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Name: Ryan B.
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 2001


Question:
Hello, on many occasion's you have referred to the act a 'subliming' or 'sublimation' as going from solid directly to gas without an intermediate liquid state.

However, my dictionary says that to sublime is: "to cause to pass directly from the solid to the vapor state and condense back to solid form"

The extra step of condensing back into solid form is what interest's me. My question is, do you know of a natural or artificial process that will cause something to go from solid, directly to gas, and then directly to solid form?

I am a studious alchemist and the very word 'sublime' was created by alchemists. I eagerly await your response.


Replies:
Two examples of molecules subliming from the solid directly into the gas phase are:

1. "dry ice" that is frozen carbon dioxide.

2. snow on a black top road or driveway, when the temperature is less than 0 C. It is a common observation that the snow tends to "disappear" without the formation of a liquid puddle.

The reverse process is the formation of frost on the windows of an automobile, or a window pane, when the temperature is several degrees below the freezing point ( 0 C.). If you observe this, you will see the frost form without prior condensation of the water vapor into dew and subsequently freezing. However, this latter vapor ----> liquid ----> ice also happens under proper conditions.

Vince Calder


Sublimation often refers to the change of a solid substance directly to a vapor without first going through the liquid state. If one looks at the pressure-temperature diagram of any pure substance (see any thermodynamics text book), one sees a particular temperature-pressure combination where all the three states of matter, namely vapor, liquid, and solid coexist. Three lines drawn from that point mark the solid-liquid, solid-gas, and liquid-gas equilibrium conditions. These lines have been determined empirically or theoretically.

As such there is a range of temperatures and pressures that a solid can be converted to vapor without going through the liquid phase. These often occur at low pressures and temperatures and that is why there are not many commonly-encountered phenomena.

One common example, however, is dry ice (solid carbon dioxide). It sublimes at -78.5C. Another is when air saturated with water vapor is suddenly cooled below the freezing point of water. That is how snow is made.

The term sublimation is also used to describe the reverse process: a gas changing directly to the solid upon cooling. I know it is a little confusing but as far as I know the precise direction of the conversion (solid to vapor or vapor to solid) will have to be deduced from the context. Such difficulty does not exist with liquid/gas transition (condensation and evaporation) or liquid/solid transition (melting, solidification). I have seen the term condensation used to indicate vapor to solid phase (such is chemical vapor deposition when a metal is vaporized to then be condensed on a cold surface to generate a coating). But this is not a specific term.

Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory


Good luck with your alchemy. Let us know if you meet with any success.

Every substance has a "vapor pressure", that is, the pressure exerted by the gaseous form of the substance in equilibrium with its liquid or solid form. When weather reports speak of "relative humidity", it means the pressure of the water vapor in the air as a fraction of the vapor pressure. The vapor pressure changes with temperature.

Just as a liquid can pass into the gaseous state by evaporation, a solid can do the same thing. Solid carbon dioxide ("dry ice") evaporates (sublimes) without meating, because liquid carbon dioxide is not stable at pressures below 5.11 atmospheres. Water will behave like that at low enough pressures; the liquid phase water is not stable below 0.006 atmospheres pressure. Below that pressure, cooling the gas changes it into solid, and warming the solid changes it into gas. It is not even necessary to make the pressure of the whole system lower than this cutoff, just of the vapor phase of the substance of interest. For example, you may notice that ice cubes left in the ice cube tray of your freezer will slowly shrink. This is because they are evaporating (subliming), even though they never melt!

To change the cold vapor back into a solid, all that is required is to lower the temperature or to increase the vapor pressure. This also happens with water in your freezer. Ever wrap something in your freezer in a plastic bag? After a while, did you notice ice crystals on the inside surface of the bag? What happened was that water evaporated (sublimed) from the frozen food inside the bag and condensed (sublimed) on the bag itself.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois



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