|There are 3 things that go from solid to gas
without going through a liquid stage: Dry ice and
moth balls. Can you tell me the third?
|Question Date: 2006-11-02|
Well there are many materials that bypass the
liquid state. It is all a matter of pressure,
temperature and material. For example, even water
ice will sublimate. Try this experiment. Fill up
an ice cube tray in the freezer . Then leave the
tray in the freezer and watch its progress for
weeks and even months. BE PATIENT. Because the
humidity in the freezer is so low (esp. true if
you are not opening the freezer a lot and letting
moist air in), you will notice over time that the
ice volume in the tray will decrease through time!
That is because the vapor pressure of
H2O in equilibrium with solid
crystalline ice exceeds that in the freezer box.
You see the way the freezer works is to take warm
moist air in and then via the refrigeration cycle,
cool and de humidify the air... and that cold dry
is used to chill everything in the freezer. Evert
solid material exudes off an equilibrium vapor
pressure. For most materials is far below the
atmosphere pressure. But for a few its higher. And
that is the case for the example you cited.
Actually the list is larger that the two you
named, but iodine is the other common material
that goes from solid to gas without going through
a liquid. Remember that this is at normal
pressure, and under conditions where you change
the pressure, the story might be different.
The process you are describing is called
sublimation, in which a substance passes from the
solid to the gas state without first becoming a
liquid. Actually, there are a large number of
materials will sublime; it is just a question of
the temperature and pressure at which this will
occur. For example, water will sublime if you
lower the pressure and temperature enough. This
phenomenon is often used to freeze-dry things.
Iodine will sublime at room temperature if you
lower the pressure just a bit.
If you want
to be more quantitative about it, you can look at
the triple point of a material. On a
pressure-temperature phase diagram, this is one
pressure and temperature point. Below either the
triple point temperature OR the triple point
pressure, the material is capable of sublimation.
For example, the triple point of mercury, found in
older thermometers, is roughly -39 o C and a
pressure of 0.2 mPa so if you have solid mercury
at -50 o C and 100 mPa, it should sublime.
Alternatively, if it were at 10 o C and 0.1 mPa,
it should also sublime.
There are a lot more than three, and it all
depends on the pressure and temperature you are
doing this under. In central Antarctica, water ice
evaporates directly into a gaseous phase, without
going through being a liquid first.
room conditions sublimes? Anything that you can
smell, but which is still solid, for one thing.
You already bring up mothballs. Rubber is another
example, although rubber doesn't evaporate to the
point that a rubber band will exactly disappear.
I'm not sure the example you're looking for, but
there are multiple possible answers.
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