The thermometric property of mercury is best on the change in

Analysis

The question asks for the specific physical property of mercury that allows it to function as a thermometric substance in a liquid-in-glass thermometer.

Concept Explanation

A thermometric property is any physical property of a substance that changes linearly and predictably with temperature. For a liquid-in-glass thermometer (like the one containing mercury), the thermometer works because the liquid expands when heated and contracts when cooled.

• Volume expansion: As the temperature of the mercury increases, the kinetic energy of its atoms increases, causing them to move further apart. This results in a measurable increase in the volume of the mercury, which causes the liquid level to rise in the narrow capillary tube of the thermometer.
• Why others are incorrect: While density does change with temperature (as volume increases, density decreases), the direct observable change that indicates the temperature on the scale is the change in volume. Pressure is the primary thermometric property for gas thermometers, and resistance is the thermometric property for platinum resistance thermometers.

Step-by-Step Solution

1. Identify the device: The question refers to a mercury thermometer.
2. Observe the mechanism: In a mercury thermometer, we observe the mercury column rising or falling in a glass tube.
3. Relate to physics: The rise or fall of the column is a direct consequence of the mercury expanding or contracting. Expansion and contraction are changes in the volume of the substance.
4. Conclusion: Since the mercury occupies more space (volume) as it gets hotter, the change in volume is the property used to calibrate the temperature scale.

Final Answer

The correct option is B.

Exam Tip: Always remember the "thermometric property" rule:

• Liquid-in-glass thermometers (Mercury/Alcohol) $\rightarrow$ Volume
• Resistance thermometers $\rightarrow$ Electrical Resistance
• Constant volume gas thermometers $\rightarrow$ Pressure
• Thermocouples $\rightarrow$ Electromotive Force (e.m.f)