I recently was part of a conference where young girls in grade 6 are brought to the University to take part in science experiments. The lab that I personally teach with them is "Cryogenics". This lab is a lot of fun and involves investigating dry ice and liquid nitrogen. So today's blog is dedicated to the young lady who asked me about the discoverer of liquid nitrogen.
Nitrogen was discovered by Joseph Priestley (1733-1804) between 1773-1780. In this time he also discovered ammonia (a nitrogen based gas), carbon monoxide, sulfur dioxide, silicon tetrafluoride, and his most famous discovery: oxygen. On the periodic table, nitrogen is element 7, and is symbolised by the letter "N". Its boiling point is -196 C.
Nitrogen exists as a diatomic molecule, N2, making up over 75% of the Earth's atmosphere. The two atoms are held together by a triple bond that has a bond energy of over 800 kJ/mol. (This is A LOT, making this bond very favourable!) In the form N2, nitrogen is completely inert; however, other molecules that contain nitrogen are not inert. Many explosive compounds contain nitrogen, the most notable being TNT: TriNitroToluene. Anything that has a high nitrogen content risks becoming explosive due to the extreme favourability in the formation of N2.
The Haber Process: NH3 is ammonia and is an important fertilizer for crops (plants need nitrogen). However, very few plants actually have the ability to draw nitrogen from the air and convert it into a usable form. They require a symbiotic relationship with microorganisms in their roots to do this. This is called "nitrogen fixing". One of the most important discoveries in human history is the Haber Process. While N2 may be inert, ammonia is not and in 1908 Fritz Haber worked out the necessary conditions for converting nitrogen from the air into ammonia. This process became instrumental to Germany's World War I effort because it allowed for the production of inexpensive explosives. Of course today this process is still used to generate ammonia and is celebrated for the feeding 1/3 of today's population with ammonia-fertilised crops. (Interesting Historical Side Note: Haber also worked with chlorine gas, and was instrumental in developing the practice of using this poisonous gas by German troops in the trenches during World War I. In 1933 this Jewish scientist, who had contributed to the German war effort considerably in WWI, was driven from his work by the Nazi regime.)
Back to nitrogen: liquid nitrogen was first obtained in 1895 with the invention of the liquifaction of air. This process also allowed for the separation of the components of air by distillation. Nitrogen is the most volatile (lowest boiling), boiling at -196 C. This is followed by oxygen at -186 C. And lastly, argon at -183 C. Liquid nitrogen is often used in flash freezing. Anything that has a high water content will freeze extremely quickly. In the lab I ran, I froze flowers, tomatoes, bananas, and latex gloves which the girls then smashed, much to their delight. (My favourite are always flowers as they shatter like light bulbs.) Being inert, liquid nitrogen is relatively safe and easy to work with. I actually freaked out some of the girls by showing that you can pour liquid nitrogen over the hand of a person (I had another lab helper not one of the kids do this) and it will not freeze them because the temperature of a person's hand at +37 C causes the liquid nitrogen to immediately evapourate into gas before touching the skin. Not unlike dropping water on a hot skillet. The big danger is with its ability to liquefy oxygen. This is a very reactive compound, to the point of scary! My favourite thing to do with liquid nitrogen is to make ice cream. If you ever have the chance to have liquid nitrogen ice cream, take it!
References:
Gray, T. The Elements 2009 Black Dog & Leventhal Publishers Inc., New York, NY.
Petrucci, R. H.; Harwood, W. S.; Herring, F. G. General Chemistry 2002 Prentice Hall Inc., Upper Saddle River, NJ.
Balchin, J. Quantum Leaps: 100 Scientists Who Changed the World 2010 Arcturus Publishin Limited, London.
