Assertion–Reason (Thermometry): Assertion (A): Alcohol, rather than mercury, is used in a thermometer to measure very low temperatures around –60 °C. Reason (R): Alcohol has a much lower freezing point than mercury.

Introduction / Context:This Assertion–Reason problem tests conceptual understanding of thermometric liquids and why different fluids (alcohol vs mercury) are selected for different temperature ranges. The focus is on low-temperature measurement near –60 °C and how material properties drive instrument design.

Given Data / Assumptions:

• Assertion: Alcohol (e.g., ethanol or dyed spirit) is preferred for thermometers at very low temperatures such as –60 °C.
• Reason: Alcohol freezes at a much lower temperature than mercury.
• Objective: Decide truth values and whether the reason explains the assertion.

Concept / Approach:Thermometric liquids must remain liquid across the working range and expand sufficiently with temperature. Mercury solidifies at around –38.8 °C, so it cannot measure temperatures below that point. Common alcohols (ethanol) remain liquid well below –60 °C and possess a high coefficient of thermal expansion, yielding good sensitivity.

Step-by-Step Solution:1) Check A: At –60 °C mercury would be solid; thus alcohol thermometers are used. A is true.2) Check R: Alcohol has a far lower freezing point than mercury (ethanol ~ –114 °C), so R is true.3) Causality: The very reason alcohol does not freeze in that range is precisely why it is chosen. Hence R correctly explains A.

Verification / Alternative check:Beyond freezing point, note ancillary factors: alcohol wets glass better and allows visibility via dye; mercury, though better for moderate ranges due to linearity and non-wetting behavior, is unusable when solidified.

Why Other Options Are Wrong:“Both true but not explanation” ignores the direct causal link; “A true, R false” contradicts known freezing points; “A false…” is inconsistent with practice.

Common Pitfalls:Confusing boiling point with freezing point; assuming mercury’s wide usability implies suitability at all temperatures.

Final Answer:Both A and R are true, and R explains A.