International temperature scales and fixed points: identify the pair that does not belong to the classic list of seven fixed-point substances historically selected for calibration.

Introduction / Context:
Standardized temperature scales use reproducible phase-change points of pure substances to define calibration anchors. Historically, sets of fixed points (e.g., boiling or freezing points of specific metals and gases) were selected for international temperature scales before modern ITS-90 refinements. This question asks you to spot the pair that does not fit the classic selection and conventional use of those substances.

Given Data / Assumptions:

• “Seven substances” refers to an earlier, widely taught list of agreed materials.
• Typical fixed points used: freezing points of pure metals (tin, zinc, silver, antimony), and reference points for water and certain gases like oxygen or sulphur.
• We focus on whether the specified phase (freezing vs. boiling) matches conventional fixed-point usage.

Concept / Approach:
In metrology, freezing (melting) points of metals are favored because they are more reproducible than boiling points at normal pressure. Zinc’s freezing point, not its boiling point, is the standard fixed point. Mercury’s commonly used reference is its triple point (or carefully controlled points), not the ordinary “freezing point” in older barometer contexts. Hence, the pair “zinc (boiling point) and mercury (freezing point)” is mismatched with accepted fixed-point practice, making it the outlier.

Step-by-Step Solution:
Review typical fixed points: water (ice/steam), oxygen or sulphur points, and metal freezing points for antimony, silver, tin, zinc.Check each option’s phase usage.Identify the pair that uses a nonstandard phase (zinc boiling) or an atypical mercury reference.Select the out-of-list pair.

Verification / Alternative check:
Modern ITS-90 emphasizes triple points and metal freezing points; historical lists align with those principles and avoid boiling points where possible due to pressure sensitivity.

Why Other Options Are Wrong:
Ice/steam: Classical reference points for Celsius scale.Oxygen/sulphur: Recognized in earlier scales for low and mid-high anchors.Antimony/silver and tin/zinc (freezing): Standard metal fixed points.

Common Pitfalls:
Confusing triple points with freezing points; overlooking that boiling points are pressure-dependent and thus inferior as fixed points without strict pressure control.

Final Answer:
Zinc (boiling point) and mercury (freezing point)