ELI5: Why does sublimation happen? Why do things that sublimate like dry ice not melt into a liquid before turning into a gas?
ELI5: Why does sublimation happen? Why do things that sublimate like dry ice not melt into a liquid before turning into a gas?
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Not really an easy ELI5 explanation other than “that’s just kinda how physical chemistry works”. Chemical compounds exist in phases, each compound has its own “phase diagram” which tells you what form (solid, liquid, gas, etc) that thing exists in at every given temperature and pressure. Every phase transition (solid->liquid, liquid->gas, solid->gas, etc) has a specific set of temperatures and pressures associated with it. Some compounds sublime (solid->gas) at temperatures and pressures that are close to what we experience all the time (e.g. naphthalene). Every chemical in existence sublimes at some combination of temperatures and pressures, but it may require particularly extreme temperatures and pressures that are hard to achieve.
There is a special pressure and temperature called the triple point where a substance is gas liquid and solid. If conditions change just so, it will not transition as you would expect. Pressure is what most people don’t consider. Substances don’t just react to temperature.
Chemistry flunkie here. I think it’s because, at “normal” atmospheric conditions such as in your home, chemicals such as carbon dioxide are too weakly held together at room temperature to be maintained as a liquid. It’s all or nothing. It takes a lot of pressure, i.e. abnormal atmospheric conditions, to keep the molecules close together enough for them to be a liquid.
Basically, water is a solid or liquid because the forces between the molecules are strong enough to keep them together in normal atmospheric conditions.
When you take a solid and increase its energy AT CERTAIN PRESSURE CONDITIONS it can either undergo melting or it can undergo sublimation. You’re un-freezing the material. But after un-freezing what’s next? Molecules or atoms that were not moving much suddenly start moving more and more. That is actually what temperature is – it’s a measurement directly related to how much movement energy those molecules have.
Why sublimation happens versus melting depends on how strongly the molecules attract each other, as well as what is going on in the gas pressure around that solid. Now that some of those atoms/molecules are more free to move (unfrozen) what do they want to do? If they can, they will gladly jump up to the gas phase if they aren’t too tightly attracted by their neighbors.
If it can’t sublimate, it could be because there’s too much pressure smacking it back down, or it just has a strong sense of self-attraction, then it will instead become liquid… still moving but not quite as “free”.
(Don’t forget “triple point” where it sublimates, boils, freezes, melts all in perfect equilibrium)
We are mostly used to things happening close to human living conditions… something close to 25C, 1 atmosphere pressure. Under those conditions there are a few things that sublimate. That might seem weird, unusual.
But really, it’s not unusual. It’s only unusual to you because you’re normally around 1atm and not that many common things sublimate around that exact pressure that your body likes.
Because the conditions in the environment around it are so far away from allowing it to be a liquid.
It has something to do with temperatures and airpressure.
Both affect the status of a chemical element.
If you look at something more common like water, it freezes into a solid at 0° and boils into a liquid at 100° but these temperature turning points only apply to normal air pressure.
If you boil water at the top of Mount Everest, the boiling point is much colder more like 80°, if you use a pressure cooking pot the boiling point raises to 120° due to the increased air pressure.
You can take this into the extrem and even reach a point where water vaporizes, freezes and is a liquid at the same time, at an exact temperature and exact pressure.
For Dry ice which is frozen CO2 you need a super cold temperature, so the temperature difference between the frozen CO2 and the air temperature is insanely high.
But when the pressure isn’t high enough, once the CO2 is getting too warm, it just turns into a gas and floats away.
So in order to get liquid CO2 you need the right temperature and the right pressure.
But there is more, depending on the element, it may be unstable and fuse with elements from the air immedeatly to form new molecules. Thats how certain elements are only found in a lab environment under a vacuum or noble gas atmosphere.
Solid and gas phases are easy to understand – it is temperature dependent – a solid becomes a gas when the energy (from heat) is able to overcome the forces holding the solid together (latent energy of sublimation) and break free
Liquid phase requires temperature and pressure – it is an ‘in between’ phase where the molecules are partially free – temperature is required to allow the molecules to break from the solid, and ‘compression’ is required to keep them from being completely free (like a gas)
To get liquid carbon dioxide, you need a lot of pressure (>5x atmospheric pressure) to be able to maintain the liquid phase – so you can get dry ice to melt, but only under compression
Below that pressure, the liquid phase cannot exist – as demonstrated by the phase diagram – so the solid (dry ice) sublimates directly to gas (carbon dioxide)
Each substance has a triple point, which is the only set of conditions where solid, liquid, and gas can coexist. If the surrounding pressure is lower than the triple point pressure, the substance can’t exist as a liquid and it skips right from a solid to a gas.
For example, the triple point of CO2 (dry ice) is about 5.1 atm. At normal atmospheric pressure (1 atm), CO2 can’t become a liquid, even if heated. So when solid CO2 warms up at room pressure, it sublimes. The molecules have enough energy to break free from the solid structure, but there’s no stable condition for them to form a liquid so it goes straight to a gas.
Look up “triple point” on Wikipedia, it’s easier to understand on a graph.
Imagine molecules are sheep that when sleeping (low energy) they like to be close together and when awake (higher energy) they like to run around and be as far apart as possible.
Say you have two groups of these sheep; one inside a closed barn packed so tight that they are touching and the other huddled closely together in an open field.
When a sheep in the barn wakes up it will try to move around but it will constantly be bumping into the walls of the barn and other sheep. This is like a solid turning into a liquid.
when a sheep in the field wakes up they will immediately leave the herd of sleeping sheep and run around in the field leaving the group smaller but otherwise undisturbed. This is like a solid turning directly to gas.
Water only exists as a liquid on earth because the weight of the atmosphere provides confining pressure that holds the molecules together the same way the walls of the barn held the sheep together. Basically a liquid is a solid that has enough energy that a significant part of it would turn to gas if only the confinement was removed.
Out in space there is no confining pressure so as soon as a water molecule gains enough energy to separate from its companions it does so unimpeded and can travel very far away carrying the energy with it leaving the rest a solid.
Going further with this analogy a super critical fluid is what happens when all the sheep in the barn have woken up. They all have enough energy to be a gas and so would all leave if the door was opened.