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Name: Regina
Status: student
Age: 15
Location: N/A
Country: N/A
Date: Around 1999 


Question:
I stumbled across the boiling point of iodine and I found it funny. Since iodine is a subliming substance, how come it has a boiling point? It has already turned into a gas at 114 degress Celsius (m.p.), so at what state will it be when it is at 184 degress Celsius (b.p.)?


Replies:
Dear Regina,

Allow me to compliment you on asking such an excellent question. Your ability to critically analyze data found in the literature is to me a sign of a discerning intellect.

All of your facts are correct. However, I must point out that all solids sublime at room temperature, at least to some extent. For example, a piece of ice at (0C,1atm) both melts and sublimes. A piece of wood also sublimes, although imperceptibly.

The phrases "melting point" and "boiling point" refer specifically to the temperatures when a complete phase transition first occurs at fixed pressure. But that does not mean that there is no appearance of the new phases below the temperatures of interest. For example, water has a finite vapor pressure at room temperature. This vapor pressure gradually gets higher and higher, and at the boiling point the vapor pressure is equal to the atmospheric pressure. To prove this point, put an open glass of water in a room, having first carefully measured how much water there is to start, and come back a week later and see how much water is left. There will be less water, because some of it has evaporated. This is despite the fact that the water was not heated to the boiling point. However, if you do the same experiment with a closed container, some water will end up in the vapor phase but most will stay in the liquid phase. By LeChatelier's principle, the more vapor you remove, the more will evaporate.

The same is true of iodine. At (25C,1atm) solid iodine can indeed sublime; the equilibrium constant for sublimation is 4 x 10^-4. This is small, but not tiny at all, and the more purple fumes that escape, the more are generated (again LeChatelier). So there's not a conflict between all the data you cited.

very best regards,

prof. topper
department of chemistry
the cooper union
new york, ny


It is true, iodine sublimes at room temperature. However, the vapor pressure of iodine at room temperature is less than one atmosphere. As the temperature increases, the vapor pressure increases as well.

When solid iodine is heated, eventually it will melt. The boiling point is the temperature at which the liquid is in equilibrium with the vapor at one atmosphere of pressure, in other words, the temperature at which the vapor pressure is one atmosphere.

Richard Barrans, Ph.D.
Assistant Director
PG Research Foundation, Darien, IL



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