Refrigerants – Lowest normal boiling point at 1 atm Among ammonia (R-717), carbon dioxide (R-744), sulphur dioxide, and R-12, which has the lowest normal boiling point at atmospheric pressure?

Introduction / Context:
Normal boiling point (NBP) helps compare refrigerants’ volatility and typical evaporating pressures at atmospheric reference conditions.

Given Data / Assumptions:

• NBP is defined at 1 atm (101.3 kPa).
• CO₂ does not exist as a liquid at 1 atm; it sublimates at −78.5 °C.

Concept / Approach:
Typical NBPs: ammonia ≈ −33 °C; R-12 ≈ −29.8 °C; sulphur dioxide ≈ −10 °C. CO₂ lacks a normal boiling point at 1 atm (no liquid phase), so it is not comparable on this basis.

Step-by-Step Solution:
List NBPs for those with liquid at 1 atm.Ammonia has the lowest (most negative) NBP among such candidates.Therefore select ammonia.

Verification / Alternative check:
Handbook tables confirm the stated NBPs; the CO₂ exception is due to its triple point at 5.18 bar.

Why Other Options Are Wrong:

• R-12 and SO₂ have higher (warmer) boiling points than ammonia.
• CO₂ does not have a normal boiling point at 1 atm; it sublimates instead.

Common Pitfalls:
Confusing sublimation temperature of dry ice with a boiling point; overlooking the pressure dependence of phase transitions.

Final Answer:
Ammonia