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Sublimation
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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