Thermodynamic Celsius scale – definition and interval Which statement correctly describes the thermodynamic Celsius scale used for practical temperature measurement?

Introduction / Context:
Although the modern SI definition uses the kelvin linked to the Boltzmann constant, the practical Celsius scale for teaching and many instruments still references the traditional fixed points: the ice point and the steam point under specified pressure. Understanding the wording matters for exam accuracy.

Given Data / Assumptions:

• Ice point: equilibrium of ice–water at 1 atm.
• Steam point: equilibrium of water–steam at 1 atm (condensation/boiling point correspondence).
• Thermodynamic link to kelvin preserves 100 equal divisions between these points.

Concept / Approach:
The classical Celsius scale assigns 0 °C at the ice point and 100 °C at the steam point, establishing an interval of 100 degrees. The appropriate wording is “condensation (steam) temperature” for the upper fixed point. Therefore, statements (b) and (c) together correctly describe the traditional definition. Statement (a) uses “evaporation temperature,” which is less precise and not the standard phrasing for the upper fixed point in fixed-point calibration.

Step-by-Step Solution:

Verification / Alternative check:
Classic ITS-90 descriptions use triple points and fixed points; historically, 0–100 °C between ice and steam points are the educational anchors.

Why Other Options Are Wrong:

• (a): Ambiguous wording; the standard upper point is the steam (condensation) point.
• (b) or (c) alone: Each is correct but incomplete compared with the combined correct option.

Common Pitfalls:
Confusing evaporation with condensation terminology; the fixed point defines equilibrium of water with its saturated vapor at 1 atm.

Final Answer:
Both (b) and (c)