Chemistry Question #1704

Michael Doggett, a 41 year old male from Kingston, ONTARIO, CANADA asks on November 24, 2003,

We have an older pop machine at work with canned soft drinks. Sometimes, when a pop is opened that comes right from the machine, the contents slightly freeze and the pop overflows from the can. It appears that the pop is not frozen in the can. Why does the contents freeze when opened? We talked about it in the lunch room and I suggested that the pop was very cold and when opened, the carbon dioxide bubbles carried any heat to the surface of the liquid thus allowing the temperature to fall below zero and create some freezing of the liquid. Someone else offered that PV=nRT must be respected and pressure is changed when the can is opened which impacted the temperature of the liquid. Are we both on track? Can you please explain this currious situation? I will post your response close to the pop machine.

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The answer

John Jones answered on December 8, 2003

The problem with Michael's explanation is that there's no obvious reason for the carbon dioxide bubbles to be any warmer than the rest of the liquid, so why would losing the bubbles take heat away?

Michael's friend's explanation, is a bit closer, but the effect of opening the can on the temperature of the gas is not a consequence of pV=nRT, which only applies to ideal gases, but of the departure of the carbon dioxide from ideal behaviour. Specifically, a real gas will be cooled when it is allowed to expand through a valve to a lower pressure in proportion to the Joule-Thompson coefficient for that gas. Carbon dioxide has a relatively high Joule-Thompson coefficient, so it gets cooled quite a bit. The fact that the carbon dioxide is dispersed through the fluid in little bubbles will mean that the liquid is also cooled.

My own explanation is as follows: solutes tend to lower the freezing point of liquids. When we open the can, carbon dioxide comes out of solution, so, the freezing point of the liquid goes up. So, even if its temperature remains the same, some of the liquid now turns to ice (releasing enough heat as it does so to prevent the rest of the liquid freezing.)

So there probably is a fall in temperature of the can, due to the cooling effect of expanding gas, but ice would form even if this was not a factor, due to CO2 coming out of solution. To judge the relative importance of these two effects, we would either have to do some calculations, or attach a thermocouple to the can as we opened it.

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