Ask A Scientist© Chemistry Archive

name         Nick
status       student
age          9

Question -   We are studing physical changes, and chemical changes at
school. my daddy told me to ask the teacher if a solid can go right to a
gas witout being a liquid first.she said that carbon dioxide can, but not
ice. Dad says she is wrong. would you help me with this one. thanks.
Nick.

---------------------------------
Nick,

Your daddy is correct. Whenever a solid goes directly to the gaseous condition
-- without first melting -- the process is called sublimation. As you already
know, dry ice can do this. So can solid iodine, naphthalene (moth balls),
paradichlorobenzene (moth crystals), camphor, menthol, and other compounds as
well.

Ordinarily, ice melts before it becomes a gas. Nevertheless, ice can
"evaporate"-- sublime -- just as your father says. Though it does not 
occur all
that often, when the humidity is low and the temperature is just below
freezing, even snow (which is ice) can also sublime.  Ice will sublime if
placed in a vacuum, even if the temperature is well below freezing. Your 
dad is
a smart fellow.

Regards,
ProfHoff
=========================================================
Nick, Your dad is correct. I have to disagree with the teacher. You can
even observe this happening.
1) If you leave a tray of ice cubes in the freezer for several weeks,
without letting them melt, when you take them out you will see that the ice
cubes are significantly smaller than when you put them in. This is
especially noticeable if the "frig" is a "no-frost" type.

2) I do not know where you live but if it is cold enough to get frost on car
or house windows you will frequently see it disappear as the sun comes up
and the air gets warmer without having the frost melt. The same thing occurs
when there is snow on an asphalt driveway and the temperature is even 0 F.
If the sun is bright and some of the black asphalt is exposed, it absorbs
the light, turns it into heat and evaporates the snow, without the
temperature coming anywhere near freezing.

3) "Freeze drying" is a technique used in many industrial and laboratory
operations. This technique involves pumping a vacuum on the sample in a
chamber that is held at temperatures less than the freezing point of water.
The water has sufficient vapor pressure to sublime, and be pumped away, even
when the temperature is considerably less than the melting point of ice (0
C.). I have no doubt you can find out a lot about this process by doing a
web search on the topic.

Vince Calder
=========================================================
Your teacher is partly right.  Solid carbon dioxide, is known as "dry ice"
because it evaporates without melting.  So she's correct about that.  She's
also correct that solid water (ice) melts at a lower temperature than water
boils.  However, ice can also turn into vapor without melting.  This happens
in your freezer all the time.  If you look at ice cubes in your freezer, you
will see that over time they shrink.  This is because they are slowly
evaporating.

The trick is the pressure.  We see water boiling when the gas it gives off
(water vapor) has the same pressure as the air around it.  Ice gives off
water vapor too, but at a much lower pressure.  Solid carbon dioxide gives
off carbon dioxide vapor at a pressure equal to that of the air at a much
lower temperature.

If you keep it in a pressure tank, solid carbon dioxide will become liquid
(melt).  In the same way, if you put it in a vacuum, ice will turn into
vapor without melting.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
=========================================================
Hello  Nick :

         Your teacher and your dad are both right. If you consider reactions
at atmospheric pressure, carbon dioxide does indeed go directly from a solid
to a gas (a process called sublimation), whereas ice first melts to a liquid
and then boils to a gas. Your dad is right in saying that ice can go
directly from a solid to a gas, but this happens at pressures different than
atmospheric.

         Whether a solid goes directly to a gas, or changes first to a liquid
depends on the value of the triple point pressure, relative to atmospheric
pressure. If the triple point pressure is higher than atmospheric, then it
is solid ---> gas. If the triple poiint pressure is less than atmospheric
pressure, then the reaction is solid ---> liquid ---> gas.

Jim Rubin
=========================================================
Generally, when discussing phase changes, such as solid to liquid to gas, we
are referring to an experiment where the air pressure is kept constant and the
temperature is raised from the temperature that the material is solid to the
temperature at which it is a gas.

Under normal pressure conditions, for example on a typical spring day, carbon
dioxide will evaporate directly from a solid to a gas without becoming a 
liquid
first.  There is no temperature at which you can have liquid carbon 
dioxide (at
normal pressures)..  Water (ice) first melts to become a liquid then 
evaporates
to become a gas.

It is true that under VERY high pressure conditions solid carbon dioxide will
first melt and then evaporate.  There are probably conditions under which 
water
will evaporate directly from the solid to a gas without going through the
liquid state.  Perhaps this is what your father was referring to?  Perhaps he
could explain what he means?

Greg Bradburn
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