In the last weeks, during the Science Holidays, an initiative organized by the Ministry of Education, Science and Research to entertain children at Universities during the summer, we organized two different programs where liquid nitrogen is involved. The first one deals with a magnet floating above a superconductor that is cooled down below its critical temperature with liquid nitrogen. The second one aims at explaining the kids the concept of a liquid gas and show them not only interesting and funny applications, but also the dangers of working with this liquid. The highlight of our program is making nitrogen ice cream.
How is nitrogen liquified?
Almost everyone knows the different physical states of matter for water: solid (ice), liquid (water) and gas (steam). In the same manner, nitrogen can change its physical state from gaseous to liquid when it cools down. The liquefaction process of nitrogen can be explained in a very simplified way as follows: Air consists of approximately 78 percent of nitrogen. For liquefaction, air is first highly compressed in a container. During this process, the pressure rises and so does the temperature, according to the ideal gas law. One can picture this when compressing a balloon: the air inside the balloon creates a higher pressure to the outside and therefore the balloon can even burst. But the temperature rise in this case is so small that you do not notice it. In the case of the liquefaction process, the air pressure is increased up to about 200 bar. For comparison, the normal air pressure is one bar.
The heated compressed air now gives off heat to its surroundings. The air is then expanded again and cools down. The temperature of the air is thus far below the original initial temperature and close to the boiling point. This is the temperature at which the aggregate state changes from gaseous to liquid. For nitrogen this happens at minus 196 Degree Celsius (77 Kelvin). Below this temperature nitrogen is liquid and above it, it is gaseous. Liquid nitrogen is kept in containers that function similar to a thermos flask. These are double-walled containers, called vacuum flasks, where a vacuum is created between the two walls to prevent heat exchange from the environment with the nitrogen, this means to isolate it. In an open container the nitrogen, depending on the amount, will evaporate within a few hours. In a closed vacuum flask, the liquid nitrogen will last a few weeks due to the slow seepage of the vapour.
When liquid nitrogen gets in contact with air, it vaporizes immediately, creating a cool fog. This can be very nice in the summer. However, liquid nitrogen is not harmless. The extreme low temperature of liquid nitrogen can lead to freezing of the skin and eye tissue. The damage is similar to that of a burn wound. Therefore, when handling with liquid nitrogen wearing goggles and protective low-temperature gloves is highly recommended.
When working with liquid nitrogen in a room, one should make sure the room is well ventilated. This is to prevent asphyxiation, due to the nitrogen vapour, which displaces the oxygen in the room. An oxygen content below ten percent can already lead to asphyxiation.
A frozen rose and smoking popcorn
How a material will react when placing it in contact with liquid nitrogen, depends on the material structure and its properties. For example, let us consider an air balloon. If it is dipped in liquid nitrogen, because of its elastic material, the balloon will shrink in size compressing the air inside. After the balloon is pulled out of the liquid nitrogen, it will gain its normal size and the air inside will expand.
Dipping a rose, which consists mainly of water, into liquid nitrogen freezes it out. Then one can destroy the rose into pieces by striking it on a table. The small parts left will slowly thaw. In contrast, if you take popcorn, which contain almost no water inside and are porous, then these will not freeze and the nitrogen vapour will be stored in the pores. One can eat the popcorn and slowly chew it while exhaling nitrogen vapour through your nose and mouth.
So now we come closer to the applications of liquid nitrogen in the kitchen. A Russian thermodynamic professor during my physics study in Mexico once told us that he and colleagues used to freeze vodka during the long low-temperature measurements in the lab until it would get the consistency of butter. Then they would spread it onto bread and have an easy lunch. Whether this works, I cannot tell.
Nougat nitrogen ice cream
A further application of liquid nitrogen in the kitchen is ice cream. One can get an ice cream done in only a few minutes.
Ingredients (for 12 portions)
- Approx. 1 l liquid nitrogen
- 250 g yogurt
- 250 ml whipped cream
- 4 spoons nougat cream
Mix the yogurt, the whipped cream and the nougat cream with a mixer until you obtain a homogeneous mixture. Then slowly pour liquid nitrogen while constantly mixing until you obtain the desired consistency. Ready! Important: wear protective low-temperature gloves while mixing. It is also recommended that a specialist is around to avoid accidents.
Here you can watch how making nitrogen ice cream looks like.
Liquid nitrogen can be applied not only in physics experiments but also in the kitchen, for example in Vodka-butter-bread and nitrogen ice cream. During the Science Holidays, a girl asked why physics was so exciting? The answer from another girl, who was happily enjoying her nougat nitrogen ice cream, was “Well, because it tastes so good!” (Andrea Navarro-Quezada, 1.9.2020)
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