[dutch]

Hi,

I have done some research on Iodine because I am trying to design a chemistry game, and I have some questions about this substance.

(melting point: 386.85 K, 113.7 °C, 236.66 °F
Boiling point: 457.4 K, 184.3 °C, 363.7 °F)

I understand that many people think that iodine sublimes to gas without going through the liquid phase. I read that this is not completely true. Partly is sublimes, but partly it does become a liquid. But because of the dark purple vapor (that indeed sublimed) it is often not seen. This sounds logical to me because when you look at the triple point of iodine, it seems impossible to go from solid to liquid without passing the liquid phase (that is, under normal pressure of 1atm).

http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/FG13_18.JPG

Still I wonder, when looking at this picture, how come it sublimes? for CO2 for instance it is easy to understand.

http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/FG13_19.JPG

If you heat solid iodine just below its melting point, and you wait long enough, will all the iodine sublimes without going through the liquid phase? Or is it not possible to sublime everything because you are still below the melting point and thus part of vapor will deposit back to solid crystals again.

So what I would actually like to know is, at normal temperature, and normal pressure, when exactly will iodine start to sublime? And, the other way around, when you have heated iodine gas, at what temperature exactly will it start to deposit back again to solid. (when it is just below the the melting point? I assume that solid iodine does not exist above melting point?)

Furthermore if you would heat or cool iodine until exactly 150 degrees Celsius, will iodine only exist in the liquid phase, like you would expect, or in multiple phases. (since iodine behaves so weird anyway

Thanks for helping me!

[Borek]

I think you are misreading the phase diagram. We are talking about equilibrium at temperatures well below 100 deg C, liquid doesn't show till you get above 113.

Iodine sublimes all the time, no need to heat it up.

[dutch]

Hi Borek,

Thanks for your response. I do know that iodine sublimes well below room temperature, I just dont understand why, since the pressure should be well below normal room pressure as well, according to the diagram..

[Borek]

You are misunderstanding the diagram then. It tells you the pressure of IODINE, not total pressure of all gases present.

[dutch]

aah, ok I didn't know that!  So the pressure of iodine is normally lower then 1atm? Then it suddenly all makes sense

But now I still dont know how to find an answer to my question: at which exact temperature does iodine starts to sublime (or starts to deposit back). Even if I knew the exact pressure of iodine (or some average), the diagram is not scaled so I cannot read the temperature. Do you know where I can find such information?

And could you maybe explain to me if iodine can stay at the same temperature when subliming (like ice when melting for instance) or does the temperature has to be rising? If it can stay the same temperature, like I expect after reading all these experiments, will iodine crystals then deposit at the same time as other solid parts are subliming?

This makes me wonder: when water is melting, the temperature will stay 0C, but how does the water 'know' that it is going from ice to water, and not from water to ice?

I really do appreciate your help by the way!

[Borek]

But now I still dont know how to find an answer to my question: at which exact temperature does iodine starts to sublime (or starts to deposit back).

It does both all the time. Take a look at diagram - no matter what temperature is (as long as we are not in the area where iodine melts) in each temperature there exist equilibrium pressure of gaseous iodine. It doesn't tart at some temperature, it isn't like boiling - more like evaporation.

Even if I knew the exact pressure of iodine (or some average), the diagram is not scaled so I cannot read the temperature. Do you know where I can find such information?

No. Chemical databases or handbooks. You will need to consult a scientific library in college or university.

And could you maybe explain to me if iodine can stay at the same temperature when subliming (like ice when melting for instance) or does the temperature has to be rising? If it can stay the same temperature, like I expect after reading all these experiments, will iodine crystals then deposit at the same time as other solid parts are subliming?

No need for temperature changes, but it is not a process that goes in a constant temperature the way melting does. Both sublimation and deposition occur at the same time, what is important is which process is faster.

This makes me wonder: when water is melting, the temperature will stay 0C, but how does the water 'know' that it is going from ice to water, and not from water to ice?

It doesn't matter, both melting and freezing go at the same temperature. Which process dominates depends on the system heat balance.

[dutch]

Thank you so much for all your help. I must be very annoying for you, but I still do not completely understand.

You say iodine sublimes all the time - no matter what the temperature is, as long as we are not in the area where iodine melts. Do you mean with this 'area', all temperatures above 113C?  Because no matter what the pressure is, when the temperature is above 113C, iodine cannot exist in solid form, right?
So when you would have iodine in gas form at 200C and you start cooling it down, it will first start to sublime when reaching temperatures below 113C...right?  I really hope so because then I finally understand!! (or does it condense first?)

Thank you again!

[dutch]

wait, I think I understand now. When you have iodine gas at 200 degrees, the temperature and thus pressure as so high, that when cooling down you enter the liquid phase. And when some of this gas then condenses, the pressure drops. So the remaining part of the gas can sublime.
yes? yes?

[Borek]

200 deg C is not the whole story, as you can't ignore pressure. If the iodine pressure is low enough, it will resublime on cooling without going through the liquid phase. If the iodine vapor pressure is higher than its triple point, then it has no choice but to condense to liquid first.

Imagine a horizontal line through the diagram - it means constant pressure. Now imagine you cool down your system at constant pressure - you either move through the liquid, or not. Same about going the other direction, heating the solid at constant pressure.

Note that "constant pressure" can be a little bit misleading, as it requires a specific setup that will help keep pressure constant. If you have a rigid tank filled with iodine vapor at some T, P, and you cool it down, it is not a constant pressure experiment. But if you have a cylinder with a piston, you can keep constant pressure condition through the cooling.

[dutch]

Sorry, I should have explained that before: I have always been speaking about a closed system (this is part of the application that I am designing), so the pressure will indeed change and not stay constant. I want to simulate what happens when you heat up or cool down iodine in a closed beaker. Maybe you understand my last assumption better now? (about cooling down iodine gas of 200C at high pressure).

[Borek]

So behavior of the system will depend on the amount of iodine present. If there is enough iodine to build pressure high enough, you will see a liquid.

[dutch]

That is exactly what I wanted to hear, thank you!