Gas Law question

[NeuroNP]

OK, this isn't directly an Anesthesia question, but it has to do with the gas laws, so it's at least tangentially related.

The other day a friend of mine had a bottle of soda that she took out of the fridge and opened. Upon opening it, the top layer "flash froze" and became slush. It was totally liquid before, it only became slushy AFTER opening. She was asking around at the hospital to see if any of the docs could explain this and no one could. Finally, one of the other nurses said that he knew why. This was his explaination.

"PV=nRT. When you open the lid, you drastically reduce the pressure in the system. All other factors are equal (V,n, and R stay the same), the only thing that can change is T. Since P decreased rapidly, T does the same and since it was cold enough allready, the liquid partially froze and turned to slush."

At first I thought that was a good explaination, but then I began thinking, when you open the lid, doesn't the volume technically get rapidly larger? Wouldn't that offset the P decrease? Or do I not understand the Ideal Gas law?

bryan

[BAndersonRN]

Hey Bryan...

saw your post and thought about it for a bit (thinking back to good ol chemistry), gonna take a stab at trying to answer it. PV=nRT... the pressure inside the bottle rapidly decreased and therefore so did the temperature is a true statement.. P is equal to the amount of pressure being exerted on the gas (CO2 in this case). V is equal to the volume of the container that the gas occupies, n of course is the number of moles of the gas, R= constant, and of course T=temp. The Volume in this case hasn't changed, that is to say the volume of the soda bottle has remained the same. Since the pressure has decreased (volume stays the same), that would mean that the Temp. would have to decrease on the otherside of the equation. So, when a gas rapidly expands due to a decrease in pressure it also cools rapidly...which would explain why the soda became slushy when the top was opened.:) Hope I explained it right.. hehehe..

Take care

Brian

[NeuroNP]

That's what I was thinking, but I wasn't sure if you considered that because the system is now open to air, the volume was also increasing. (It's been awile since Chem!)

bryan

[sandman1]

These posts are correct. This is called the Joule-Thompson effect. Related to Gay-Lussacs ideal gas law. Look it up on the net and you will find all the info you need.

[BAndersonRN]

Good point.. but, we're just looking at the bottle of soda. If you were to look at the volume expanding by taking into account open air outside the bottle, then everything in the equation would change. You would have to take into account all of the partial pressures of gas in the atmosphere, the individual gasses, new temp, new volume, etc... The only system thats important in this case is the bottle itself...too many variables outside the bottle for me to even attempt to try to explain hehehe...

[NeuroNP]

Good point.. but, we're just looking at the bottle of soda. If you were to look at the volume expanding by taking into account open air outside the bottle, then everything in the equation would change. You would have to take into account all of the partial pressures of gas in the atmosphere, the individual gasses, new temp, new volume, etc... The only system thats important in this case is the bottle itself...too many variables outside the bottle for me to even attempt to try to explain hehehe...

Yeah. that makes sense

[sandman1]

This is the same law that explains a gas cannister becoming cold when you release the pressure out of it too quickly...........

[athomas91]

it is the "latent heat of evaporation" ...

[mwbeah]

Latent heat of vaporization (Lv): Refers to the heat lost by the air when liquid water (or other liquid) changes into vapor. This is also commonly known as the latent heat of evaporation. Lv= -2500 Joules per gram (J/g) of water or -600 calories per gram (cal/g) of water.