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Elemental iodine's often used to demonstrate sublimation,
which is when something goes directly from the solid to the gas phase without a liquid intermediate
There are a few other common examples out there like dry ice or mothballs,
but iodine tends to work better for demonstrations because it's so strongly colored
At room temperature its sublimation rate is really slow, so we can't see the vapors coming off of it
However, if we put it in an enclosed space the vapors can build up and get more concentrated,
and we can see a faint purple color
This isn't very impressive though, so for the demonstration the iodine is usually heated
As the iodine warms up more molecules are able to make the transition from the solid to the gas phase,
and a lot more vapor is given off
Then I place a round bottom flask filled with cold water on top of the beaker
The iodine vapors will deposit on this colder surface
and reform the iodine crystals
Okay so at this point you're probably thinking, "Great. That's a pretty straightforward
demonstration, what exactly is the myth that this video is titled after?"
Well, a lot of people, scientists included,
think that because iodine sublimes, that it doesn't or can't melt, which is totally wrong
What I find interesting is that despite it being incorrect, it's still often taught in schools and
sometimes written in textbooks. It's definitely up there as one of the most widespread
misconceptions in chemistry
For example I did a quick google search, and I easily found a few textbooks which propagate this myth
One of the saddest parts is that this book is literally just called "Iodine," but it still gets it wrong
Just as a comparison, here are some other textbooks which correctly state the complete opposite thing
What's kind of funny is that this one is also just called the "Iodine," but it actually gets it right
Anyway, at this point I'm kind of just telling you to not believe some textbooks and to believe others, without any real evidence
However, I can actually prove it to you really easily
I'm just gonna do the exact same demo that I did earlier, except this time I'll crank the heat up a lot higher
When I do this, you can easily see that the iodine starts melting
As I continue heating, everything eventually liquefies and from that point on it's technically
evaporation and not sublimation
So if you actually want to be demonstrating sublimation and not just deceiving the students,
it's really important to keep the temperature below the melting point
The boiling point of iodine is about 184° C,
but the hot plate is a lot hotter than that
So the iodine starts to boil off and the bottom of the beaker clears up a little
About a centimeter above the hot plate, the temperature of the glass is below the boiling point, but still higher than the melting point
Because of this, the vapor condenses back into liquid iodine and doesn't immediately solidify
Then, it drips back down and gets revaporized
Okay, so for me, this is pretty good evidence that it melts,
and that a liquid phase does in fact exist;
but to really just drive it home, I'm gonna do one last thing
I add a whole bunch more iodine to a beaker, crank on the heat, and quickly melt it
Then, while it's a liquid, I pour it into another beaker
If this doesn't prove that iodine melts, then I really don't know what will
Okay, so considering this myth is clearly wrong, you really have to wonder
Where did it come from and why is it so common that it even makes it into textbooks?
Well to explain this we need to get into a little bit more detail as to what sublimation is
In all forms of matter, molecules are constantly vibrating and the speed that they do this at is related to their kinetic energy
In solids the molecules are constrained by a lot more forces, but they do still vibrate
However, a given solid doesn't just have one uniform kinetic energy,
it's actually a mix of molecules all with different levels
In sublimation, some of the high energy ones are vibrating fast enough that they can overcome the forces keeping them as a solid,
and they escape as a gas
For them to do this though, they must be both near the surface and moving in just the right direction at the right time
This process generally occurs in all solids, but the rate is quite variable
It depends mostly on the strength of the forces that are keeping the solid together
For example, iodine's held together by relatively weak ones,
so they're pretty easy to overcome
Whereas in something like gold, the forces that are keeping it together are much stronger so the energy threshold is a lot higher
This in turn means it's less likely for molecules to be above it
In any case though, regardless of the substance, if we want to increase its rate of sublimation we just need to get more
molecules above the threshold, which is easily done by increasing the temperature
So this is exactly what we saw with iodine
I raised the temperature, the number of molecules above the threshold increased, so the sublimation rate also went up
Now, to take the explanation a little bit further, I need to talk about something called a phase diagram
Basically, it lets us easily work out the phase a substance should be in, given the temperature and pressure
