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I thought the answer was obvious, but endless google searching has revealed nothing but speculation and people who have completely conflicting opinions. Some people say $\ce{CO2}$ absorbs no heat, others say it can cool things extremely rapidly. Anyone who has ever had compressed $\ce{CO2}$ sprayed on themselves or on hands knows it can cause very rapid drop in temperature on what ever it hits very fast.

So, if I have a really hot mass of liquid, would spraying compressed $\ce{CO2}$ on it be an effective means of freezing/solidying the mass? - Assuming the liquid is well above room temperature (100+) and turns to a solid after a 20-30 degree drop in temperature?

I understand why the $\ce{CO2}$ is cold coming out of the tank, as it is decompressed, but is it effective at absorbing heat?

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Celebrity chef Alton Brown produces ice cream using a device called the Jet Cream. He sprays $\ce{CO2}$ from a fire extinguisher at a stream of chocolate milk, instantly producing ice cream. The only difference in your scenario and his is the starting temperature of the liquid.

To say that $\ce{CO2}$ absorbs no heat is absurd. If it is colder than what it touches, it should absorb heat.

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Yes, blowing $\ce{CO2}$ over a hot object would cool it. The second law of thermodynamics says that heat cannot move from a colder material to a hotter material. So if you had a hot object and applied any material cooler than it, the hot object would cool down. The $\ce{CO2}$ offers some advantages because it's a gas. As the gas heats, it expands. By the ideal gas law, any expanding gas must cool down, assuming a constant pressure. However, this effect should apply to any gas, and compressed air would be cheaper than pure $\ce{CO2}$. You might also apply dry ice. This could be considered really compressed $\ce{CO2}$. The dry ice offers a further advantage because of the phase transition from solid to gas, not to mention its much colder initial temperature. As the dry ice sublimates its temperature remains constant throughout the phase transition, allowing to draw much more energy out of the hot object. However, water would also offer this advantage, as it has a high heat capacity, and is much cheaper than dry ice or compressed $\ce{CO2}$. It should also be safer. Suddenly heating a block of dry ice in an enclosed space could be explosive. Even if it doesn't explode, the large amount of $\ce{CO2}$ released could displace the air from the room and choke anyone in the room.

tl;dr, use compressed air or liquid water.

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  • $\begingroup$ Yes, but carbon dioxide will be colder than compressed air or water so might do a better job and, since it is a gas, won't leave as much mess as water nor will it affect the warm material as badly. Air won't work as well as compressed air is a gas but carbon dioxide is a liquid and will be colder when emitted. $\endgroup$ – matt_black May 30 '17 at 0:08

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