Mercury freezing limitation: A mercury-in-glass thermometer cannot be used below the freezing (solidification) point of mercury. What is that temperature in °C?

Introduction / Context:
Mercury thermometers function only while mercury remains liquid. Below its freezing point, the column fractures or cannot move, producing erroneous or no readings. Knowing the freezing limit is essential for selecting a suitable low-temperature instrument.

Given Data / Assumptions:

• We seek the solidification temperature of pure mercury on the Celsius scale.
• Standard atmospheric conditions are assumed.
• No special additives (e.g., thallium) are considered.

Concept / Approach:
Pure mercury freezes at approximately −38.83°C; this is commonly rounded and presented as −38.9°C in engineering handbooks. Below this temperature, alternatives such as alcohol-in-glass thermometers, RTDs, thermistors, or thermocouples must be used.

Step-by-Step Solution:

Verification / Alternative check:
Datasheets for mercury thermometers specify lower usable limits around −38 to −39°C, often with warnings about column separation damage if cooled below that point.

Why Other Options Are Wrong:

• −11.9, −60.9, −80.9, −100: Do not correspond to the known mercury freezing point; some are much too low or too high.

Common Pitfalls:
Assuming antifreeze additives are present; pure mercury thermometers do not contain such additives and must respect the freezing limit.

Final Answer:
-38.9