ELi5: How is liquid nitrogen super cold and in the form of liquid when the container itself is in a room temperature environment?


ELi5: How is liquid nitrogen super cold and in the form of liquid when the container itself is in a room temperature environment?

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The outside of the container is room temperature, or a bit lower then room temperature. The inside of the container is the same temperature as the liquid nitrogen, or maybe a bit higher. The walls of these containers are built of mostly insulating materials, mostly vacuum. So there is very little energy transfer from the outside of the container to the inside. This means that the ambient air is able to heat up the container to room temperature without it heating up the inside of the container and boiling the liquid nitrogen.

It won’t stay liquid for long, just like water won’t stay liquid for long in the oven.

But this transition takes a while, especially in an insulated container. If you left it alone for a few hours and came back your bucket of liquid nitrogen would be empty.

> the container itself is in a room temperature environment?

Almost irrelevant. You can take a bottle of water out of the freezer and it won’t instantly melt when room temperature air meets the bottle, right?

Heat transfer takes time, and it takes more time if it needs to go through something that is not good at conducting heat. Even if the LN was just chilling in a bowl it would take time for it to boil away (a fairly short time, but it woudln’t be instant regardless). The non-pressurized dewars you see people pouring liquid nitrogen out of are specifically made to minimize heat transfer. They are double walled with a vacuum between the layers, the cork at the top is made of thermal insulating foam, etc. They cost a couple hundred dollar apiece, they aren’t your average water bottle.

The liquid still boils away, but at a massively slowed rate because it is almost completely cut off from the energy (the outside heat) it needs to boil into a gas.

The container is a really efficient thermos. Its hollow walls are evacuated, which insulates the contents from ambient heat. It isn’t perfectly insulated though l, and so the liquid nitrogen eventually evapourates away.

Essentially, once liquid nitrogen exists, it must be actively rechilled or it is just a matter of time before it evapourates.

BTW, to make liquid nitrogen you compress gaseous nitrogen, refrigerate the compressed gas, then rapidly decompress the gas. Expansion cooling disperses enough energy to cause the gas to condense into a liquid.

Do an experiment: Go to your refrigerator and pull an ice cube out of the freezer section.

It’s a solid, even though the environment around it is waaaaay above the melting point of water. How can this be?

We’re seeing the difference between kinetics and thermodynamics. If you wait a while, the system will come to equlibrium, the room will be (slightly) cooler, and the ice will now be water. But heat transfer takes time.

Eventually, your liquid nitrogen will turn to a gas. How fast depends on ambient conditions and what container it’s in.