Will liquid nitrogen evaporate if left in an unopened ...

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Does it start to evaporate as soon as it is exposed to oxygen?

Exposure to oxygen has nothing to do with it.

Goto Chengde Energy Technology to know more.

Actually the liquid will evaporate whether the lid is off or on. But, the rate of evaporation depends on how well the container is insulated. Evaporation is what keeps the liquid cold.

Leaving the lid off allows heat from the room to enter the container at a greater rate, and the liquid will evaporate faster in proportion to that rate. The open lid allows thermal radiation from above to "shine" down into the container, and it allows the cold gas near the surface of the liquid to mix with the warm room air. The rate of that mixing will depend on the height of the liquid in the container, and on how much the building's ventilation system stirs the air.

P.S., I presume that the room has some kind of forced air ventilation. You wouldn't want to walk into the room in the morning if the "air" was all "evaporated" nitrogen, and no oxygen. At least, not unless you were wearing SCBA gear.

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