What is the nature of the dense fog that forms over the surface of liquid nitrogen?

Asked by: Pedro

Answer

This is a very good question. My students ask this very same question when I bring Liquid Nitrogen into my classroom.

As you know Liquid Nitrogen is very, very cold. It boils at -196°C (or -321°F, or 77K)! As you also know the air in the room you are sitting in reading this has water vapor in it. Water vapor is invisible.

When you take a shower and your bathroom fills with fog you are actually seeing condensed water vapor. This is really tiny specks of liquid water suspended in the air in your bathroom. This happens because the hot water you used to take a shower came out in a fine spray and some of it evaporated. Because your bathroom was cooler outside of your shower that evaporated water quickly condensed. Lots of it condensed on your mirror, enough to make small drops.

This same thing happens above the Liquid Nitrogen. This stuff is so cold that the water in the air above it is able to give away enough of its energy so that water vapor will condense out of the air. When it does so these tiny bits of condensed water stay suspended in the air making the dense cloud of fog above the liquid nitrogen.

This same thing happens with dry ice. The reason the fog stays close to the ground is because, well, it is cold and because water vapor is more dense than air.

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Answered by: Tom Young, B.S., Science Teacher, Whitehouse, TX