To determine the phase of carbon dioxide (CO₂) at a specific temperature and pressure, such as -70°C and 1 atm, we refer to its phase diagram. A phase diagram displays the state of matter (solid, liquid, or gas) based on temperature and pressure.
1. Understanding Temperature and Pressure Conditions:
- The given temperature is -70°C.
- The given pressure is 1 atm.
2. Interpreting the Phase Diagram:
- On a phase diagram, the boundaries between phases (solid, liquid, and gas) are marked by lines called phase boundaries or phase transition lines.
- The diagram typically includes regions labeled as solid, liquid, or gas, indicating the conditions under which CO₂ exists in those states.
- The triple point of CO₂ occurs at -56.6°C and 5.1 atm; below this pressure and at temperatures below the sublimation point at 1 atm (-78.5°C), CO₂ exists as a solid.
3. Locating the Given Conditions on the Phase Diagram:
- At -70°C and 1 atm, the temperature is lower than the triple point temperature (-56.6°C), and the pressure is significantly higher than the sublimation point pressure of CO₂ at 1 atm (-78.5°C).
- These conditions fall within the region where CO₂ exists as a solid.
4. Conclusion:
- Under the specified conditions of -70°C and 1 atm, CO₂ is found in the solid phase.
Therefore, the correct answer is:
D. It is in the solid phase.
