Demonstration of the pH changes during the electrolysis of water

edited bv -~ GEORGEL. GILBERT Denison University Granville. Ohio 43023

Demonstration of the pH Changes during the Electrolysis of Water

guish the plus from the minus pole of any strong enough source of direct current.

SWMITIED BY

A. Hablch and H. R. HBusermann Kanbnss~huleRamibohl Chemlsches lnstitut RBmIslrase 54 CH-8001 Zurlch. SwHzerlsnd

CHECKED BY

Methane Pistol ~ l mBYo

James F. Sklnner Williams College Williamsto~n,MA 01287

CHECK- BY

George Wollaston

Leonard C. Grotz

Clarion Unlvsr~lty Clarion, PA 16274

V. W. Center-Waukssha

I t is well known that during the electrolysis of water the region around the anode becomes acidic, the region around the cathode basic. We describe a very simple and quick method to demonstrate these changes in pH. A piece of pure white cotton is soaked (but not oversoaked) with tap water, placed in a Petri dish, and spotted with some drops of a suitable pH indicator. Using the cable plugs as electrodes, the electrolysis is now performed by a direct current of 10 to 15 V, pressing the plugs in an appropriate position away from one another onto the indicator spot. Within seconds, the acidic color of the indicator is visible at the anode, the basic one a t the cathode. We have got the best result with the van den Burg universal indicator (Siegfried AG, CH-4800 Zofingen, Switzerland). turnine from ereen to red and then to blue. Also suitable is th;~erck 6175 or the B.D.H. universal indicator. Even bromthvmol blnemav beused, if one takes care to start with the neutral green col&. Reolacina the cotton bv a piece of paper, preferably indir&ult, heia&e the paper cannot cator'paper; shows a hold ent,uah water and the contact area between the plugs and the paper is too small. As a source of direct current, a pocket battery of 4.5 V can also be used, pressing its contacts directly onto the cotton. However, because of the low voltage, color changes are rather slow. Obviously, this simple method is also suitable to distin-

Humor, a touch of theater, and the unexpected are essential components of effective teaching, particularly for a subject such as chemistry, which is not inherently interesting to all students. The simple demonstration described below should leave a lasting impression of the importance of intermolecular forces and hydrogen bonding. The instructor brings two empty plastic water pistols to class. With a bit of rubber tubing and a disposable Pasteur pipet, he or she proceeds in view of the class to flush one pistol with methane from a gas outlet, pointing out that the pistol is now filled with methane. He then points the pistol a t the class and conspicuously pulls the trigger several times, nerhaos once near his nose or ear toconvince them that eas is cuming out of the pixtol. While commenting that methane pistolsare not likely to be murhofa hit on the toy market, he writesan expanded structureof methane on the blackboard. Now the second ~ i s t o is l filled with water in front of the class and squirted'out over their heads several times. An expanded structure of the water molecule, with the two nonbonding lone pairs of electrons clearly indicated, is drawn on the blackboard. While the two substances have similar molecular weights, their properties are clearly very different, one a dilute gas, the other a condensed liquid a t ambient conditions. A discussion of electronegatives of C, H, and 0 , bond polarity, dipole moments, hydrogen bonding, and molecular association can now proceed.

Waukesha, WI 53188

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Volume 64

Number 2

February 1987

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171