Carbon dioxide property check: The commonly cited temperature −78.3°C for CO₂ at 1 atm corresponds to which “boiling” or phase-change point?
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A−20.5°C
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B−50°C
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C−63.3°C
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D−78.3°C (sublimation point at 1 atm)
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E−33.3°C
Answer
Correct Answer: −78.3°C (sublimation point at 1 atm)
Explanation
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:
Recognize that “boiling point” at 1 atm is not applicable to CO₂.Identify −78.3°C as the sublimation temperature at 1 atm.Relate to dry ice behavior: it “smokes” due to sublimation, not melting then boiling at 1 atm.Conclude that the correct interpretation is the sublimation point.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)