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Guys, I was looking at boiling point and melting points of the following and got this info from wikipedia.

 

Gas / Boiling Point (oC) / Melting Point (oC)

Nitrogen / -195.79 / -210

Oxygen /-182.95 / -218.79

Argon / -185.85 / -189.35

Carbon Dioxide / -78 / -57

 

What I don’t understand is, why is the melting point of CO2 is higher than the boiling point! Bit confused! Can you help?

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the vapour pressure of carbon dioxide reaches 1 atm before it becomes liquid. therefore it sublimates rather than melt then boil. carbon also does this i believe.

 

this is due to the balance of kinetic energy of the molecules and intermolecular bond strength. its unusual but not unknown.

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I looked all over the internet to find an explanation for why sublimation occurs, and couldnt find any.

 

You say that they aren't 'sticky' - then why do they form a solid eventually? Why dont they just remain gas. I personally would like to know the scientific explanation.

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Every compound forms a solid eventually when the temperature goes down/pressure increases. It's just a matter of mass and intermolecular forces. CO2 molecules aren't polar so there's very little attraction between molecules but their mass is huge compared to H2 for example, hence the higher melting point. I suppose you can call polar molecules like H2O or NH3 "sticky", or perhaps compare them to tiny magnets as opposed to tiny, essentially nonmagnetic objects.

 

Edit: Also, note that basically everything in this crazy place we call the universe is "intrinsically sticky" due to the Casimir effect. But that's more complicated physics perhaps outside the scope of the topic at hand.

Edited by Gilded
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exactly, there is very little electrostatic attraction between the molecules. that doesn't mean there is none though.

 

compounds form liquids when intermolecular forces overcome momentum to keep them close together but not enough to keep them in the one place. when they are enough to keep them in the one place as well as preventing them from flying off on their own they form a solid.

 

In CO2, unless you apply a fair amount of pressure, the intermolecular forces will be insufficient to stop them flying off before it lets them move around close to each other.

at standard pressure there just isn't an inbetween. you can see this on the phase diagrams of any substance, even water. take the pressure down really really low and there will be no liquid phase just solid and gas.

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Right, now I am confused.... melting point is higher than the boiling point! That is what the kids will stick to and it’s gonna be tough to explain in plain English!! Come on guys, help me out here!

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no, at normal pressure, the melting point does not exist

 

it only has a melting point when you compress it. at the pressure where it has a melting point, the boiling point is higher than the melting point. infact, at that pressure it is the same temperature so you have a mixture of all three phases in equilibrium. apply more pressure and they will seperate into the 'normal' distribution.

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great!! how am i going to explain this to kids in plain english?

 

usually in simple classes, the melting point of carbon dioxide is ignored, and it is pretended that it only sublimes. FYI the opposite of sublime is deposit

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The simple answer is that, for CO2, the melting and boiling points in that table are not measured at the same pressure so it's not comparing like with like.

Most materials do what CO2 does, it's just that we happen to chose atmospheric pressure as our standard.

If we lived on a planet with a very low atmospheric pressure (below about 0.006 Atm) we would think of liquid water as some weird state.

(I'm ignoring the difficulty of life in the absence of liquid water).

 

Solids have a finite vapour pressure at any given temperature- if that pressure excedes the external pressure then the stuff will sublime. At a lower external pressure the temperature required to get the stuff to sublime will be lower.

For a low enough external pressure that will happen at a lower temperature than the melting point.

There's nothing magic about CO2 except that it happens to behave this way near normal temperatures and pressures and is common enough that we hear about it.

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Lookup the "critical point" or critical temperature. the phase transition you are worried about (liquid) only occurs at elevated pressure. At atmospheric pressure, CO2 will go straight to solid (and vice-versa) at -78 °C.

 

In the cylinder, the CO2 is supercritical (in excess of the critical point) and it is a liquid. When opened to atmospheric pressure, it will go straight to solids (dry-ice flakes, for example). If dispensed into a sealed container (which can tolerate the pressure), you will have liquid.

 

This liquid is used for extractions (of, say caffeine from coffee to mafe decaff--yark!), which is nice because opening to STP rapidly eliminates the solvent (dichloromethane was used before).

