Among the refrigerants listed (ammonia, carbon dioxide, sulphur dioxide, and R-12), which one has the highest freezing point (that is, the least negative temperature at which it solidifies)?

Introduction / Context:
Refrigerant selection considers many properties, including freezing point (solidification temperature). A higher freezing point (closer to 0°C) indicates the substance solidifies more easily and may limit low-temperature applications. This question checks knowledge of comparative freezing points for common historic/industrial refrigerants.

Given Data / Assumptions:

• We compare typical reference freezing points: CO2 has a triple point at about -56.6°C, ammonia freezes near -77.7°C, sulphur dioxide near -75.5°C, and R-12 near about -157°C.
• “Highest freezing point” means the largest (least negative) temperature value.

Concept / Approach:
Rank the freezing points numerically. The substance that solidifies at the warmest temperature has the highest freezing point. Because CO2’s triple point is around -56.6°C, it lies significantly above the freezing points of ammonia and sulphur dioxide, and far above that of R-12.

Step-by-Step Solution:

Verification / Alternative check:
CO2’s phase behavior is widely known in cryogenics and refrigeration: below its triple point temperature, it exists as solid (dry ice) and gas; liquid CO2 requires pressure above the triple point. This characteristic aligns with its comparatively higher freezing point.

Why Other Options Are Wrong:

• Ammonia and sulphur dioxide: Both freeze at lower temperatures than CO2.
• R-12: Very low freezing point compared to CO2, hence not the highest.
• Propane: Not in the original list and has a much lower freezing point than CO2.

Common Pitfalls:
Confusing boiling point with freezing point, or overlooking the special triple point of CO2 that constrains its liquid region at atmospheric pressure.

Final Answer:
Carbon dioxide