A diagram like this exists for pretty much all substances, but for this explanation I'm just going to use water
So the first step to using it is to choose a temperature and pressure
So for example, at a normal atmospheric pressure of one atmosphere,
and a temperature of -20° C or -4° Fahrenheit, water is solid, which makes sense
As I mentioned before though, all solids sublime and this includes ice, even at -20° C
If the pressure stays the same, and I just increase the temperature, the rate of sublimation will also increase
right until we reach the line separating the solid and liquid phase, which is at 0° C
At this point, the ice melts because the molecules have enough energy to overcome the forces keeping them together as a solid
From here on it's a liquid, but if I keep heating it its rate of vaporization will continue to increase
Technically though, it's now called evaporation, just because it's in a liquid,
even though it's pretty much the same process as sublimation
Eventually, we get to the boiling point where the pressure of the vapor coming off of the water is equal to the atmospheric pressure
At this threshold, most of the molecules have enough energy that they're ready to make the jump to the vapor phase
Ok, so that was just for water, but now let's take a look at the diagram for iodine, which actually looks pretty similar
I'm just going to be starting at room temperature and normal atmospheric pressure, which lands us squarely in the solid zone
When the temperature is increased the sublimation rate also increases,
until we reach the melting point at around 114° C
Then, if we keep heating and evaporating it, we'll eventually reach the boiling point which is exactly what we saw earlier
Okay, so from what we've seen so far, it really doesn't make sense to say that if something sublimes, then it can't melt
However, that's because in both the cases we've looked at, the atmospheric pressure is above the triple point
So what happens if the atmospheric pressure is below the triple point like we have with dry ice?
Let's say we start at around -100° C
As the temperature rises, as usual the sublimation rate also increases
When it gets to around -78° C, the vapor pressure is already equal to the atmospheric pressure
And it wants to turn into a gas
This is exactly like what we saw with the boiling point of water and iodine,
except this time, the boiling point is below the melting point
This occurs because the melting point is dependent almost entirely on the substance alone,
whereas the boiling point depends on the substance as well as the outside pressure
It just so happens that the atmospheric pressure we live in places it along this line
If I wanted the dry ice to melt first and then boil, I would have to increase the pressure to somewhere above the triple point
In any case though, when it gets to this boundary between the two phases, the process that it undergoes
is similar to what boiling is for liquids, but to differentiate it we call it sublimation
But wait a second, earlier on I told you guys that sublimation was a process that happens in all solids, and it's similar to evaporation
But apparently now, I'm telling you it's similar to boiling. How does that Make any sense?
Well, basically it doesn't. And that's exactly the problem.
With liquids, we have two different words, evaporation and boiling,
which describe two distinct processes
But with sublimation, for some reason we only have one
So because of this, it's kind of true to say that if something sublimes it can't melt,
and it all depends on which process we're actually referring to
Some resources try to get around this problem by saying that below the melting point, it's still called evaporation,
claiming that sublimation should only be used to describe the proper phase change when something's below the triple point
However, I think that even if everyone did use these definitions, it would mean that we have one term, evaporation,
applying to both liquid and solid phases, which might also cause some issues
I'm gonna use Wikipedia as a quick example of how wishy-washy the definition is.
On the page for sublimation, it starts by defining it as only occurring below the triple point
Okay, so that's off to a good start,
but then it goes on to say that it's also just the generic transition from solid to gas
And, for the rest of the article, it just uses both definitions interchangeably.
I think to solve all this confusion, we just need to make a new term that exclusively describes the evaporation of solids
The term sublimation would then only describe the proper phase change that occurs below the triple point
Just for fun, I took two seconds to make up a word based on my channel, and I called it "Nilation"
This way, iodine no longer sublimes, it would "Nilate" instead
Dry ice would "Nilate" at temperatures below -78° C, and then it would sublime
In my opinion, this could prevent a lot of confusion, and it would also wipe out the iodine myth
Let me know what you guys think of my made-up term, and let me know if you think something else would be better
In theory if we all started to use a new term, whether it's "Nilation" or something else,
we could probably eventually change things
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