Cold as Neptune - Frozen Nitrogen | Pēnā i te Mātao o Tangaroa

Introduction

Abstract
Pumping on liquid nitrogen causes it to freeze.

Principles Illustrated
As the vacuum chamber is pumped, the most energetic molecules of nitrogen, those in the vapour phase, are removed. Thus the average energy per molecule is lowered and the temperature is lowered. The liquid nitrogen boils rapidly as it cools and actually freezes, forming snow-like solid nitrogen.

Content

Video

Instructions

We strongly recommend you ask a university lecturer to work with you on this demonstration.
Put a small amount of liquid nitrogen in a small beaker. Be sure to use a layer of insulating foam, and a black background (just black paper) is helpful. Pump. Nitrogen will boil rapidly. Solid will form and melt repeatedly, and finally form and remain solid. With use of a video camera this demo is suitable for a large lecture theatre. We have found a video camera pointed at the apparatus with image displayed on a data projector to be very effective. The video camera seems to work best pointed at the side and angled down somewhat with our particular vacuum desiccator.

Other Information

Safety

There are many safety issues with cryogenic liquids. Be sure you use protective gear and handle the liquid nitrogen correctly. We strongly recommend any teacher planning to use liquid nitrogen seek assistance and advice from a university physics department.
For this demonstration be sure to use a fairly large chamber that can withstand cooling. Be sure to insulate the bottom of the liquid nitrogen beaker from the base of the vacuum chamber. Be sure to use a sturdy vacuum chamber and keep students at some distance. As discussed above, we have found a video camera pointed at the apparatus with image displayed on a data projector to be very effective. The video camera seems to work best pointed at the side and angled down somewhat with our particular vacuum desiccator.
Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.

Related Resources

See Cold Boil (pumping on water to cause boiling and cooling).
The superconductor demonstration (Floating on Air) uses liquid nitrogen.

Notes, Applications, and Further Reading

This demonstration is related to PIRA 4C20.40
See also the UT-Austin Version

Credits

This teaching resource was developed by the Te Reo Māori Physics Project with support from

• Te Puni Kōkiri
• The MacDiarmid Institute
• Faculty of Science, Victoria University of Wellington
• School of Chemical and Physical Sciences, Victoria University of Wellington

• The New Zealand map shown on the poster frame above is used with permission from www.nz.com.
• The image of Neptune is from NASA, Courtesy NASA/JPL-Caltech .
• This teaching resource was developed in collaboration with Mabel Stewart, a New Zealand Science, Mathematics and Technology Teacher Fellow, 2008, hosted by Victoria University School of Chemical and Physical Sciences. Mabel teaches at Bishop Viard College in Porirua, New Zealand. See NZSMT Teacher Fellowships for more information about the Teacher Fellow Programme.