Cold Extremes.
One litre of liquid nitrogen becomes about 700 litres of gas. Boiled off in a small, closed cold room, it can push the oxygen below the level at which people lose judgement — without any smell or warning.
Liquid nitrogen boils at −196 °C. Poured over a stirred ice-cream base it freezes it in a minute or two; because freezing is so fast, the ice crystals stay tiny and the texture very smooth (X.10). Dropped onto a warm surface it skitters on a cushion of its own vapour (the Leidenfrost effect), which is why a brief splash on the skin may do no harm while prolonged contact, or liquid trapped in a glove or a mouth, causes deep frostbite. Dry ice, solid CO₂ at −78.5 °C, sublimes directly to gas; it is used to chill, to carbonate and to make fog in drinks.
The dangers are specific and have hurt people. Food or drink served while liquid nitrogen is still present can burn the mouth and gut, and the rapid expansion of gas inside the body has ruptured a stomach: in 2012 a teenager in England lost her stomach after drinking a liquid-nitrogen cocktail, and in 2018 the US FDA warned against eating or handling "dragon's breath" snacks prepared with it at the point of sale. Every litre of liquid nitrogen expands to about 694 litres of gas, which displaces air: in a small closed room or walk-in it can lower the oxygen to dangerous levels with no smell or warning. Dry ice in a sealed container can burst it, and in drinks its pellets must never be swallowed. The workshop releases nitrogen into a room.
Below: the oxygen left in a room of the size you choose after the litres of liquid nitrogen you choose have boiled off and displaced its air (exact gas expansion ratio 1 : 694 at room temperature; well-mixed room, no ventilation), against the 19.5 % oxygen-deficiency limit and the levels at which judgement and consciousness fail.
- Explain how liquid nitrogen and dry ice are used.
- Know the injuries they have caused.
- Calculate oxygen displacement.
- Set safe rules for use.
Respect the cold, respect the gas
Five litres of liquid nitrogen boiled off in a 20 m³ room brings the oxygen to about 17.3 %; ten litres in a 10 m³ walk-in cold room, about 6.4 % — deep in the danger zone. In a 20 m³ room, only about 1.9 L takes the air below the 19.5 % limit. Nitrogen has no smell, and low oxygen gives no sense of breathlessness, which is why oxygen monitors are fitted where it is stored and used.
Rules for professional use: trained staff only; insulated gloves (loose-fitting so they can be shaken off), eye protection and closed shoes; store and use only in well-ventilated rooms, with an oxygen monitor where large quantities are kept; transport in vented dewars, never sealed bottles; never serve any food or drink until all liquid nitrogen has boiled off, and never serve it to be eaten "smoking"; never put dry ice in a sealed container, or a pellet in a drink that will be drunk. At home, there is no need for either: a good ice-cream maker or the freezer does the job (X.10).
Instead of liquid nitrogen, freeze two small batches of ice-cream base, one stirred in a bowl over salted ice and one left still in the freezer. Compare the crystals.
Terms
- Liquid nitrogen
- Nitrogen liquefied at −196 °C.
- Dry ice
- Solid carbon dioxide at −78.5 °C, which sublimes.
- Oxygen deficiency
- Air with less than 19.5 % oxygen.
- Leidenfrost effect
- A liquid riding on its own vapour over a much hotter surface.
Sources
- Liquid nitrogen boils at −196 °C and expands about 694 times from liquid to gas at room temperature (compressed-gas safety data sheets); OSHA treats air below 19.5 % oxygen as oxygen-deficient. U.S. FDA advisory, 30 August 2018: avoid eating, drinking or handling foods prepared with liquid nitrogen at the point of sale. In 2012 a teenager in Lancaster, England, lost her stomach after drinking a cocktail made with liquid nitrogen.
- A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.