Physics/chem question

1. Sep 20, 2009

microfracture

bare with me here, this is kind of a physics/chem question.

if you had a chunk of dry ice (solid co2) at one atm and heat it rapidly enough so it expanded, how hard of a "push" would it make? D:

2. Sep 20, 2009

Staff: Mentor

Push? What push? You mean how much pressure would it generate if enclosed?

Dry ice is at its sublimation point. It won't expand if you apply heat to it, it'll just sublimate.

3. Sep 20, 2009

Bob S

Hi microfracture-
I think the vapor pressure of CO2 is about 70 atm at 30 degrees C. Above this temperature, the CO2 becomes a supercritical fluid.
See
http://encyclopedia.airliquide.com/images_encyclopedie/VaporPressureGraph/Carbon_dioxide_Vapor_Pressure.GIF [Broken]
Bob S

Last edited by a moderator: May 4, 2017
4. Sep 20, 2009

microfracture

let me refine the question while letting you know i have the following in mind: if you put dry ice in hot water it bubbles more violently then in cold water.

so the question is: if you could apply heat to the entire solid simultaneously therefore making it expand. would it be enough force to say knock over your buddy

5. Sep 20, 2009

Bob S

hello micro-
If you put a pound of dry ice (about 10 mols) in a 1-liter bottle and heat it up, it will reach a pressure of perhaps 30 atm (1 mol = 22.41 liters at STP). If the bottle is not rated for that pressure, it will explode. I do not think there will be sufficient force unless the expanding CO2 gas is contained.
 As I write this, I am sitting 10 feet from a roughly 10-liter bottle containing 5 pounds of liquid/gas CO2 (fire extinguisher).
Bob S

Last edited: Sep 20, 2009
6. Sep 20, 2009

Staff: Mentor

Yes, if you apply heat faster, it will sublimate (turn from solid directly to a gas) faster.
Well like I said, it doesn't expand when heat is applied.

7. Sep 28, 2009

microfracture

so it just goes "poof"?

then please explain why co2, when it sublimates, in a closed container creates pressure.

8. Sep 28, 2009

Staff: Mentor

pV=nRT

9. Sep 28, 2009

redargon

Russ is talking about the fact that if you heat the dry ice just enough to get it to sublimate that the pressure will remain constant. If, however, you continue to heat it, then you would increase the pressure up until the critical tmeperature, by approximately pV=nRT.

You might be able to knock down your buddy, by melting enough dry ice to fill a room with enough CO2 to render your "buddy" unconcious. Otherwise you'll need some sort of driver to convert the pressure into a force ie. a piston in a cylinder or something along those lines.

10. Sep 28, 2009

redargon

if you keep the volume constant, ie. a closed bottle. (forgot to menton that... oops)

11. Sep 28, 2009

Bob S

When you put dry ice in hot water, a layer of gaseous CO2 separates and insulates the solid CO2 from the hot water, limiting the heat transfer rate. Thus the rate of sublimation is limited. The expansion rate will be fast, but not instantaneous.
Bob S

12. Nov 13, 2009

microfracture

what if you use light, ie a laser to heat it. therefore having no physical heat sink to make the process slow.

13. Nov 16, 2009

redargon

No, not unless you threw a block of the ice at him or let it melt in a big enough closed container to invoke an explosion from the pressure build up as it melts. Sticking a piece of dry ice in front of your buddy and heating up, even with lasers attached to sharks heads (or wild sea bass), is not going to knock him over.