As should be obvious my my interactions in various threads, my knowledge of chemistry has regressed since I did my GCSEs.
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In fact, the white dragon's breath weapon wouldn't actually need to be chemically different to just breathing out regularly at all; just mechanically different (like, physics mechanics, not game mechanics). All it would need is some specialized organ to store and compress and eventually decompress carbon dioxide; and the breath weapon would just be the equivalent of a CO2 fire extinguisher; I understand that the gas leaves those kind of extinguishers at around -66 degrees celcius
http://surreyfire.co.uk/co2-fire-extinguishers/
http://safety.eas.ualberta.ca/?page_id=197