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Sublimating Metals
Name: Weiyoong
Status: student
Grade: 9-12
Location: Singapore
Question:
I saw the link.
http://www.newton.dep.anl.gov/askasci/chem03/chem03444.htm
Since it is stated that ice evaporates, does
metal in solid form evaporate too?
Replies:
Hi Weiyoong,
The general answer to your question is, no, at anything like normal
temperatures and pressures, metals (at least common metals) do not
sublime. But under certain circumstances, especially high temperatures
and low pressures, metal sublimation is known. A common example is a
normal light bulb. The white hot tungsten filament slowly evaporates
(or sublimates), eventually resulting in its failure. The dark spot
often seen on a burned out light bulb is the tungsten filament that
has sublimated and condensed directly to a solid on the cooler glass
shell.
Regards,
Bob Wilson
Weiyoong,
Good way to infer a question. As you may have noticed from the answers
given in that link, the rate of evaporation is very much a function of
temperature and the vapor pressure of that substance at that particular
temperature. Water has a boiling temperature at sea-level of 100degC.
Thus, we can say that at sea-level and 100degC, the vapor pressure of
water is 760torr. If we go down to just above 0degC, the vapor pressure
is down to 4.6torr. If we go down another 90 deg to -90degC we find that
the vapor pressure is barely measurable at 0.00007torr.
Translate this pressure to number of moles (using the ideal gas
equation; PV=nRT or PV/RT = n) and we find that assuming a volume of 1L
above the substance, at 100degC (760torr), there are 0.0326mol of water
vapor. At 0degC and 4.6torr, there are 0.00027mol. At -90degC (and
0.00007torr) there are 6e-9 mol. Even if we assume that all these number
of moles are removed by the blowing of air above the solid such that
this is the number of moles removed per second, it would take around
30sec to blow away 1mol (18g) of water at its boiling point, about an
hour for ice at 0degC, and over 5 years (!) to blow away 1mol (18g) of
ice at -90degC.
Now think of a metal (such as iron) that has a melting point that is
around 1500degC. At room temperature, this metal must have an
infinitesimally small vapor pressure. It would take a long time indeed
to blow away such a substance. Thus, we say that some solid objects,
such as metals have a non-appreciable vapor pressure, or an
insignificantly low vapor pressure.
Greg (Roberto Gregorius)
Under the proper conditions it is possible to sublime metals as well as
many other compounds. Perhaps the "classic" example is the sublimation of
CO2 ("dry ice"). Every pure substance has three physical forms -- solid,
liquid, vapor. Some substances have more than one solid form. It is possible
given the proper conditions to convert one to the other without involving the
third physical form.
Vince Calder
Just as with water molecules in ice, atoms from metals can vaporize too,
although for most metals the rate is very slow at room temperature (so slow,
it is essentially zero). This is because the amount of energy it takes to
break free from the crystal lattice of the metal is so high (much higher
than required for a water molecule to break free).
The main principle to learn here is that the atoms or molecules in a
substance don't all have the same energy. Some have more energy than others,
and are therefore more likely to 'break away' from a solid and 'evaporate'.
However, the amount of energy required is different for different
substances. For metals, that amount is much, much higher, so many, many
fewer molecules will break free, so few that (for a bar of platinum, for
instance) you wouldn't be able to notice a change in its mass for a very,
very long time. There are instruments that generate large amounts of energy
to drive metal atoms from a solid, though, and they can evaporate metals
quickly.
I bring up platinum because the official standard for the kilogram has been
a bar of platinum-iridium -- if it evaporated quickly, it would be worthless
as a standard of mass!
I hope this helps,
Burr Zimmerman
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Update: June 2012
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