How do you get liquid nitrogen

How to make ice cream with liquid nitrogen

In the last few weeks we have organized two experiments with liquid nitrogen during the Science Holidays, an initiative of the Federal Ministry of Education, Science and Research for the holiday care of children at universities in the summer. One of these was the levitation of a magnet over a superconductor, which was cooled below its critical temperature with liquid nitrogen. The other experiment tries to explain the concept of liquid nitrogen and not only teach the children about exciting and funny applications, but also the dangers of this interesting liquid. The highlight of our program is the production of nitrogen ice cream.

How is liquid nitrogen made?

The various states of aggregation of water are known: solid (ice), liquid (water) and gaseous (steam). In the same way, nitrogen can change its physical state from gaseous to liquid when it cools down. The liquefaction process of nitrogen can be explained very simply as follows: Air consists of around 78 percent nitrogen. To liquefy it, air is first strongly compressed in a container. During this process, the pressure and thus also the temperature increases according to the ideal gas law. The same thing happens when you squeeze a balloon: the air in the balloon creates a higher pressure outwards and therefore the balloon can even burst. In this case, however, the temperature rise is so small that you do not notice it. During the liquefaction process, the air is increased to a pressure of around 200 bar. For comparison: the normal air pressure is one bar.

The heated compressed air now gives off heat to the environment. Then the air is relaxed again and cools down. The temperature of the air is thus far below the original starting temperature and close to the boiling point - that is the temperature at which the physical state changes from gaseous to liquid. For nitrogen, this happens at minus 196 degrees Celsius (77 Kelvin). At this temperature nitrogen is liquid and above it is gaseous. Liquid nitrogen is stored in containers that work much like a thermos. They are double-walled containers, called dewar vessels, in which a vacuum is created between the two walls in order to avoid heat exchange with the environment, i.e. to isolate them. In an open container, depending on the amount, the liquid nitrogen will evaporate within a few hours. In the closed dewar vessel, the liquid nitrogen will also hold for a maximum of a few weeks due to the slow seepage of the steam.

Not entirely harmless

If liquid nitrogen comes into contact with air, it evaporates immediately and a cool mist is created. That can be very pleasant in summer. However, liquid nitrogen is not harmless. The extremely low temperature of the liquid can cause frostbite on contact with the skin and eyes. The damage is similar to a severe burn. Therefore, protective goggles and low-temperature gloves are usually necessary as safety measures.

When working with liquid nitrogen, it is important that the room is very well ventilated because the high concentration of evaporated nitrogen can displace the oxygen in the room and there is a risk of suffocation. Even an oxygen content of less than 15 percent in the air can lead to suffocation.

Frozen roses and steaming popcorn

What happens to a material when it is immersed in liquid nitrogen depends heavily on the structure and properties of the material. In the case of an inflated balloon, for example, the resilient plastic from which the balloon is made will contract when it is immersed and the air inside is compressed. If you take the balloon out again, the balloon expands again and the air relaxes. Thus the balloon returns to its original shape.

If you immerse a rose, which consists to a large extent of water, in liquid nitrogen, the rose freezes completely. If you hit it on the table, the rose breaks into small pieces, as can be seen in the picture below. The small parts then slowly thaw again later. In contrast, popcorn, which contains very little water and is porous, does not freeze when immersed in liquid nitrogen, but instead stores nitrogen vapor in the pores. You can eat the popcorn and watch the nitrogen vapor blow out through your nose and mouth.

A Russian thermodynamics professor told us while I was studying physics in Mexico that he and colleagues freezed vodka in a styrofoam bowl with liquid nitrogen to a butter-like consistency during the long low-temperature measurements in the laboratory. Then they spread the vodka butter on bread and thus had a quick snack. I can't say whether that actually works.

Nougat nitrogen ice cream

Another application of liquid nitrogen in culinary art is nitrogen ice cream, which can be made within a few minutes.

Ingredients (for about 12 servings)

  • About 1 liter of liquid nitrogen
  • 250 g natural yogurt
  • 250 ml whipped cream
  • 4 tablespoons of nougat cream

preparation

Mix yogurt, whipped cream and nougat cream to a homogeneous mixture. Slowly add the liquid nitrogen, stirring constantly, until you have the desired consistency. The nitrogen ice cream is ready! Important: use low temperature gloves while mixing. It is also recommended to have a knowledgeable person with you to avoid accidents.

When a girl asked during the Science Holidays why physics is so exciting, the answer from another girl, who was licking her sticky ice cream with relish, was: "Well, because it tastes so good!" (Andrea Navarro-Quezada, September 1, 2020)

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