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Q: Chemistry ( Answered,   2 Comments )
Question  
Subject: Chemistry
Category: Science > Chemistry
Asked by: jaquata-ga
List Price: $4.00
Posted: 13 Sep 2006 05:58 PDT
Expires: 13 Oct 2006 05:58 PDT
Question ID: 764794
Both sublimation and evaporation bring material into vapor phase, How
do the two methods of separtion differ?
Answer  
Subject: Re: Chemistry
Answered By: livioflores-ga on 13 Sep 2006 09:46 PDT
 
Hi!!


Sublimation:  The transition of a substance from the solid phase
directly to the vapor phase, or vice versa, without passing through an
intermediate liquid phase.


Evaporation:  The conversion of a liquid (water) into a vapor (a
gaseous state) usually through the application of heat energy during
the hydrologic cycle; the opposite of condensation.

Both above definitions come from "Groundwater Glossary":
http://www.groundwater.org/gi/gwglossary.html


Take a look to the following paragraph:
"Sublimation/evaporation:
For the purpose of this study, freeze-drying is defined as a method of
drying wetted archival and library materials by freezing, then under
vacuum conditions by converting the solid to the vapor phase; the
liquid phase is by-passed.
When a liquid is converted to the vapor phase by heating, the action
is called evaporation. As a solid (frozen), its Conversion to the
vapor phase by the application of heat without going through the
liquid phase is called sublimation. An example of sublimation:
preservation in nigh mountainous regions where water in meat freezes
then evaporates (sublimes) with no liquid phase to produce unwanted
side effects.
The process of both sublimation and evaporation depends ultimately on
the relationship of temperature and pressure to the kinetic energy
(energy of constant motion) present in water as a liquid or solid. For
example, in a kettle of water, as the temperature is increased the
kinetic energy increases and permits molecules from the liquid to
escape as vapor. Maximum vaporization occurs at the boiling point,
100C (212 F) at 1 atmosphere. Conversely, the vapor pressure of
water, that is, the pressure of the vapor in equilibrium with the
liquid, is 760 mm Hg at 100 C. If the atmospheric pressure is reduced
to say, 525.8 mm Hg, as would be the case if the same kettle is boiled
on the top of a mountain, the boiling point will be reduced by several
degrees to 90 C (194 F). The reason: at this temperature the vapor
pressure of water is 525.8 mm Hg.
Water as a solid behaves in much the same way as water as a liquid
except that the temperature at which the energy of motion of the solid
(ice) permits the release of water vapor molecules begins at the
opposite end of the thermometer scale: the freezing point and below.
The vapor pressure scales become numerically very low. By using
selected temperatures below the freezing point together with suitable
pressures, the sublimation of the solid will take place. ..."
From "Vacuum freeze-drying":
http://www.unesco.org/webworld/ramp/html/r8707e/r8707e06.htm


Read also:
"Clapeyron equation:
Clapeyron equation gives the relation between T and p in a two-phase
pure-substance system. ...
Clapeyron equations may also be applied to the solid-vapour phase
change (sublimation) by changing the values of the variables
accordingly. Substances with sublimation points are not common, the
best known being carbon dioxide (CO2, with Tsubl=195 K), called dry
ice, and less known acetylene (C2H2, with Tsubl=189 K), sulfur
hexafluoride (SF6, with Tsubl=209 K), naphthalene (C10H8, with
Tsubl=321 K), uranium hexafluoride (UF6, with Tsubl=329 K), aresenic
(As, with Tsubl=887 K), carbon (C, graphite, Tsubl=3910 K), and some
mercury compounds. Notice, however, that the strict meaning of
sublimation-point used here, i.e. the solid-to-vapour phase-change of
a pure substance with its triple-point pressure below normal pressure,
p0=100 kPa, is rarely followed not only in common language but in
scientific literature, where sublimation is synonymous of evaporation
from a solid, usually in the presence of ambient air, and thus, one
talks about sublimation of iodine, I2, the has a normal boiling point
at Tb=387 K, sublimation of naphthalene, C10H8, the has a normal
boiling point at Tb=424 K, etc. Evaporation in solids in air is a
general rule, forced by the gradient of chemical-potential and thus
proportional to its vapour pressure, but their vapour pressure at
normal temperatures is very low and, although all solids sublimate,
their sublimation is usually neglected except in the examples given.
Water ice sublimates too, below freezing temperatures, as in a
freezer."
From "Thermodynamics of phase change":
http://imartinez.etsin.upm.es/bk3/c06/Phase%20change.htm


Finally I suggest you the following lecture:
"Sublimation:
As long as the vapour pressure of a fluid is less than the external
surface pressure, it only evaporates at the surface and without
appreciable motion of the surface and the vapour diffuses into the
air. Also solid substances in contact with the free atmosphere
evaporate (mostly only in small quantities) gradually at their
surface; it is known to everyone from musk, camphor, naphthalene,
ammonium chloride by their fragrance as well as from ice, which also
evaporates at the severest cold into the air. Thus, also every solid
has a definite vapour pressure (however, often it is so small that is
can only be measured indirectly. The evaporation of a solid and the
solidification of its vapour, without the solid of the vapour passing
through the liquid state, is called sublimation. Every solid substance
sublimes very slowly in the atmosphere; in contrast, it sublimes very
violently when the sublimation pressure exceeds the atmospheric
pressure. If the sublimation point (which can be compared to the
boiling point of fluids) is below the melting point of the solid
substance, it sublimes when it is heated; in order to be able to heat
it to melting, it must be heated in a closed container. However, as a
rule, the sublimation pressure of solids lies close to the melting
point and far below the atmospheric pressure. The quantity of heat,
which 1 g of a substance uses for sublimation, is called its
sublimation heat; at the melting point, corresponding to the energy
principle, it is equal to the melting heat plus the evaporation heat
of the melted substance. For the preservation of thermodynamic
equilibrium between the solid and the melted part of the substance,
the substance, solid or melted, must have (according to the theory) at
the melting point the same vapour pressure. In the cases of benzol and
water, the theory and measurements agree satisfactorily."
From "J11 Heat - Sublimation":
http://kr.cs.ait.ac.th/~radok/physics/j11.htm#Sublimation


For a better understanding of the key concepts I also suggest you the
following articles at Wikipedia:
"Water vapor - Wikipedia, the free encyclopedia":
http://en.wikipedia.org/wiki/Water_vapour

"Sublimation (physics) - Wikipedia, the free encyclopedia":
http://en.wikipedia.org/wiki/Sublimation_%28chemistry%29


Search strategy:
I used the following keywords at Google.com
define: sublimation
define: evaporation
sublimation evaporation difference


I hope this helps you. Feel free to request for a clarification if you
find something unclear and/or incomplete.


Best regard,
livioflores-ga
Comments  
Subject: Re: Chemistry
From: neilzero-ga on 13 Sep 2006 06:07 PDT
 
As far as I know the only difference is sublination happens to a
solid; while evaporation happens to a liquid.   Neil
Subject: Re: Chemistry
From: damu22-ga on 28 Sep 2006 11:42 PDT
 
sublimation occurs for the compounds which are volatile and
evaporation can can be done by just heating.

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