# 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. Commented May 29, 2015 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. Commented May 29, 2015 at 12:15
• please avoid duplicate questions on SE sites Commented May 29, 2015 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. Commented May 29, 2015 at 22:24
• I'm voting to close this question as off-topic because it was cross-posted and answered on Earth Science. Commented May 29, 2015 at 22:26

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.