A METHOD OF TEACHING EQUATION WRITING C m m s H. STONE.ENGLISH HIGHSCHOOL, BOSTON, MASSACHUSETTS
The importance of the equation in any course in chemistry is obvious. The ability to write chemical equations correctly is of the first importance, hut is often acquired by the beginner only with difficultyand after continued practice. Too often the method of presentation is uninteresting and dry and the student comes to regard equations as a necessary evil and a bugbear. But the subject can he presented in such a way as to become interesting instead of unattractive to the student. The writer has used the method described below for a number of years with good results. Previous work on valence and formula writing has, of course, been taken up. Arrange upon the lecture desk a dozen or more 50-cc. graduates and fill each about half full of water. Behind each graduate place two bottles containing solutions of reagents which, when mixed, will a precipitate. For example, the first pair of bottles might contain silver nitrate and hydrochloric acid. The bottles should be arranged in such order that the successive equations will increase in difficulty but not too rapidly. The following pairs of reagents may he used: Mercurous nitrate and potassium iodide Silver nitrate and sodium chloride Silver nitrate and potassium iodide Copper sulfate and sodium carbonate
Barium chloride and zinc sulfate Lead nitrate and potassium iodide Mercuric chloride and potassium iodide Lead nitrate and potassium chromate
Many other pairs will suggest themselves to the interested teacher. Such difficult combinations as ferric chloride agd potassium ferrocyanide or aluminum sulfate and barium chloride should be deferred until the students have acquired a considerable mastery of the simpler equations. When all is ready, the teacher may explain briefly the method of procedure and then call Johnny down to the desk to perform the first experiment. He pours together into the first graduate silver nitrate and potassium chromate, let us say. A red precipitate is formed. He is then asked to write the equation on the blackboard, the teacher making needful suggestions at first. Then comes up the question of color. From the equation it is apparent that two products are formed, silver chromate and potassium nitrate. Which one of these is the red one? At this point the teacher may put forward a bottle containing solid potassium nitrate and ask the class to note the color of it and then to draw their conclusions. At once they will arrive a t the conclusion that the red precipitate must he silver chromate. The color is then indicated on the board by writing the word "red" above or after the formula for the silver chromate. Johnny then sits down and Jennie is called to the desk and the process repeated with the next pair of solutions. Since a little skill in arranging the pairs of bottles will prevent the continued recurrence of the same color, an expectant attitude regarding the next color develops in the class. 1332
VOL.7, No. 6
TEACHING EQUATION WRITING
1333
With the more difficult equations, such as the reactions between the chlorides of arsenic and antimony with hydrogen sulfide, the question of coefficients will of course come up. A spirited class discussion may be started and the question threshed out to the satisfaction of the students. Some surprising things may be introduced to heighten the interest. For example, the reaction between a little lead nitrate and hydrogen sulfide produces a black precipitate. Addition of hydrogen peroxide will change this to white lead sulfate by oxidation. The precipitate formed by potassium iodide and mercuric chloride will disao~earif more ootassium iodide is added, as will, also, the white precipitate formed by the action of sodium hydroxide on soluble lead salts when more sodium hydroxide is added, the plumbite being formed. This series of experiments can be continued through several recitations a t the teacher's option, gradually leading up to the more difficult equations for reactions in solution. The class exercises should be supplemented by similar exercises in the laboratory. It is not unusual to hear students in the laboratory remark: "Gee! This is the most interesting experiment yet!" A A
Cadmium Oleate a s an Impremating Material. Dr. H. J. B u m describes . (Chem.-Ztg., Nov. 29, 1929) the use of cadmium oleate as an impregnating material. Cadmium hydroxide after treatment with olein and dehydration gives a brown product which is soluble in many organic solvents, especi+llybenzene, toluene, xylol, and amyl acetate, yielding, a t suitable concentrations, almost colorless jellies. Colloidally dispersed in ammonia, in which i t gives stable dispersions, forms an excellent impregnating material. Tiles, as well as linen of suitable mesh, become impervious t o water after impregnation and drying a t the ordinary temperature. Filter paper or ordinary hlotting paper shaped into bags can be used t o carry water. Tests of the durability of the impregnating material are, as yet, incomplete, hut tiles and pieces of unglazed clay have remained unchanged for years after impregnation. Results with cellulose lacquers, however, are less satisfactory. Other cadmium compounds of the fatty acids such as the palmitate and the stearate show similar but graduated properties.-Chem. & Ind. Richard Chenevix. There were two Richard Chenevix, if the plural of the proper noun is thus correctly rendered; we are concerned with the chemist and mineralogist, son of the bishop's nephew. Born in 1774. probably in Dublin, he began in 1800 to publish scientific communications in Nicholson's Journal, and in 1801 in the Philosophical Transactions of the Royal Society, of which society he was in the same year elected a Fellow. He examined Hatchett's newly discovered "columbian," he investigated the magnetic properties of nickel, and made a special study of palladium and platinum. The former precious metal was regarded by him as an alloy of platinum with mercury, but i t was subsequently classed as a new metal by Wollaston, who had also discovered platinum. Chenevix' contributions t o chemical science were by no means insignificant, nor were they unrecognized, for in 1803 he was awarded, far his various researches, the Copley gold medal of the Royal Society. It may he remembered that in February of last year we referred t o the centenary of the death of WoUaston, Chenevix' successful rival, and indulged in some thoughts concerning the survivor. Time passes, and it is the survivor's death, on April 5, 1930, that we now recall.-Chem. & Ind.
