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.
