Fixed Points Define Celsius Scale in Thermometers

Steps:

• Identify the purpose of fixed points: they calibrate thermometers by setting reproducible temperature references.
• Recall that mercury-in-glass thermometers use the Celsius scale, standardized globally.
• Note the lower fixed point is 0°C (water's freezing point) and upper is 100°C (water's boiling point at 1 atm).
• Confirm these points are practical and consistent for most thermometers, including mercury ones.

Why B is correct:

• The Celsius scale, used in mercury-in-glass thermometers, is defined by water's freezing point (0°C) and boiling point (100°C) as fixed points per international standards.

Why the others are wrong:

• A: Mercury's freezing (-38.8°C) and boiling (356.7°C) points are impractical for everyday calibration and not standard.
• C: Room temperature varies (around 20-25°C) and lacks precision as a fixed point; mercury's boiling is irrelevant.
• D: Room temperature is not fixed or reproducible; water's boiling is only one point, not a pair.

Final answer: B