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.
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ABoth A and R are true, and R is the correct explanation of A
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BBoth A and R are true, but R is not the correct explanation of A
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CA is true but R is false
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DA is false but R is true
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ENone of these
Answer
Correct Answer: Both A and R are true, and R is the correct explanation of A
Explanation
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.