# At what temperature CO2 becomes solid at normal pressure? [closed]

I have met contradicting evidence. The phase diagrams show that $\ce{CO2}$ solidifies at $-78.5\ ^{\circ} \mathrm{C}$:

Wikipedia confirms this

At 1 atmosphere (near mean sea level pressure), the gas deposits directly to a solid at temperatures below $−78.5\ \mathrm{^\circ C}$ ($−109.3\ \mathrm{^\circ F}; 194.7 ~\mathrm{K}$) and the solid sublimes directly to a gas above $−78.5\ \mathrm{^\circ C}$.

Yet this article claims it happens at $133\ \mathrm{K}$ (otherwise $\ce{CO2}$ would showfall at Earth's poles).

Which claim is correct?

• I'm interested. For as far as I can remember, -78 degrees Celsius is (or was) the only temperature mentioned. Now we have to prove the second article wrong. – It's Over May 29 '15 at 12:12
• @MARamezani the lowest recorded temperature on Earth is -90 C (Vostok station). But I never heard about any CO2 snowfalls on Earth. I think if the sublimation temperature was indeed -78.5 C the CO2 would start depositing on Earth's poles. – Anixx May 29 '15 at 12:15
• please avoid duplicate questions on SE sites – bummi May 29 '15 at 12:42
• Please do not cross-post. You've been around long enough on the network to know that. If you had catered/varied the question for each site, it would have been okay. – jonsca May 29 '15 at 22:24
• I'm voting to close this question as off-topic because it was cross-posted and answered on Earth Science. – jonsca May 29 '15 at 22:26

## 1 Answer

The phase diagram you have shown is correct and shows the sublimation point at $1~\mathrm{atm}$ as $-78.5~\mathrm{^\circ C}$ . So if you have a pure $\ce{CO2}$ atmosphere, it will solidify at $-78.5~\mathrm{^\circ C}$.

Temperatues in Antarctica are known to get below $-78.5~\mathrm{^\circ C}$ so you might wonder why whe don't find dry ice in Antartica (like we do at the Martian poles [Note: they are a lot colder]). The reason is that the partial pressure of $\ce{CO2}$ in the atmosphere is so low that at $-78.5~\mathrm{^\circ C}$, dry ice is forming but it is subliming faster than it forms because the vapor pressure of $\ce{CO2}$ in the atmosphere above it is well below the saturation vapor pressure required for ice to accumulate.

The vapor pressure must reach the saturation vapor pressure for dew or frost to form. This happens at the dew point or frost point temperature, which is dependent on atmospheric pressure and the absolute amount of vapor in the air. As atmospheric temperature increases, the dew/frost point temperature increases. As atmospheric pressure increases, the vapor pressure increases. At very low temperatures, the dew/frost point temperature is very low.

http://wattsupwiththat.com/2009/06/13/results-lab-experiment-regarding-co2-snow-in-antarctica-at-113%C2%B0f-80-5%C2%B0c-not-possible/

https://stevengoddard.wordpress.com/2014/06/03/antarctica-gets-cold-enough-to-freeze-co2/

• Thank you. I think it is better to stress that this is all about partial pressure that is on the diagram. And after all, the article is correct, one needs -140 C for a CO2 snowfall. – Anixx May 29 '15 at 13:05