Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
8. Chemistry in the World
What happens when you play with liquid nitrogen?
Liquid nitrogen boils off into the air, where it came from, at room temperature.
Liquid nitrogen is made by compressing and cooling air. Air is mostly nitrogen. The oxygen in the air becomes a liquid at a higher temperature than nitrogen, so it liquefies first and boils off last, when air is liquefied. When the gases that liquefy first are removed, almost pure nitrogen is left.
Liquid nitrogen boils at -320° Fahrenheit. So having a bowl of liquid nitrogen in your kitchen is similar to having a bowl of water in an oven at 600° F. The water would boil away. But as long as there was water left in the bowl, the water itself would still only be at a temperature of 212° F—the boiling point.
The same thing happens with liquid nitrogen. As long as there is liquid in the bowl, it cannot get hotter than the boiling point. So anything we put in the liquid will be cooled to the boiling point of the liquid.
When a rose is put into liquid nitrogen, the liquid vigorously boils around the rose, since the rose is 400° F hotter than the liquid’s boiling point. The rose transfers its heat to the nitrogen until they are both at the same temperature: -320° F.
At that temperature, the water in the rose petals is frozen solid. The petals become as brittle as thin sheets of glass. If the rose is then dropped onto a table before the air can melt the ice, the rose will shatter into hundreds of tiny shards. But you have to act quickly—the air is 400° F hotter than the rose petals and can melt the ice quickly.