Refrigerant property of ammonia: The __________ point of ammonia (NH₃) at standard pressure is −77.7°C.

Introduction / Context:
Ammonia (R-717) is a widely used refrigerant in industrial systems. Knowing its key thermophysical properties—boiling, freezing, and critical points—is essential for safe and reliable design.

Given Data / Assumptions:

• Standard atmospheric pressure conditions.
• Ammonia’s normal boiling point is approximately −33.3°C.
• The stated temperature is −77.7°C.

Concept / Approach:
Compare the given temperature with known characteristic temperatures. Because −77.7°C is far below the normal boiling point, it aligns with the freezing (melting) point of ammonia, the temperature at which liquid ammonia solidifies at 1 atm.

Step-by-Step Solution:

Verification / Alternative check:
Consult standard property tables for R-717 to confirm the numerical values; industrial design texts list these constants consistently.

Why Other Options Are Wrong:

• Boiling point is −33.3°C at 1 atm, not −77.7°C.
• Critical point temperature is above 100°C, not negative.
• Dew point refers to saturation temperature at a given pressure for vapour mixtures, not a fixed constant for a pure substance at 1 atm.

Common Pitfalls:
Confusing boiling and freezing points for substances with negative values; always cross-reference known constants.

Final Answer:
freezing (melting)