 

So, also see "supercritical fluid extraction (SFE)".

 

Cheers,

 

O3

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The simple answer is that, for CO2, the melting and boiling points in that table are not measured at the same pressure so it's not comparing like with like.

Most materials do what CO2 does, it's just that we happen to chose atmospheric pressure as our standard.

If we lived on a planet with a very low atmospheric pressure (below about 0.006 Atm) we would think of liquid water as some weird state.

(I'm ignoring the difficulty of life in the absence of liquid water).

 

Now, that I can explain to the kids….

 

So is there a pressure range that CO2 can be in all 3 states relative to temperature?

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Yeah, liquid CO2 is some pretty neat stuff. Got a chance to see it when some Dry ice was put under pressure and it actually melted. It's pretty neat seeing it when you were always taught that it only sublimes.

 

Liquid Iodine is neat too. You don't even need a lot of pressure to see that. Just put a lot of I2 in a narrow test tube and heat it a little bit. The standard pressure of gaseous I2 will cause the iodine to form a liquid.

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There's nothing special about molten I2. It melts at 114C and boils at 184C. Between those temperatures, at normal pressures, it's a liquid.

 

What do you mean by "The standard pressure of gaseous I2 will cause the iodine to form a liquid."

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iodine is unusual, for sure, but contrary to popular belief, it doesn't really sublime at normal pressures. It is a solid at room temperature, and strangely for a solid, it has an appreciable vapour pressure, which something most solids are considered not to have. As you raise the temperature, that vapour pressure increases, but as long as you have plenty of iodine, you can raise its temperature to the melting point and melt it just like nearly any other solid. Of course by that time there'll be an awful lot of vapourised iodine hanging around. Not unlike bromine there.

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There's nothing special about molten I2. It melts at 114C and boils at 184C. Between those temperatures, at normal pressures, it's a liquid.

 

What do you mean by "The standard pressure of gaseous I2 will cause the iodine to form a liquid."

 

Take some ice and put it on a piece of paper and heat it up. You'll see the ice melt into a liquid.

 

Take some iodine and put it on a piece of paper and heat it up. You will NOT see liquid iodine form. It will go right from a solid to a gas.

 

If you heat up I2,the standard vapor pressure of I2 over the solid will cause a liquid to form. If you put the I2 into a wide open area, that "standard pressure" never exists and the iodine never melts.

 

If you can show me a video of I2 out in an open container melting, then I'll admit I'm wrong. I'll just be stunned if you can give me that proof.

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iodine is unusual, for sure, but contrary to popular belief, it doesn't really sublime at normal pressures. It is a solid at room temperature, and strangely for a solid, it has an appreciable vapour pressure, which something most solids are considered not to have. As you raise the temperature, that vapour pressure increases, but as long as you have plenty of iodine, you can raise its temperature to the melting point and melt it just like nearly any other solid. Of course by that time there'll be an awful lot of vapourised iodine hanging around. Not unlike bromine there.

 

It does sublime, because "sublimation" is just the process of solid turning into vapor. In a closed system with sufficient I2 present, it will not completely sublime, that is true. But for example ice also sublimes at -5 deg C, which has little to do with its boiling point.

 

If you heat up I2,the standard vapor pressure of I2 over the solid will cause a liquid to form. If you put the I2 into a wide open area, that "standard pressure" never exists and the iodine never melts.

If by "standard vapor pressure" you mean "the vapor pressure at 298K", then no liquid will form. But at 113 deg C, it melts. The vapor pressure of I2 at its melting point is about 130 mbar. It boils at 184 deg C, and (by definition of boiling point) its vapor pressure is then 1 atmosphere (1.01325 bar).

 

I've attached some data so we can talk about CO2 again, and leave the iodine.

 

Here's the CO2 phase diagram

 

And the vapor pressure of I2 at different temperatures is found below:

iodine.JPG

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iodines triple point is 386.65K and 12.1kPa

 

it has a liquid phase at standard pressure. although unless there is appreciable iodine in the atmosphere it will evapourate too quickly to be noticable.

 

case closed, back to CO2

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