[smghz]

I have two quick question about dry ice.

My book says, "It has an interesting property: at normal pressures it passes directly to the gaseous state without first melting to the liquid. This property, together with the fact that this change occurs at -78C, makes solid carbon dioxide useful for keeping materials very cold."

So, if it sublimes at such a low temperature, what's the use, then? Would not the solid just be a gas? How would that help in the freezing? I get the part about the liquid; we don't want liquid touching our stuff. But what about the sublimation thing?

The authors also said this: "When liquid carbon dioxide evaporates (changes to vapor), it absorbs large quantities of heat, cooling as low as -57C."
How in the world does that make sense? How can absorption of heat lead to...cooling???

thanks.

[AWK]

Sublimation is an endothermic phase transition.

[smghz]

Sublimation is an endothermic phase transition.

thanks for your response. its still not helping though! I don't get why dry ice is helpful when it sublimates and thus becomes gas (not solid anymore) at such low temp. And how is it that evaporated liquid CO2 actually cool by absorbing heat. Not so clear or elucidated!

[Arkcon]

thanks for your response. its still not helping though! I don't get why dry ice is helpful when it sublimates and thus becomes gas (not solid anymore) at such low temp. And how is it that evaporated liquid CO2 actually cool by absorbing heat. Not so clear or elucidated!

Question isn't clear.

Say you have water at 35 C.  You add water ice, which starts to melt.  Ok, soon you have a little more water, but eventually, until all the ice melts, you have some water at 0 C.

Say you have some methanol at 35 C.  You add dry ice, it sublimes, bubbling like crazy.  Before it all disappears, you have some methanol at -77 C.

Except for the numbers, what's the difference?

[smghz]

Okay!

So I believe what is happening is that, on one hand, water has a 0C melting temperature, so that if you use ice to cool stuff, it'd cool it to 0C by the time it evaporates. And even when it evaporates the air around it would be cooled likewise.

Not so with dry ice. Dry ice has both the property of sublimation and of doing so at such a low temperature: -78C. So that, when it is used to refrigerate items, by the time the dry ice sublimes, the items cool down to such extremely low comparable temperatures, a condition sustained by the corresponding now-sublimated CO2 which would also cool down the air and create a kind of apparatus.

Am I right? I think it just clicked in me! Thank you very much!

[Enthalpy]

Looks good. "Cooling" is what happens to the other materials when dry ice sublimes. The surroundings provides the sublimation heat to dry ice. -57°C is just nonsense to me: depending on he conditions, you nearly reach the sublimation temperature.

[insertwittyname]

to the OP: If I understand your doubt correctly, your problem is how does dry ice ABSORBING heat cool stuff, right?
Basically, the dry ice takes the heat FROM the surroundings and thus cooling stuff down. It just takes the heat away from whatever you want to cool. I hope that helps

[P]

The authors also said this: "When liquid carbon dioxide evaporates (changes to vapor), it absorbs large quantities of heat, cooling as low as -57C."
How in the world does that make sense? How can absorption of heat lead to...cooling???

thanks.

As the last 2 posts suggested, this cooling is of the surroundings (what you are trying to keep cool). As the CO2 absorbs heat and sublimes into a gas and leaves, it take the heat with it leaving what is behind and the surroundings cooler.

This is why we sweat for example. The sweat turns to gas as it evaporates taking energy with it from it's surroundings (the skin) and cools it. The sweat itself that has evaporated is warmer, but has left the area.

Any clearer?

[snorkack]

Looks good. "Cooling" is what happens to the other materials when dry ice sublimes. The surroundings provides the sublimation heat to dry ice. -57°C is just nonsense to me: depending on he conditions, you nearly reach the sublimation temperature.

-57°C is not a pure nonsense - it is the freezing point of carbon dioxide.
So, if you have liquid carbon dioxide and release pressure on it, it cools. At pressure of slightly over 5000 mbar, it reaches temperature -57 degrees, at which point it freezes.
The solid continues to sublime. At pressure of 1000 mbar, it reaches temperature -78 degrees.
But note that the partial pressure of carbon dioxide is not 1000 mbar! It is 0,4 mbar - used to be 0,3.
The vapour pressure of carbon dioxide reaches 0,4 mbar at around -140 degrees.

Does dry ice remain at -78 degrees where its vapour pressure 1000 mbar is far in excess of its partial pressure, or does it continue cooling until it reaches its partial pressure, at -140 degrees?

[Enthalpy]

-57°C is not a pure nonsense - it is the freezing point of carbon dioxide.
So, if you have liquid carbon dioxide and release pressure on it, it cools. At pressure of slightly over 5000 mbar, it reaches temperature -57 degrees, at which point it freezes.
The solid continues to sublime. At pressure of 1000 mbar, it reaches temperature -78 degrees.
But note that the partial pressure of carbon dioxide is not 1000 mbar! It is 0,4 mbar - used to be 0,3.
The vapour pressure of carbon dioxide reaches 0,4 mbar at around -140 degrees.

Does dry ice remain at -78 degrees where its vapour pressure 1000 mbar is far in excess of its partial pressure, or does it continue cooling until it reaches its partial pressure, at -140 degrees?

The freezing point is irrelevant because the liquid doesn't exist at atmospheric pressure.