Liquid nitrogen is one of my favorite compounds in the science lab because it is so cold (-321 deg F) and readily available. Nitrogen is all around us, in fact the air we breathe is about about 78% Nitrogen! But, at standard temperature and pressure (STP) nitrogen is a gas. When it is pressurized and cooled way down it will turn to a liquid, making our favorite freezing formula - Liquid N2. Nitrogen is always shows as the diatomic compound N2 because it is a diatomic element - it doesn't like to live alone so it attaches to another Nitrogen atom (much like the other diatomic elements: H, O, F, Cl, Br, and I).
This science demonstration, of freezing a balloon full of warm breath, clearly outlines one of chemistry's gas laws: Charles's Law. Charles's Law states that as the temperature of a gas increases, so does its volume. Likewise, if you decrease the temperature of a gas, the volume will also decrease (see diagram). This explains why the balloon, filled with warm breath, shrinks drastically as liquid nitrogen is poured over the top.
Exhaled breath is actually a combination of several gaseous compounds with the following composition (rounded averages): 78% Nitrogen, 15% Oxygen, 5% Carbon Dioxide, 5% Water, and 1 % Argon and other gases. Each of these elements and compounds has a different freezing, melting and boiling point so the breath in the balloon undergoes several phase changes as it warms - this is the sound you hear in the video.
When the balloon is cut open, nitrogen escapes right away as a gas because most of it does not cool below -321 deg F and turn to a liquid. Next the Oxygen begins change phases from liquid to gas at -297 deg F, you can clearly hear it boiling in the video. Then the Carbon Dioxide, which is intermixed with the other gases begins to solidify as the oxygen and nitrogen leave and the water freezes solid. The CO2 begins to sublimate (turn from solid to gas) as the air in the cup rises above -109 deg F. When the frozen breath is finally eaten (or drank, depending on how you look at it) the CO2 quickly turns to a gas and the ice water turns to a liquid. And, the drinker gets to experience his own breath, concentrated in flavor, bubbling with CO2 and tasting like nasty morning breath...only a whole lot stronger...and colder...and way more disgusting.
Where to buy liquid nitrogen?
Liquid nitrogen can be purchased at an industrial gas supplier or medical gas supplier. You may have to do some searching in your area but these suppliers usually carry oxygen gas for medical use, acetylene gas for welding and liquid nitrogen for medical and agricultural uses. The container you need to carry liquid nitrogen is called a Dewar (find one here) which are quite expensive. Most places will not sell you liquid nitrogen if you don't have a Dewar although some suppliers will rent or loan you one.
How much does liquid nitrogen cost?
Liquid nitrogen is sold by the liter (or gallon) and varies from $0.10 (US Dollars) for bulk purchases up to $5.00 per liter for a small Dewar.
Is liquid nitrogen dangerous?
Yes. It can be very dangerous. Liquid nitrogen is -320 degrees F (-195.75 degrees C) so it can cause frostbite on skin very quickly and can damage eyes, nose, nose, mouth and internal organs on contact. Liquid nitrogen also evaporates very quickly and the ensuing gas takes up more space than the liquid. If the liquid is left in a container that cannot vent off this gas, pressure builds up and a pressure explosion can occur - this is why only an approved Dewar container with a pressure valve and loose lid can be used to transport consumer Liquid Nitrogen.