MadSci Network: Other

Re: Can a rock 'evaporate' or become a gas?

Date: Tue Feb 2 09:05:11 2010
Posted By: Uncle Al Schwartz, Organic synthetic chemist
Area of science: Other
ID: 1265093335.Ot

Short answer:  Everything has a vapor pressure.  It can be extremely small.
   The lab joke is, "tungsten's vapor pressure at room temperature is one
atom/universe."  Higher temperatures promote higher vapor pressures.

All phases have a thermodynamic "chemical potential", sometimes called
"fugacity."  In a closed system chemical potentials come to equilibrium by
having the phases enter one another.  The necessary amounts can be
extremely small (the tungsten joke) to 100% (mixing different gases).  In
an open system chemical potential runs until something exhausts.  Vapor
pressure is one mechanism among many.  Clausius-Clapeyron equation.

Consider a rock in air.  There is a chemical potential for both and
contingent vapor pressures.  A strongly bound solid will have an
*extremely* small vapor pressure (thermodynamics).  It will evaporate
*extremely* slowly (kinetics).  There is nothing to measure.

Increase the temperature: increase the vapor pressure (thermodynamics),
increase the rate (kinetics).  Hot rock does evaporate!  Fused silica
softens to workability at about 1900 C (eye-searing brilliant white-hot). 
White-hot silica slowly evaporates then quickly condenses to dust in the
air.  Quartzblowers wear face masks or work in an exhaust hood.  That dust
destroys lungs (silicosis).  Quartzblowers also wear welder's goggles.

Atmospheric pressure is 760 torr.  The vapor pressure of ice at 0 C is
about 11 torr.  Solid ice evaporates!  Slowly.  Put some ice in a 0 C
vacuum chamber and turn on the pump.  As the pressure drops below 11 torr
the ice will rather quickly evaporate (sublimation).  This called
"freeze-drying" or "lyophilization."

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