Question

What phase of `"CO"_2` exists at `-57^@ "C"` and `"2 atm"`?

Answer 1

This is just asking you to use your finger to trace where you are on the phase diagram. Think back to when you learned about graphs. Treat `T`, the temperature, as your `x` value, and `P`, the pressure, as your `y` value.

Now point to where `T = -57^@ "C"`, and `P = "2 atm"`. Did you point here?

So then, what single phase is here?

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For further practice with phase diagrams, see here.