Thermodynamic fixed points: Which of the following values is NOT a representation of water’s triple point on a temperature scale?

Introduction / Context:
The triple point of water is a unique thermodynamic state where ice, liquid water, and water vapor coexist in equilibrium. It defines a precise temperature on absolute scales: 273.16 K or 491.69 °R. Recognizing correct and incorrect representations across scales prevents confusion with ordinary freezing points or rounded values.

Given Data / Assumptions:

• Exact triple-point temperature: 273.16 K.
• Equivalent Rankine value: 491.69 °R (≈ 492 °R when rounded).
• 32 °F is the standard freezing point at 1 atm, not the triple point.

Concept / Approach:

On absolute temperature scales, the triple point is fixed by definition (historically used to define the Kelvin). Conversions: °R = 1.8 × K. Hence 273.16 K × 1.8 ≈ 491.69 °R. Some engineering texts round to 273 K or 492 °R for quick estimates, but these refer to the same physical state. In contrast, 32 °F is not the triple point; it is the freezing/melting point of water at standard atmospheric pressure, a different condition.

Step-by-Step Solution:

Verification / Alternative check:

Thermodynamic tables distinguish the triple point and 1 atm freezing point, with small but real differences. Instruments like triple-point cells are used for calibration at 273.16 K, not at 32 °F.

Why Other Options Are Wrong:

273 K / 492 °R: Though rounded, they represent the triple-point temperature on absolute scales approximately. 32 °R: (if considered) is far from the triple point; however, the explicit incorrect choice among those listed as common representations is 32 °F.

Common Pitfalls:

Equating freezing point at 1 atm with triple point; ignoring that absolute scales provide consistent fixed points for calibration.

Final Answer:

32 °F (Fahrenheit)