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Name: Mandy H.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 11/6/2004


Question:
Why does I2 sublime first before going through a melting stage?


Replies:
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 an example.

Vince Calder



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