Identify the refrigerant: Which common refrigerant has a normal boiling point closest to −10.5°C at approximately 1 bar?

Introduction / Context:
Each refrigerant has a characteristic saturation temperature at a given pressure. Knowing approximate normal boiling points helps with quick identification tasks and sanity checks in design or troubleshooting.

Given Data / Assumptions:

• Normal boiling point refers to saturation temperature at about 1 bar.
• Target value is approximately −10.5°C.
• We compare common historical refrigerants.

Concept / Approach:
Ammonia (R-717) boils near −33°C at 1 bar; R-12 near −29.8°C; carbon dioxide does not boil at 1 bar—it sublimates at −78.5°C; R-134a boils near −26.3°C. Sulphur dioxide has a normal boiling point close to −10°C (commonly quoted around −10.0 to −10.5°C), matching the query most closely.

Step-by-Step Solution:

Verification / Alternative check:
Consulting standard refrigerant property tables will confirm these typical values across reputable handbooks.

Why Other Options Are Wrong:

Ammonia/R-12/R-134a: all have significantly lower normal boiling points than −10.5°C.Carbon dioxide: does not have a liquid phase at 1 bar; it sublimates, so “boiling point” is inapplicable at 1 bar.

Common Pitfalls:
Assuming all older refrigerants boil near −30°C; SO2 is notably warmer at ~−10°C and was historically used in early systems despite toxicity concerns.

Final Answer:

Sulphur dioxide (SO2)