Question

Describe all the phase changes that would occur and estimate the temperatures at which they would occur when CO2 is heated from -80 degrees celsius to -20 degrees celsius at a constant pressure of 3 atm; and then 6 atm

Answer 1

To understand better, you need the phase diagram of CO₂ which is the following:

Now let's see what happens at 3 atm. In this case, you are almost midway between 1 atm and 5.11 atm in the graph.

If you follow a line from 3 atm, you'll see that we are in the solid phase, until it touches the diagram, you'll see that there's a first change of phase, that would be between the solid and gas phase. This means that it's experimenting a Sublimation phase (solid to gas), and this occurs at a temperature near -56.7 °C (Maybe about -61 °C). If you keep increasing temperature til -20 °C it will still be in the gas phase.

Now if you increase pressure up to 6 atm, things change, according to the diagram, if you follow a straigh line from 6 atm, the changes of face will be from solid to liquid at a temperature a little higher from triple point temperature (around -55 °C or -53 °C). This phase change will be melting (or fusion). Increasing temperature up to -20 °C, yo'll see another change of phase, this time from liquid to gas at a aproximate temperature of -48°C or -47 °C. This will be boiling.

Hope this helps