, not on some universal “high temperature” threshold.
Also, the white fog visible with dry ice is not CO₂ gas — CO₂ is colourless and invisible. The fog is condensed water vapour from the atmosphere (like the “steam” above hot food). Confusing the CO₂ vapour with the visible white fog is a very common misconception.
CBSE Class 9 (Matter in Our Surroundings) tests: definition of sublimation, examples (dry ice, camphor, iodine, naphthalene), and the separation technique using sublimation. For Class 11 (States of Matter), expect questions on phase diagrams, the triple point, and why sublimation occurs below the triple point. JEE occasionally asks about phase diagrams and the conditions for sublimation.