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Helium-4 boiling point
4.2221 K
Measured
Normal boiling point of liquid ⁴He at one standard atmosphere.
Liquid ⁴He boils at 4.2221 K at 101 325 Pa on the EPT-76 table. Labs say 4.2 K. That is the 4.2 rail. It is not 42.
What it supports
A cryogenic fixed point. NIST TN 1334 prints 4.2221 K. The Chemistry WebBook later prints a normal boiling point of 4.2238 K. Both are helium. Neither is 42. Lowest elemental boiling point. ⁴He stays liquid to T → 0 at 1 atm. Pair with the lambda point 2.1768 K so the helium ladder is visible.
What it does not
Not 42. Not 4.2238 rounded or shifted into 42. Kamerlingh Onnes liquefied helium on 10 July 1908. That date is history, not the number.
ExactnessApproximate
Form4.2
UnitK
Strengthhard