Liquid Nitrogen/BLEVE

Required Training	Required PPE
UC Lab Safety Fundamentals Online Cryogen Safety at UC Davis	Lab coat, safety glasses/goggles, hearing protection, cryo-gloves (blue fabric)
Performers Required:
Equipment	Chemicals
Insulated cryogen transfer dewar (1, 4, or 10 L)	Liquid nitrogen (LN2)
Ice Bucket
Metal Container
Tongs
Items to freeze (flower, balloons, rubber tubing, etc.)
Electric kettle filled with water

Procedure:

1. Carefully pour LN₂ into the ice bucket from the transfer dewar.
2. Dip items into the LN₂ until frozen. Use tongs and cryogen gloves to hold items to be frozen. Do not immerse cryogen gloves in liquid nitrogen. When freezing items, immerse them slowly to minimize splashing from boil-off.
3. Once frozen, items can be broken by hand or by hitting them on the ground. Hitting tubing against an edge, like the tongs, greatly improves the chance of shattering.

Boiling Liquid Expanding Vapor Explosion (BLEVE)

1. Carefully pour LN₂ into the metal container from the transfer dewar and the remains in the ice bucket.
2. The electric kettle should be heating throughout the show to ensure the water is boiling for this final demo.
3. The audience should be greater than 20 feet away and should be warned to cover their ears. The performer should have on hearing protection. The performer dumps the boiling water into the container of liquid nitrogen. This will cause an impressively large plume of water vapor to erupt from the container, and the expansion can be quite loud. There is a technique to maximize the size of the plume of water vapor – the container of boiling water is held at chest level, with the performer standing at arm’s length from the metal container. The performer quickly inverts the container of water and forcefully directs the contents into the center of the cooler in a single downward motion, such that all of the water hits the LN₂ once. To prevent ice aggregates from forming in the container, the performer should tip over the container producing a fog effect for the audience.

Clean-up: Never pour LN₂ into a drain or other confined space. LN₂ left in the transfer dewar will boil off completely. Be sure to clean up any pieces of shattered items.

Hazards: LN₂ boils at -196 °C (77 K), and will cause frostbite on contact. Do not allow LN₂ to become trapped against skin – ensure that pants are uncuffed and cover any permeable openings on the tops of shoes. Do not store or transport LN₂ in enclosed spaces (i.e. elevators, cars with windows rolled up), as the vaporized nitrogen will displace oxygen and can create an asphyxiation hazard. Furthermore, never keep LN₂ in sealed containers, as the pressure build-up will eventually cause the vessel to rupture, potentially causing great physical harm to anyone nearby.

Principle: Freezing items in LN₂ is a physical process in which the low temperature of the LN₂ liquefies or solidifies whatever object it comes in contact with, thus increasing the viscosity of the object. If the object is hydrated, such as a flower, then the LN₂ will freeze the water and make the flower shatter like glass. Liquid nitrogen is constantly boiling, as it is exposed to conditions much warmer than its boiling point. As liquid nitrogen is exposed to the air, it condenses any water vapor into a white mist that can be seen at the top of the container.

Notes: Never pour liquid nitrogen on any part of a person, as it may become trapped by clothing and cause frostbite. Never throw liquid nitrogen up into the air – any liquid that does not boil off will fall, and poses a hazard to anyone standing nearby. Additionally, do not eat frozen food items (graham crackers, pretzels, marshmallows) or offer them to the audience. This has been done in previous years, but there is a risk of frostbite if a significant amount of liquid nitrogen is absorbed into the food. This also violates food safety regulations since our liquid nitrogen is not food grade. It also sends the wrong message to the audience – there should not be any consumption of food or drink during a chemistry performance. Avoid using Styrofoam containers as they tend to brittle over time or break from the BLEVE.

Examples of items to freeze:

• Flowers or other plants can be frozen and then broken by hand, or on a surface.
• Elastomers (rubber bands, Tygon tubing) can be shattered when frozen, but become flexible again when warmed. Caution – when freezing Tygon tubing, placing an open end of the tube into the liquid nitrogen will create a fountain from the other end of the tube. Ensure that the open end is not pointed at anyone. Also take care when breaking Tygon tubing to prevent shattered pieces from flying towards the audience; it is better to crush it with a hammer than to smash the tubing on the ground.
• Balloons (air-filled) can be immersed in liquid nitrogen and shrunk down as the air inside liquefies. The balloon will re-inflate when warmed, provided the latex was not cracked when frozen. Helium balloons will not fully collapse, as helium condenses at 4.2 K, much lower than the boiling point of LN₂.
• Bananas, if frozen for > 2 minutes, can be used to hammer nails into wood or other objects. The ripeness of the banana affects its ability as a hamer, less ripe = better. Caution – never position your hand directly over the nail, as the banana may shatter on impact and your hand could continue downward onto the nail, causing significant injury.
• The gaseous nitrogen can be used to blow up a balloon or produce sound when passing through a noise maker. Pour 20-30 mL of liquid nitrogen into a glass beverage bottle. Either affix a balloon or a noise maker (may require fittings). To produce gas at a greater rate, rotate the glass bottle.