sublimation
/ sub-li-MAY-shun /
Leave a block of dry ice on a table and it doesn't melt into a puddle — it just gives off swirling white fog and slowly shrinks, turning straight into gas without ever becoming liquid. That direct leap from solid to gas, skipping the liquid stage entirely, is sublimation.
Sublimation happens when a solid's molecules can break free into vapor at a pressure below the substance's triple point, so the liquid phase has no window to exist. It is the solid-to-gas member of the phase-transition family, the mirror image of deposition (gas going straight to solid, as when frost forms on a cold window). Like all phase changes, it costs latent heat — the heat of sublimation, which equals the heat of melting plus the heat of vaporization.
Sublimation is more common than it seems. Snow on a sunny, freezing day shrinks without melting; mothballs slowly vanish; freeze-drying preserves food and coffee by sublimating away ice in a vacuum; and dry ice creates stage fog precisely because it sublimes. It is also a gentle purification trick: heat a solid so it sublimes, then catch the vapor on a cold surface, leaving non-volatile impurities behind.
Solid carbon dioxide (dry ice) sublimes at about −78 °C under normal pressure: it never melts, only smokes away into gas, leaving no liquid mess behind.
Dry ice goes solid-to-gas directly, skipping the liquid stage.
The reverse of sublimation — gas turning straight to solid — is called deposition (sometimes 'desublimation'), seen when frost or snowflakes form directly from water vapor. By energy bookkeeping, the heat of sublimation equals the heat of fusion plus the heat of vaporization.