Electrolysis of Water, Old School Syle

Introducing the class to the Hoffman apparatus.

Sometimes you have to pull out a classic demo to keep things real.  A few years ago I found a Hoffman apparatus in a back corner of the stock room.  I cleaned it up, ordered new stopper electrodes, and gave it a try.  This classic demonstration of the electrolysis of water is a great way to “see” the chemical formula of water.  We all learned at an early age that water is “H-2 O”.  But do you really believe it?  Now my students can say with confidence that water is definitely 2-parts hydrogen to 1-part oxygen.

Bleeding out the rest of the air.

Pouring in the sodium sulfate solution.

Today I asked for student volunteers to set up the Hoffman apparatus.  The blue liquid is water with sodium sulfate dissolved in it (a catalyst) and bromothymol blue (a pH indicator).  The apparatus is designed to collect hydrogen in one side and oxygen in the other.  Each side is marked off in milliliters so you can quantify the amount of gas produced in each chamber.   The rubber stopper in the bottom of each of the two tubes contains a metal electrode that I connect to a dc power supply.  The electric current that is passing through the water causes a decomposition reaction to occur.  Oxygen is collected at the anode, while hydrogen is collect at the cathode. 

Connecting the electrodes to the dc power supply.

Watching the color change.

You can see right away the ratio of hydrogen to oxygen produced.  At first one of my student volunteers thought he had done something wrong.  He said, “Mrs. Geyer, I think this side is leaking.”  I waited a while longer to see what the other students thought about the reaction.  As the volumes increased, constantly in a 2 to 1 ratio, another student said, “No, that’s what’s supposed to happen.  I have seen this before in my science class in Germany.”  He proceeded to explain the demo to the class.  Perfect!  I love it when a plan comes together.

The bromothymol blue is a nice addition to the demo.  Not only does it give the solution a beautiful color, but it also distinguishes the two electrodes.  This indicator turns yellow when it is in an acid solution and blue when it’s in a basic solution.  The color change happens very quickly after turning on the power supply. As you can see in the photos, the side with the small amount of gas is yellow and the larger volume of gas is blue. The reaction at the anode produces oxygen gas and hydrogen ions, which cause the pH to drop to the acidic range.  The cathode side produced hydrogen gas and the hydroxide ion, which increases the pH to the basic range.

Here you can see the 1:2 ratio of the gases.

The culminating event is the test of the two gases.  The video shows our simple test of the oxygen and hydrogen gases.  We opened the stopcock at the top of each tube and collected the gas in a test tube.  A glowing splint will flare up in the presence of oxygen.  A burning splint will cause the classic “hydrogen pop” with the hydrogen gas.  My student volunteers were eager to do the tests, even if one had to conquer her jitters about the pop.

 video of our test of the gases

Waiting for the color to change again.

When it was all done, we set it up again, but switched the electrodes.  We wanted to see if the pH indicator would change colors if we switched the anode and the cathode.  We waited for a while and finally the blue turned yellow, but we did not wait long enough to see the yellow one turn blue.  Time was running short and two more students were ready to test the oxygen and hydrogen produced.

This classic demonstration with the Hoffman apparatus is a great way to connect my students to the chemistry class that their parents took back in the day.  I hope that a few of these kids will mention it over the dinner table tonight.