A Simple Demonstration of the Law of Conservation of Mass LOUIS W E I S S Metropoliton Voeotionol High School and New Y o r k University, New Y o r k City
HE REACTION between lead acetate and potasT s i u m chromate in a closed system is used by most teachers in demonstrating the law of conservation of mass. Some deviate from this practice by precipitating another substance or by decomposing sugar or ammonium dichromate in an evacuated flask (1). The precipitation is preferred to the decomposition reactions because of simplicity of preparation and increased safety of operation. Recent articles (2) suggest modifications of the apparatus used to demonstrate the precipitations. Most courses of study used in secondary schools and colleges present the law of conservation of mass after the study of oxidation and Lavoisier's experiments on metals and their oxides. The concepts of solution, suspension, precipitation, etc., follow much later. In using the precipitation of lead chromate to demonstrate conservation of mass, the author has found that the questions asked by the students are chiefly concerned with the names, formulas, and uses of the reactants and products, as well as the definition and theory of preci~itation. The entire Dnrpose of the lesson is obscured b; this untimely introdu&ion of new substances and phenomena. This practice is pedagogically unsound.
To overcome these difficulties, a photoflash bulb was counterbalanced, ignited by means of the electric current, and weighed again after cooling. The entire experiment can be performed during the lesson in a few minutes without time-consuming preparation. Special bulb holders with dry cell batteries, such as used by photographers, may be purchased or easily constructed. In spacious lecture halls where larger balances are used for increased visibility, several photoflash bulbs may be used a t once. The bulb which is attached to the source of electricity may be held next to the others when igniting. The light energy will set off the others within a fraction of a second. This simple experiment is much more spectacular and stimulating than precipitation or decomposition reactions. It is more easily understood by the students and serves as excellent motivation for the review of the oxidation of metals previously studied. LITERATURE CITED
(1) Au~nun,"Lecture Demonstrations in General Chemistry." McGraw-Hill B w k Company, Inc., New York. 1939, pp.
(2)
42-3.
,.
editor's basket."
19, 394 (1942), .,Out of the
