[TheIce]

Hi,
I am not sure if its the right forum, i am confused with the amount of subjects u have here, so just move it to the correct forum if its not the appropriate location.

I had this thought in my head... it kills me trying to figure out what will happen in such case....

Lets pretend we have a sphere of steal, and in the middle, there is a core of 6CM in size, the distance between the core and the surface of the ball (Radius) is 20CM, and as u can see thats some VERY thick and firm sphere.

now lets pretend that we've put a piece of dry ice in the core (technical matters are not important), the dry ice is at size of 5CM, which leaves 1CM of space volume for gas (the carbon dioxide),

now... what happens when there is no more room for the gas to expand in? and of course the gas released from the dry ice piece takes up a lot more volume than volume available...
well, logic tolld me the ice will remain solid?(not sure if its true).

But lets pretend we heat up this steal sphere (metal can transform heat very well through 20CM) up to 300degC.
it should "force" the ice to turn into gas... but still there is no room for the gas to be contained in....

claiming the gas will break 20CM thick steal seems rather unlogical.... and even if it could, we can always pretend we use bigger sphere... its not the point.

What will happen then? the ice cant stay solid at -78 deg-C while the sphere is so hot...
but at the same time, there is no more room for the gas in the core...
what am i missing? there got to be some logical answer to it.
I dunno where i should look for an answer, so i just posted it here

*Also excuse my english, i am not a native speaker.

thanks in advancve
Avi Hillel

[mike]

I think you are missing pressure

[TheIce]

I think you are missing pressure

Sorry, i dont think i fully understand (I am not a chem guru),

Can u please give me a more detailed information?
For example in what way the pressure plays a factor in here?

And what actually happens to the ice and gas in this case,

Thanks for the fast reply and for your patience.

[constant thinker]

Check out this phase graph.

http://upload.wikimedia.org/wikipedia/commons/6/6a/Phase_changes_of_CO2.png

Maybe it will help you understand how pressure effects the state of a substance. This one relates directly to CO₂.

If you still don't understand, then just ask. This kind of graph actually helped me to understand pressure, and its effects on the state a substance is in.

[mike]

The ideal gas law is:

PV = nRT

(P)ressure
(V)olume
n = numebr of moles
R = gas constant
(T)emperature

So this shows you how the different values affect each other, in your case V is constant as is R and I guess n.

It is not really possible for a gas to take up more space than the volume it is in, this is in essence why you might see a pressure increase if you were measuring the pressure inside you sphere. The ideal gas equation shows us that for a certain number of gas molecules in a fixed volume if you increase the temperature you will also increase the pressure.

Now you can look at the phase diagram that constant posted and see at what temperature and pressure the CO₂ will be a solid, liquid or gas. You can not simply say that because you are heating it it will convert to gas because it is in a sealed vessel so as you are heating you are also increasing the pressure, which in turn means the compound is harder to turn into a gas (if you get what I mean). It is like a pressure cooker, it takes a higher temperature for water to boil in a pressure cooker.

[TheIce]

Thanks for this graph....

In regarding to my question, that means that as pressure builds the gas will be forced to turn into solid again?
err lets pretend the ice is heated and the pressure reaches 10,000atm
it would be at 60 deg-c and should be solid.

Now in a passive situation, where i dont heat it up... the dry ice's starting point is at -78 deg, and the pressure is 1atm.
by "default" it would transofrm into gas slowely, pressure will be build up.... and.... where does it "balance" and  stop?
as liquid on 0 deg-c?

[mike]

It will stop at the saturated vapour pressure, this is when the forward a backward phase change between the solid and vapour is equal. You can maybe estimate this from the graph constant posted.

[TheIce]

Thank u guys!

You really helped a lot :] everything is much more understandable now :] thanks!