Carbon dioxide property check: The commonly cited temperature −78.3°C for CO₂ at 1 atm corresponds to which “boiling” or phase-change point?

Introduction / Context:Unlike many refrigerants, carbon dioxide (R-744) has a triple point above atmospheric pressure. Therefore, at 1 atm CO₂ does not “boil”; it sublimates, transitioning directly between solid and gas.

Given Data / Assumptions:

• Pressure is 1 atm.
• Triple point of CO₂ is at about 5.18 bar; critical point is around 31.1°C and 73.8 bar.
• Given temperature is −78.3°C (often quoted as −78.5°C).

Concept / Approach:Below the triple point pressure, liquid CO₂ cannot exist. Thus, at 1 atm the phase change occurs directly between solid and vapour at the sublimation temperature, approximately −78.3°C.

Step-by-Step Solution:

Verification / Alternative check:Phase diagrams of CO₂ show 1 atm lying left of the triple point, confirming no liquid phase is possible and the change is solid–gas.

Why Other Options Are Wrong:Other temperatures listed do not match standard constants for CO₂ at 1 atm; −63.3°C is closer to oxygen’s boiling point, not CO₂.

Common Pitfalls:Using the term “boiling point” for substances that sublimate at 1 atm; always check the triple point.

Final Answer:−78.3°C (sublimation point at 1 atm)