There is video circulating tonight of a summer party in Mexico where organizers poured liquid nitrogen into the swimming pool. They weren’t messing around, either—you can see a number of ~10 L dewars being upended into the water.

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Of course, this is a very, very bad idea. Under these conditions, the nitrogen (b.p. = −196 °C) quickly boils and displaces all of the oxygen present, causing people in the area to asphyxiate . Remember, it only takes 28 g of liquid nitrogen to displace 22.4 L of air at standard temperature and pressure. The fact that you are asphyxiating is masked by your ability to exhale carbon dioxide normally, so the burning sensation you experience when holding your breath or drowning is absent. Furthermore, the thick fog produced upon mixing the liquid nitrogen and water conceals any distress.

It looks like multiple people were hurt at the party and needed to be rescued.

News reports, updated 21 June 2013: Daily Mail (corrected most of the wrong chemistry, but a caption referencing a reaction with chlorine remains), Latin Rapper Blog (corrected to get the chemistry right; gives ChemBark a shout out, see comments), International Science Times (gets it right, credits ChemBark), Excelsior (gets the chemistry wrong), Fox News (gets the chemistry wrong), MSN Now (gets the chemistry wrong), Huffington Post UK (gets chemistry mostly right, credits ChemBark, but still needs to correct photo captions), NY Daily News (gets the chemistry wrong), The Sun (gets the chemistry wrong), The Telegraph (gets the chemistry wrong in the headline), Popular Science (gets the chemistry right, includes interview and link to ChemBark), Slate (corrected the story to get the chemistry right).

Deborah Blum covered the misreports by the media of the chemistry at play on the Knight Science Journalism (KSJ) site. The post links to ChemBark.

Updated note to media: Many outlets have reported that liquid nitrogen will react with chemicals in swimming pools to generate a poisonous gas. This is almost certainly incorrect. Molecular nitrogen is relatively inert and should not react with anything present in the pool, like “chlorine” (mostly, NaOCl) or water. The danger of adding liquid nitrogen to the pool stems simply from the nitrogen’s boiling and pushing away all of the oxygen around, leaving none for the swimmers to breathe. The cloud that is present has nothing to do with poison gas. Rather, it is the same thing as fog. The liquid nitrogen is cold and cools the air close to the pool to the point that water vapor in the air condenses into very tiny droplets that stay suspended. The cloud is fog, generated from the same effect responsible for your being able to “see your breath” when you exhale on a cold day. The visible cloud is not nitrogen and it is not smoke; it is droplets of water suspended in an atmosphere of air that is heavily enriched in nitrogen.