Lecture Demonstration of a Phase Transition in a Solid Considerable time is spent in most general chemistry courses discussing transitions between solid, liquid, and gas phases, hut little mention is made of solid-solid phase transitions. Allotropy of elemental solids is usually described, but the general impression is that by varying experimental conditions different forms can be obtained and that reversible transitions from one form to another are extremely slow. Actually, some phase transitions occur rapidly and reversibly lthc basis for differential thermal analysis, DTA) and the two phases may have strikingly different physical properties. One such transition is between y-AgI (the room temperature form) and a-AgI (stable above 140°C). The high temperature farm is highly conductive because of facile silver ion migration while the low temperature form is only slightly conductive. A lecture demonstration has been devised illustrating this phase transition by making use of the electrical conduetivitv ~ r o ~ e r t i e A s . el let of silver iodide sandwiched between two silver electrodes is connected in series with an ordinar; lighi bulb and-plugged into a normal 110-V receptacle. The initial voltage drop is almost entirely across the silver iodide pellet because of its low conductivity. However, the conductivity is sufficient to cause heating ll2R) in the pellet and within a short time (a few seconds to a few minutes depending on the age of the pellet used) the pellet reaches 140"C, undergoes the phase transition to the a-form and suddenly becomes extremely conductive (conductivity is about 1 ohm-' cm-I). At this point the voltage drop across the pellet becomes small, and the light bulb "turns on". When the circuit is disconnected, the pellet will cool and undergo the reverse phase transition to the less conductive y-AgI. A wait of a minute or two allows sufficient cooling toshow that the AgI is in the y-form again. A convenient method for making such a silver iodide pellet was to press a 3-layer mixture of 200 mg powdered silver, 2 g silver iodide, and 200 mg powdered silver in a 0.5-in. diameter KBr die used for ir pellets. The pellet was then held in a plastic jig containing metal foils on the top and bottom to make connection with the silver electrodes. A 25-W bulb gave a convenient 60-sec time interval for the phase transition. When freshly prepared, the pellets have same low resistance paths which allow the bulb to light quickly; but the time increases as the pellets anneal with use. A switch should also be incorporated in the circuit for convenience and safety. J o h n H. Kennedy Fred Chen University of California SantaBarbara. 93106
Volume 50, Number 2, February 1973 / 109
