CORRESPONDENCE ANALOGIES IN TEACHING FRESHMAN CHEMISTRY

To the Editor containing one gram-atom of one of the following ele-

DEAR SIR: ments: H, C, S, Cu, E'e, Ni, Pb, Sn, Hg, etc., as well as samples of one mol of each of the following: water, I found the paper on "Analogies in Teaching Fresh-

salt, baking soda, man Chemistry" U. CKEM. EDUC., 10, 627 (1933)l interesting, and I intend to use some of the analogies in Each bottle is labeled as follows: the course I am offering.

Some of your readers may be interested in the follow- ing analogy I have been using to explain the meaning of gram-atom and mol:

Suppose you are manufacturing bead purses and you are using four kinds of heads, referred to as beads H, C, N, and 0. Sup- pose you are using these beads in equal quantities, and you wanted to order a shipment of beads f rov the glass factory.

You would not ask the manufacturer t o count the beads for you. Instead, you would weigh the individual beads, and if their weights are as 1:12:14:16, you would order 12 pounds of C, 14 pounds of N, and 16 pounds of 0 for every pound of H you order.

Similarly, if you are told that the atomic weights of C, N, 0 are 12, 14, and 16, respectively, and if you wanted to obtain as many atoms of each as there are atoms of hydrogen in one gram of that substance, you would use 12,14, and 16 grams of the three substances, respectively.

From now on, we shall refer to these quantities as one gram- atom of the respective elements. They all contain the same number of atoms and that number is approximately equal to 6 X l O S 3 ; we shall refer to that number as N or Awogadro's number.

I t is evident that N molecules of HnO will weigh 18 grams. From now on we shall refer to N molecules of any compound as a mol of that substance.

This explanation is usually followed by an exhibit consistingof a cardboard box whose volume is approxi- mately 11.1 liters and of several wide-mouthed bottles

Element :- Atomic weight :- This bottle contains: -Grams. One gram-atom. N atoms.

I agree with John R. Lewis, the author of the paper, that reactions produced in the students' minds may be entirely foreign to the ones the teacher wished to ob- tain. In this connection I should like to relate an amusing illustration.

While explaining metathesis, I used as an illustration the case of two couples, AX and BY, who shuffled ~jartners, making use of the institutions of divorce and second marriage. By analogy, the students gave the correct answer for the products of reaction between silver nitrate and sodium chloride.

On the following week I was giving the students an explanation of the tendency of chlorine atoms to

' combine in pairs to form the molecule. One of the stndents asked me whether chlorine atoms could propa-. gate their kind by union, "just as sodium chloride molecules and bacteria could."

MILTON J. POLISSAR ARMSTRONG COLLEGE

BERKELEY, CALIP.