Why substances sublime

Start with near-ice-cold water and then raise it to a boiling temperature. If you keep the heat low enough, you can sit at this point, just below boiling, for a relatively long time. This is because the rate at which you add energy may be only just overcoming radiative and conductive heat losses to the surrounding room.

Note that phase transitions are defined within the context of thermodynamics , which is, by construction, a fluid model i. Note that kinetic models can describe macroscopic parameters as well, but they do so by finding the ensemble average in velocity space of the distribution function to treat a large number of particles as if they exhibit a bulk behavior similar to a fluid.

The differences between fluid and kinetic models can be subtle, but you can think of them in the following ways.

Continuous vs. Discontinuous When I use the term discontinuous above, I am referring to a change that occurs on a smaller scale than the resolution of the specific observation. For instance, we assume that shock waves contain a discontuous jump in density, pressure, etc. For most fluid models, this spatial scale is so small we can approximate it as being infinitesimal and neglect it.

This approximation greatly simplifies many of the equations we would use to try and model such phenomena, even though the transition from upstream to downstream is not truly discontinuous.

We define the transition as being discontinuous because it is comparable to or smaller than the smallest relevant scale lengths considered for the problem at hand i.

In nature, there are few things that could be truly called discontinuous I actually know of none, but some of the quantum whisperers on this site might know of some. Thus I am trying to be careful in this statement. However, that some phenomena changes continuously on the smallest scales may not matter for the macroscopic dynamics where we assumed a discontinuous change. As in the shock wave example above, that the ramp region has a finite thickness does not render the conservation relations used to model most shocks i.

The assumption that the ramp is discontinuous works because the transition is faster than the scales i. Thus, the definition of continuous vs. So in the purest sense, yes, a phase transition is close to not exact because particles are discrete a continuous transition if we could measure things "infinitely" fast and on an "infinitely" small scale.

Interesting Side Note: The use of a model distribution function generally inserts irreversibility into any model one would evolve dynamically from this point forward. Whether one sees a solid or a liquid or a gas is determined by the correlation properties of the molecules in the substance. Solids in crystals have strong and long-range correlations, liquids have no long-range but strong short-range correlations, while gases have almost only weak intermolecular correlations.

In the part of the phase diagram below the triple point, crossing the coexistence line from solid to gas means that upon adding energy, single molecules separate from the surface in an only weakly uncorrelated way and therefore immediately behave like molecules of a gas. On the other hand, in the part of the phase diagram above the triple point, crossing the coexistence line from solid to liquid means that upon adding energy, the surface breaks into many tiny and not very well-defined regions consisting of few highly correlated molecules and therefore immediately behave like molecules of a liquid, characterized by strong short-distance correlations.

For a very small piece of solid, the distinction between gas and liquid is not very pronounced, but for solids of the size relevant on human time scales, we are already very close to the infinite volume thermodynamic limit where these effects happen instantaneously.

Thus the transition is almost instantaneously. Phase changes depend not only on temperature but also pressure. Like any substance, if you get the right combo of pressure and temperature you can keep it at a solid, liquid or gas or if you're really good keep it at the triple point and get all three phases in equilibrium with each other. For C02, the liquid phase is very likely outside of most temperature pressure combos used in engineering applications and everyday life.

Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why do some substances undergo sublimation while others do not? Ask Question.

Asked 5 years, 8 months ago. Active 1 year, 10 months ago. Viewed 7k times. Improve this question. Freelancer Freelancer 9 9 silver badges 27 27 bronze badges.

Add a comment. Active Oldest Votes. Caveats and Notes Note that phase transitions are defined within the context of thermodynamics , which is, by construction, a fluid model i. We treat fluids as a continuous blob occupying a unit volume that can deform under stress but the initial fluid element, if incompressible , will occupy the same volume. For kinetic models, we assume a model distribution function that most closely describes the discrete particles in the system.

In many cases, this function is continuous in the mathematical sense and the use of ensemble averages can yield bulk properties similar to those in thermodynamics e. Carbon dioxide exists in its solid state at The only factor that makes it impossible to naturally obtain liquid CO 2 is pressure.

CO 2 exists as a liquid at a pressure condition of 5. Though the temperature can be naturally obtained on Earth, the pressure of 5. So, to obtain liquid carbon dioxide, we need to quintuple the atmospheric pressure!

Similarly, any sublimable substance ammonium chloride, naphthalene, camphor, iodine would not be able to exist in liquid form due to unfavourable pressure conditions naturally, that is. Source Of Picture: Wikipedia. View all posts by pcbpedia. Like Like. Because the motive of my posts are to explain something of my interest in lucid language so that people would understand, as opposed to putting in a lot of technical terms which just makes it seem voluminous.

You are commenting using your WordPress. You are commenting using your Google account. You are commenting using your Twitter account. You are commenting using your Facebook account. Notify me of new comments via email. Notify me of new posts via email. Skip to content. Home Contact. Dry Ice Solid CO 2 subliming. Share this: Twitter Facebook.