A TEACHING DEVICE FOR REACTIONS RESULT-ING IN SHRINKAGE IN VOLUME

HAROLD J. ABRAHAMSPublic High Schools of Philadelphia, Pennsylvania

Among the difficulties which baffle beginning students in chemistryis the system:

2H2+02 ->2W

2 volumes+1 volume�-^"2 volumes (steam)

The balancing of the equation is straight-forward enough, but theuse of the coefficients to interpret the volume relationships leads tothe seeming fallacy that two volumes plus one volume yields two.This leaves the student with a feeling that he has wandered into alabyrinth from which there is no exit. The student is therefore con-fused by the demonstration in which ten volumes of hydrogen unitewith five volumes of oxygen in the eudiometer, producing ten volumesof steam, not fifteen, as he would have expected.The following device may be of use to teachers who like to rational-

ize such difficulties and thus demonstrate to their students that theyare not asked, in science, to take things upon faith, but rather thata little penetrating thought frequently provides logical answers toseemingly illogical phenomena in Nature.

Six wooden blocks, each measuring 2^" by 2^" and y thick, wereprepared and perforated in the center, so as to enable a two inchlength of dowel, of y thickness, to fit rather tightly into the perfora-tion. Four of these blocks were painted white and two painted black.Three dowel pins, each two inches long and y in diameter were alsoprepared. The white blocks were then assembled, two blocks on adowel pin, producing two pairs. The black blocks similarly treated,produced one pair.The explanation of the system

2H2+02 -^2H20

2 volumes4-1 volume�>2 volumes (steam)

may take the following form:The teacher may ask a student to take a place in front of the class

and arrange the blocks so that the white ones represent the hydrogenin the equation and the black ones represent the oxygen. When thisis done there will be two pairs of white blocks and one of black. Thisof course represents the left side of the equation. Now, if the studenthas not yet drawn the inference that each of the blocks in the pair

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represents one atom, or if some alert student has not yet volunteeredthe information, he may be told so by the teacher. He is now askedto change the arrangement of blocks so as to represent the right sideof the equation. To do so he will, of course, have to pull the blackpair apart and add one block (representing an atom of oxygen) to eachwhite pair (representing two atoms of hydrogen). The dowel pins havebeen made sufficiently long to more than accommodate the thirdblock. When the assembly is complete he will have two groups, eachwith two white and one black member (see Plate I).

Questions by the teacher bring out the following points:

1) There were six individual blocks to begin with.2) There were still six individual blocks at the end of the process.3) Nothing has therefore been created or destroyed (this provides

a review of the Law of the Conservation of Mass).4) The apparent fallacy of two plus one yielding two is brought

about by the fact that on the left side of the equation we are speakingof units, each of which consists of two parts (atoms), while on theright side the unit consists of three parts (two atoms of hydrogen andone atom of oxygen).

5) Our equation is therefore correct, as is the statement that twovolumes of hydrogen plus one volume of oxygen yields 2 volumes ofsteam.

This device may be used in connection with the teaching of prob-lems in Gay Lussac^s Law, the Conservation Law and wherever elsea teacher may find it applicable. It may prove useful in the presenta-tion of the principle of Le Chatelier and Braun. If the number ofblocks is changed to six white and two black, the device may be usedfor the system N2+3H2�>2NH3. Other uses will undoubtedly occurto the chemistry teacher.

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