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Q & A: boiling liquid nitrogen

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Most recent answer: 11/17/2016
When a coin is placed in liquid nitrogen, the liquid boils steadily. After about a minute, it then boils extremely vigorously before it stops boiling altogether. Why the sudden increase in boiling activity near the end?
- Kathy
North Tustin, CA USA

I think is is in the same general category as the leidenfrost effect, a peculiar change in rates of water boiling in a heated pan. Borrowing from the explanation of that effect, here's what I guess is going on. 

When the coin is much warmer than 77K (the boiling point of liquid nitrogen) it's coated with nitrogen gas. That keeps liquid from touching it. Heat leaks out slowly through the gas, slowly boiling the liquid. As the coin gets colder, some part of it first gets very close to 77K. Liquid can come up and directly touch it, with the gas formed flowing upward and leaving liquid still touching the coin. The heat flow speed up because it doesn't have to go through gas. As soon as the whole coin reaches 77K, the boiling stops.

Mike W.

(published on 11/17/2016)

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