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Offline sohail

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Dry Ice
« on: 26/09/2007 21:09:50 »
Recently I came across a picture of dry ice being poured out of a test tube. Does anyone know how this "dry ice" is created? and why the water in its gas state looks completely different (like a really heavy white mist)?


 

Offline DrDick

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Dry Ice
« Reply #1 on: 27/09/2007 16:47:24 »
From http://www.dryiceinfo.com/science.htm

Quote
The first step in making dry ice is to compress carbon dioxide gas until it liquefies, at the same time removing the excess heat. The C02 gas will liquefy at a pressure of approximately 870 pounds per square inch at room temperature. Next, the pressure is reduced over the liquid carbon dioxide by sending it through an expansion valve into an empty chamber. The liquid will flash, with some turning into gas causing the remainder to cool. As the temperature drops to -109.3F, the temperature of frozen CO2, some of it will freeze into snow. This dry ice snow is then compressed together under a large press to form blocks or extruded into various sized pellets.

The white mist you see is not water in the gas state (which you can't see).  It's actually water in the liquid or solid state.  The air around the dry ice is quite cold.  When gaseous water approaches the dry ice, it condenses into very small droplets of water (which may then freeze).  These droplets are small enough that they will remain suspended as a cloud rather than rain out (just like the clouds in the sky).

Dick
 

Offline techmind

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Dry Ice
« Reply #2 on: 02/10/2007 10:37:47 »
Recently I came across a picture of dry ice being poured out of a test tube. Does anyone know how this "dry ice" is created? and why the water in its gas state looks completely different (like a really heavy white mist)?
How dry ice is made has already been answered. I'll try to address the remainder of your question:

I'm not sure whether you're actually thinking of liquid nitrogen or dry ice? Both are very cold (nitrogen is 77Kelvin, -196Celcius, dry ice is -78Celcius). "Dry ice" is solidified carbon-dioxide is typically purchased as a solid block or small pellets (a similar size to the pencil-erasers you put on the end of a pencil). At atmospheric pressure, CO2 goes directly from a solid to a gas (there is no liquid state) - it is said to "sublime".

You can get a white mist above either CO2 or liquid nitrogen - but this is really just an atmospheric mist because the water in the nearby air has condensed out due to the cold (just like on a cold autumn morning). Nitrogen usually gives a much thicker fog. Because the "fog" is mostly cold air/nitrogen/CO2 gas, it's more dense than the surrounding air, so it sinks and can be "poured".

In the case of liquid nitrogen, you often don't see the liquid nitrogen itself for all the fog. When you do get to see it, the nitrogen itself is a clear liquid that looks much like water - but it has a lower surface tension, and it's trying to bubble and boil the whole time!
« Last Edit: 02/10/2007 10:41:54 by techmind »
 

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Dry Ice
« Reply #2 on: 02/10/2007 10:37:47 »

 

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