Submttted b y Frank J Bockhoff, Fenn College, Cleveland, Ohio C i a ked by Roger Wheaton, Michigan State University, East Lansing
PRE PARATION 11 a large test tube, (6.5 en] X 50 em) to a in. diameter spherical h of 5-7 cm with 'la ethylene pellet^.^ Provide a 2 mm jet for rom]pressed air, directed downward from'a hsta,nee of 20-25 cm above the surface of the pellets. DEWIONSTRATION GI aduiillv increase the veloritv of the air until the r n & d e s (polyethylene pellets) begin to s^lift restlessly. A few of the molecules of liighc:r-than-avcrage kinetic energy- will begin to riaie as high as 30 cm into the vapor state. Incit!ase the air velocity further. More and morc molecules will leave the liquid state and enter the vapor state, an equilibrium being reactled at each idr velocity. REMARKS Allthough other materials myA be used, commereial polyethylene pellets are particularly
satisfactory molecules because of their low density and slight irregularity. The continuous movement of pellets to and from the liquid surface affords an excellent example of dynamic equilibrium between vapor and liquid. Furthermore, any desired equilibrium condition (or vapor pressure) can be maintained as long as desired at a given air pressure (analogous to temperature), and the liquid-vapor interface.remains clearlv defined. VARIATIONS liecause of the translucency of polyethylene pellets, the demonstration is particularly effective when illuminated from beneath in a darkened room. If complete vaporization is desired, for example, to demonstrate the kinetic theory of eases, the uellet bed deuth should be decreased to f c m . * Available from ~astman~hernical Products Inc.. Kingsport, Ten"
POLYMORPHIC FORMS OF MERCURIC IODIDE
AIR
TEST *Â¥'TUS
165cmx50crnl
d2Em", T ..
~T 1
PLASTIC PELLETS
( A~~PMF...
wtted by: C . 0 VanCamp, Ramona High School, Riverside, California Checked by: Roger Wheaton, Michigan State University, East Lansing
alf fill u %in. ignition tube with red mercuric iodide.
may be carried out several times before the tube completely cools.
eat the mercuric iodide with a match or burner. en the yellow mercuric iodide forms, invert the test e and allow the salt t o cool and the red form will reIf the tube is then turned right side up, ugh heat is retained in the glass to convert the outer r of iodide t o the yellow form again. This change
REMARKS The red mercuric iodide changes t o the yellow form a t 126'C, and the yellow form melts a t 2 W C ; so care must be taken in heating. If mercuric iodide is not available it is readily prepared by precipitation from solutions of mercuric nitrate and sodium iodide.
""a1
of Chemical Education
May 1980
