Question 2 of 10
Pressure (atm)
Moting
Boling
20
point
point
15
Liquid
Solid
Temperature (C)
Using the phase diagram for CO2, what phase is carbon dioxide in at -20°C
and 1 atm pressure?
A. It is in the gas phase.
B. It is in the solid phase.
C. It is at its boiling point.
D. It is in the liquid phase.



Answer :

To determine the phase of carbon dioxide (CO2) at -20°C and 1 atm pressure, we will refer to the phase diagram of CO2. Here's a step-by-step explanation of how to read the phase diagram and determine the phase:

1. Understand the Phase Diagram Sections:
- The phase diagram of CO2 typically shows temperature on the x-axis (in °C) and pressure on the y-axis (in atm).
- It is divided into three regions: solid, liquid, and gas.

2. Identify Key Points on the Diagram:
- Triple Point: The condition under which all three phases (solid, liquid, and gas) coexist. For CO2, the triple point is at approximately -56.6°C and 5.1 atm.
- Sublimation Point: The temperature at which CO2 transitions from solid to gas. At 1 atm, the sublimation point for CO2 is around -78.5°C.
- Critical Point: The highest temperature and pressure at which CO2 can exist as a liquid. Beyond this point, it must be a gas regardless of pressure.

3. Locate the Given Conditions:
- The given conditions are -20°C (temperature) and 1 atm (pressure).
- On a typical phase diagram, this point would be higher on the temperature axis than -78.5°C and to the right of 1 atm line, above the sublimation line.

4. Determine the Phase:
- At 1 atm pressure and -20°C temperature, we note that -20°C is warmer than -78.5°C.
- According to the phase diagram, when CO2 is at a temperature higher than -78.5°C and at 1 atm (below the critical pressure), it exists in the gas phase.

Hence, at -20°C and 1 atm pressure, carbon dioxide (CO2) is in the gas phase.

The correct answer is:
A. It is in the gas phase.