# Why does liquid co2 goes back to solid at atmospheric press

1. May 19, 2012

### vlajky

I you look at the phase diagram, co2 is in liquid state at 5,11 atm and t= -56 C. But why does he convert directly to solid state at 1 atm? Again, if you go down from five to one atm, you should be in gaseous state ? Is it have something to do with temperature ? What happens with temperature during the pressure drop?

I am specifically asking about liquid to solid transition that occur in the experiment explained below and why it happens during the pressure drop. On phase diagram if pressure drop from 5 to 1 atm at t= -56,6 c you are in the gaseous state? It must be the temperature drop all so ? If it is, why it happens ?

experiment :
Dry ice is placed into an PET plastic cylinder. A valve is closed and pressure in the cylinder increases. When the pressure reaches 511 kPa it stops increasing and liquid CO2 appears. The liquid begins to boil and when all solid CO2 is gone, the pressure increases further. The valve is opened and the pressure drops again to 1 atm. Eventually solid CO2 reforms, the liquid disappears? WHY THE SOLID STATE REFORMS AGAIN ( from liquid ) RATHER THEN GASEOUS ?

2. May 19, 2012

### mathman

Without knowing the details I can hazard a guess. With the pressure drop some of the CO2 evaporates cooling the rest which becomes solid.

3. May 20, 2012

### vlajky

I think you're right. That's the only reasonable explanation. Thank you!:)