Filled-system expansion thermometers — lowest temperature capability: Which of the following filled-system/expansion thermometers can measure to the lowest temperatures reliably?

Introduction / Context:
Expansion thermometers rely on the thermal expansion of a contained medium. The choice of medium sets the usable temperature range. For very low temperatures, the working fluid must remain liquid and responsive without freezing or becoming too viscous. This question examines which common type reaches the lowest temperatures in practice.

Given Data / Assumptions:

• Compare standard media: mercury, alcohols, liquid metals.
• Focus is on the lower limit of measurement.
• Glass or steel construction affects durability but not the fundamental freezing point.

Concept / Approach:
Mercury freezes at about −39°C, so mercury-based thermometers cannot measure far below this. Alcohols (e.g., ethanol, toluene-based fluids) remain liquid well below −50°C (certain mixtures near −100°C), permitting significantly lower temperature measurement. Sodium or potassium metal thermometers target high-temperature ranges, not low. Thus, alcohol-in-glass thermometers are preferred for very low temperatures among the listed options.

Step-by-Step Solution:

Verification / Alternative check:
Calibration charts for laboratory thermometers show alcohol-filled devices reading well below −50°C, confirming their suitability for low-temperature work.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming “steel” inherently expands the range; the fill medium dominates the temperature limits.

Final Answer:
Alcohol-in-glass thermometer