You come into contact with nitrogen every day; it makes up 78% of our atmosphere. While it is very common to find nitrogen in its gaseous state, it is much more difficult to find it in its liquid state. That is because liquid nitrogen is very cold. If it gets warmer than -321°F it turns back into a gas.
Liquid nitrogen is made by compressing and expanding regular air. The air is first compressed, which makes the air warm. The compressed air is then cooled to room temperature and then released into a larger container. This transition from high pressure to low pressure results in a cooling of the air based on the ideal gas law, which you can learn morea bout here. It’s the same principle used in refrigerators or air conditioners, but is repeated many more times, allowing the nitrogen to cool off even more and separate from the other gasses in the air. We use liquid nitrogen in many of our experiments, which you can check out here!
One problem is that liquid nitrogen will slowly boil away in storage, so in the month between our fall and spring seasons, any leftover liquid nitrogen will just boil away and be wasted. Instead, we took ours to the pool!

At 86°F, the water is much warmer than the -321°F liquid nitrogen. As a result, the liquid nitrogen begins to turn into an invisible gas, which is not the interesting thing the video shows. The growing white cloud that you see is condensed water vapour from the moist humid air above the warm pool water. As the intensely cold liquid nitrogen is added, the moist, humid air almost immediately condenses, and as the cold spreads out, the cloud follows suit.  
Warning: Please note this experiment should not be done with people in the pool. This is not because of the cold (it’s very hard to change the temperature of water), but because of the layer of nitrogen hovering above the pool. The nitrogen pushes the oxygen out of the way. So any swimmers will feel like they’re breathing regular air, but will not be getting vital oxygen. In fact, the room that we dispense liquid nitrogen in has to be large enough and have adequate ventilation to prevent this from happening to people standing nearby.

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