Refrigerant property check: What is the freezing (melting) point of sulphur dioxide (SO₂) used in legacy refrigeration systems?

Introduction / Context:
Sulphur dioxide (SO₂) is an older inorganic refrigerant historically used before modern halo-carbons and ammonia dominated. Knowing key thermophysical points like freezing (melting) temperature is important for safe operation and to avoid blockages in expansion devices at low temperatures.

Given Data / Assumptions:

• Standard atmospheric pressure conditions when quoting freezing point.
• Pure SO₂ (no mixture effects).
• Freezing point refers to the solid–liquid equilibrium temperature.

Concept / Approach:
The freezing point is a fixed material property at 1 atm and is obtained from standard thermophysical tables. Below this temperature, SO₂ solidifies, which is hazardous for valves and capillary tubes in refrigeration circuits.

Step-by-Step Solution:

Verification / Alternative check:
Cross-check with chemical handbooks and refrigeration property charts listing SO₂ melt point near −75°C.

Why Other Options Are Wrong:

• −56.6°C and −77.7°C are not the standard melt points for SO₂ at 1 atm.
• −135.8°C is more characteristic of far colder substances; not SO₂.
• −10°C is close to SO₂ normal boiling point (about −10°C), not the freezing point.

Common Pitfalls:
Confusing boiling and freezing points; both are negative for SO₂ but differ by tens of degrees.

Final Answer:
-75.2°C