Difficulty: Easy
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:
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)
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