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During a series of comments here, it was suggested that taking dry ice into an airplane, the airplane gets lighter as the ice sublimes.

This was supported by the idea that when the dry-ice sublimes and CO2 is released in form of gas the pressure inside the plane rises, and as the airplane keeps the pressure at a certain level the 'extra pressure' would be released, releasing 'extra air' and thus making the airplane lighter.

I am not convinced with this idea, why would the sublimed CO2 gas rise the pressure if at the same time there is more volume available in the airplane? (the volume left behind by the dry ice which sublimed) My intuition tells me that the new volume available makes the pressure drop and the new gas makes the pressure rise, and that both drop and rise are equal.

Now, apart from some particularities the aircraft systems has with pressure and air, the more general question would be:

There is a box which you can fully close and no air escapes nor enters, you open the box and put inside dry-ice and close the door. The pressure inside the box would be equals to the room pressure, isn't it? Now, as time passes the dry ice fully sublimes, what is the pressure inside the box now?

Thanks.

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  • $\begingroup$ This is similar to the question if a vehicle with birds in it is lighter when the birds are flying as opposed to if they're sitting. $\endgroup$ – BillDOe Feb 5 '18 at 21:29
  • $\begingroup$ Note that the pressure is constantly regulated $\endgroup$ – QuIcKmAtHs Feb 5 '18 at 21:49
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    $\begingroup$ The same mass of CO2 takes up much less space as dry ice than it does as a gas. $\endgroup$ – zeta-band Feb 5 '18 at 21:56
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    $\begingroup$ To answer your final question: imagine putting the dry ice inside a box with nothing but vacuum. The starting pressure inside the box is 0. After the dry ice sublimates, there is gas in the box and the pressure is now non-zero. So, you can see that sublimation can indeed increase the pressure. Solid dry ice is much denser than gaseous carbon dioxide, meaning the same mass takes up different volumes depending on the state. $\endgroup$ – Nuclear Hoagie Feb 5 '18 at 21:57
  • $\begingroup$ @NuclearWang I like that $\endgroup$ – Nico D Feb 5 '18 at 22:05
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  1. Whether $N_2$ or $O_2$ or $CO_2$ is displaced from the aircraft when the pressure is reduced is not important. The $CO_2$ adds molecules into the air when it sublimes, this increases the pressure of the air in the aircraft, but some of this mixture of gases is ejected to reduce the pressure. So the mass of the aircraft is reduced - very slightly.

  2. Gasous $CO_2$ occupies about 1000 times the volume of the same mass of solid $CO_2$. So the extra volume available when the block of dry ice has sublimated has a negligible effect in reducing the pressure of the gases in the aircraft.

  3. The dry ice in the sealed box does not completely sublime. At a given temperature it sublimes only until the gasous $CO_2$ reaches its saturated vapour pressure (SVP) at that temperature. The SVP is almost independent of the air pressure already inside the box, so the pressure in the box increases.

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