Iodine, Sublimation, and Melting
Name: Mandy H.
Why does I2 sublime first before going through a melting
All (almost) pure substances have three phases -- solid, liquid,
vapor -- that are in equilibrium with each other. There are three possible
transitions solid to liquid (melting), liquid to vapor (boiling or
evaporation), solid to vapor (sublimation). The melting temperature is
governed by the forces holding the molecules of the substance in its crystal
lattice compared to the mobility of molecules of the substance in the liquid
state, the boiling point (or more correctly the vapor pressure) is governed
by the forces holding the molecules in the liquid state. For most substances
the forces holding the solid together, the forces holding the liquid
together, compared to the vapor (almost no forces holding the vapor
molecules together) are in that order. For some substances, however, the
forces are not in that order and it is "easier" for a molecule to break free
of the crystal lattice without undergoing a change into a liquid phase.
There is nothing 'magic' about this. The boiling temperature of substances
(at 1 atmosphere) is quite arbitrary and is a function of the "normal"
pressures that we operate in. The classic example of sublimation is dry ice
(CO2). Its vapor pressure is equal to 1 atmosphere at -78.4 C but it melts
at about -56 C and at this temperature has a vapor pressure of about 5
atmospheres. Iodine melts at 113.6 C. The liquid boils at about 185 C. It is
only because we "think" in terms of atmospheric pressure that some of these
substances seem unusual.
As an aside, there are some substances that have multiple melting points
because the solid can exist in several different crystal phases, phosgene is
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Update: June 2012