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I'm trying to get the right terminology for various forms of phase change. I am familiar with the phase change / triple point diagram for water, and we have various terms for the transition of a liquid or solid to a gas, such as boiling, evaporation, vaporization, sublimation.

There seems to be a clear distinction between 2 types of transition between a liquid and a gas: One happens when the water boils or vaporizes at it's boiling point (that varies with pressure of course) and the other type, called evaporation, which can occur at any temperature below the boiling point. So those 2 types are clear: Boiling / Vaporization vs Evaporation.

However my question relates to the 2 types of phase change which occur when ice changes to gas. I can only find the one word sublimation, however, there are also 2 distinct types of sublimation, that which occurs at the exact temperature (dependent on the pressure), and in that way is akin to boiling.

But then there is an analogous change similar to evaporation, where the process is slower and the change from ice to gas can happen at almost any temperature lower than the sublimation temperature. I don't want to make up a term such as "evaporative sublimation" unless there is really such a term. Can anyone enlighten me on this.

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  • $\begingroup$ Sublimation does not occur at one specific temperature. Furthermore, in thermodynamic terms, all first order phase transitions are a balance of the relevant Gibbs free energies, period. The various names used to describe (or not) specific conditions do not change how and why the phase transition occurs. $\endgroup$ – Jon Custer Jun 28 '16 at 22:52
  • $\begingroup$ "Boiling" and "evaporating" are physically identical - the reason we have different words is because one is 'fast enough' for human observation and the other isn't. Same as "rusting" vs "burning:" they're both oxidation, but one you can't watch happen without falling asleep or going for food. I suppose you could say boiling is a specific case of evaporation where the vapor pressure exceeds ambient pressure, but that's a change in circumstance, not in process. $\endgroup$ – Asher Jun 28 '16 at 23:15
  • $\begingroup$ However, by saying that "Sublimation does not occur at one specific temperature...", you are perhaps simply stating that there has not yet been a distinction made for sublimation as there has been for boiling vs evaporation. In other words, "boiling" occurs exactly along the liquid-vapor pressure-temperature line in the phase diagram and so there are exact pressure-temperature points on that line; whereas "evaporation" can happen at temperatures and pressures that are not on that line. So perhaps the term sublimation is more analogous to the term evaporation. I find that a pity. $\endgroup$ – ScienceNerd6 Jun 28 '16 at 23:57
  • $\begingroup$ What I mean is that, if the definition of sublimation is any vaporization that can happen anywhere below 0 C and at any pressure less than 611 pa. (for water ice) and not be on the ice-vapor line of the phase diagram, I see this as a pity, because I would hope that there was a term like "boiling", which would be pressures and temperatures exactly on the ice-vapor line. I was hoping there would be some finer distinctions in this area. $\endgroup$ – ScienceNerd6 Jun 29 '16 at 0:01
  • $\begingroup$ @ScienceNerd6 , I think you are drawing a distinction where there shouldn't be one. Water molecules move both ways across a water interface, all the time. If the relative humidity is 100%, the rate of evaporation equals the rate of condensation, and there is no NET change in the water level of an open container. For lower relative humidities, the rate of evaporation exceeds the rate of condensation. The exact same situation applies for the boiling water. $\endgroup$ – David White Jun 29 '16 at 2:48

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