How are solid, gas and vapor able to coexist at once and what would it look like?
In: Chemistry
I’m not sure if this is really an ELI5, but I think it’s easier to understand if you think about the triple point as the transition out of the region of pressure and temperature where sublimation occurs.
Sublimation is a state transition at low pressure and temperature where solids will transition to gases directly, skipping the liquid phase entirely. Basically there is no liquid state at low pressure and temperatures, it’s either a gas or a solid.
The triple point is the point beyond which sublimation stops, and liquid starts to form. If you stay right at the triple point, you get a combination of sublimation, thawing, boiling, and freezing all happening at once (this video is just the first one I found on youtube if you’re interested in what it looks like [https://www.youtube.com/watch?v=Juz9pVVsmQQ](https://www.youtube.com/watch?v=Juz9pVVsmQQ), around 1:15 is the triple point).
What? In the same element, same compound? I’m not aware of any compound that will exist simultaneously in 3 different states unless forced. Ie water becomes steam and boils with heat, and ice with the removal of said heat. Can an ice cube, boiling water and room temperature coexist without interference in the exact same spot? No.
There are some good videos on YouTube showing you what it looks likes. It’s essentially and equilibrium between the states. H20 would be freezing, melting and vaporizing all at the same rate.
It’s kind of like when you want to rest, but you just drank an energy drink beforehand.
You constantly flip between sitting down, lying down, and standing up. You’re only ever in one state at any time.
Now imagine 3 of your friends are beside you trying to do the same thing. Individually, you’re only in one state, but as a group you could be in all 3 states simultaneously.
It’s important to understand what the triple point means. First start by simplifying the discussion and let’s just talk about the melting point. At sea level on Earth, the melting (or freezing) point of pure water is 0 degrees centigrade. This means that at exactly 0 degrees and at exactly 1 atmosphere of pressure, the solid and liquid forms of water exist in equilibrium. Or as you put it, they can coexist.
Now consider that the melting point and the boiling point of any element or chemical compound will be different at different pressures. For example, the boiling point of water at sea level is 100 degrees but if you go to Denver, water will boil at around 95 degrees because the air pressure is lower. The melting point also changes with pressure, but it does so much, much more slowly at the kinds of pressures that matter to us for now.
So since the boiling temperature of something will decrease as pressure decreases, but the freezing point changes only very slightly, eventually you can get the pressure low enough that the boiling temperature and the freezing temperature are exactly the same. This is called the triple point. If pressure is lower than the triple point, **the liquid form of that matter cannot exist**. It will either freeze or it will “sublimate” directly to the gas phase. If this sounds familiar, that’s because you see it every Halloween in the form of dry ice. The reason dry ice turns directly in to gas is because it is made of a substance (CO2) that is below its triple point on Earth. If we put dry ice in a pressure chamber and cranked up the pressure to 6 times the air pressure on Earth, we could then successfully melt it in to a liquid.
So when we say that solid, liquid, and gas exist in equilibrium, it’s not much different than saying that at the melting point, solid and liquid exist in equilibrium or that at the boiling point, liquid and gas exist in equilibrium. It’s just that at a very specific pressure **and** temperature, all three can be in equilibrium or coexist.
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