LECTURE DEMONSTRATIONS IN GENERAL CHEMISTRY* UNIVBRSI~TO F WISCONSIN, MADISON, WISCONSIN JAMESH. WALTON,
I n lecture experiments the course ofa given chemical reaction i s oftenfollowed by observing the rate at which a gaseous product i s evolved. This can be done by attaching a rubber balloon to the container in which such a reaction is taking fihce and noting the rate at which the balloon i s inflated. Seoeral experiments of this type are described. Directions for the preparation of pyrophoric lead and iron and the formation of silver and lead trees are also given. Demonstrations of the absorption of certain gases and the use of glass wool in lecture exfieriments are also described.
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In connection with demonstrations in general chemistry the author has noted the desirability of showing the audience in a semi-quantitative way the rate a t which gases were being evolved from various chemical reactions. I n certain cases it was found that the demonstration could be very satisfactorily made by slipping the neck of an ordinary ten-cent balloon over the neck of the flask containing the reactants and following the evolution of the gas by watching the rate of inflation of the balloon. This simple bit of technic can be applied to a number of types of reactions, three of which will be described. The Preparation of Hydrogen To show the influence of metallic copper on the reaction, Zn
+ HlSOt = ZnS04 + HZ,
place 150 cc. of 2 N HzS04 in each of two 500-cc. flasks. To one flask add a few crystals (0.2 g.) of CuSOc5Hz0 and allow them to dissolve. Obtain two ordinary rubber balloons, place in each ten pieces of zinc (C.P.) rod '/
