NCSU – Dept. of Chemistry – Lecture Demonstrations Gas Law / IMF

Triple Point of Water

Description: Water at reduced pressure reaches the triple point when cooled with liquid nitrogen.

Materials:

Triple point apparatus (Dab 114) Safety shield Liquid N 2 and insulating gloves Ring stand

Procedure:

For larger lecture halls, project demonstration using document camera.

1. Prior to the demonstration, open the valve on the triple point apparatus and boil the water for about 15 minutes. Close the valve and cool the apparatus by placing it under running tap water. When the apparatus is close to room temperature, invert the flask and place in the ring stand.

2. To protect from implosion risks, place ring stand behind safety shield. Pour liquid nitrogen into the depression in the flask. After a short period of time, the water will boil and ice can be observed forming inside the flask.

Discussion: On a phase diagram, the point at which all three curves intersect is known as the triple point. At this temperature and pressure, all three phases are in equilibrium. The triple point of water is 273.16 K and 4.58 torr.

Safety: Wear proper protective equipment including gloves and safety glasses when preparing and performing this demonstration. Perform demonstration by pouring liquid nitrogen over the safety shield. Wear insulating gloves when handling liquid nitrogen.

References: http://sirius.ucsc.edu/demoweb/cgi-bin/?fl_pr_ht-thermo-triple_point

Andrews, L. J. Chem. Educ. 1989 , 66 , 597. (liquid phase of CO 2)

Video: http://www.maniacworld.com/water-at-triple-point.html