Temperature instrument calibration: Fixed points used to standardize temperature measuring instruments are typically the __________ points of pure substances.

Introduction / Context:
Accurate temperature measurement requires calibration at reproducible reference points. Pure substances provide well-defined phase-change temperatures under standard pressure conditions.

Given Data / Assumptions:

• Calibration should use stable, repeatable reference temperatures.
• Phase-change temperatures of pure substances are reproducible plateaus.

Concept / Approach:
Melting and boiling points of pure substances (e.g., ice point and steam point of water at 1 atm) are classic fixed points for thermometer calibration. Modern scales (ITS-90) include triple points and other defining fixed points, but historically and practically, both melting and boiling points are used widely for standardization.

Step-by-Step Solution:
Identify fixed points → melting and boiling plateaus.Use in practice → calibrate zero/upper points and interpolate for scale.Thus, both melting and boiling points are correct.

Verification / Alternative check:
Classical thermometer calibration uses the ice bath (0°C) and steam bath (100°C) at 1 atm for water.

Why Other Options Are Wrong:

• Only melting or only boiling omits commonly used counterparts.
• “Neither” is incorrect; phase changes are the archetypal fixed points.
• “Only triple points” is too restrictive for routine practice.

Common Pitfalls:
Ignoring pressure dependence; fixed-point cells must maintain specified pressures and purity.

Final Answer:
Both melting and boiling points