Hoffman Apparatus on the Go....

Hoffman apparatus

What are we doing to help kids achieve?

     We reached the topic of bonding and I wanted to do some demonstrations and/or experiments to get kids thinking. Thanks to my modeling friend and colleague Mary Palmer I decided to steal an idea from the American Modeling Teacher's Association. Here was the plan: The idea was to start with electricity.  Students first do an experiment with static electricity. They can take some small pieces of paper and rub a plastic ruler against their head or sweater. The paper then "jumps" to the ruler when it is close. Have students come up with a model or explanation with few words as possible. This serves two purposes...first, there is some formative assessment. It helps me to see where students are at in their understanding of charges and electricity. Second, it starts to plant the idea that "stickiness" and electricity are somehow connected.

     Next, I wanted to have the students work with a Hoffman apparatus. There are many places online to build a DIY Hoffman apparatus. The ACS offers an electrolysis of water lesson that includes a hand made Hoffman apparatus as part of a unit on energy that I used as a resource. For a power source I stopped by our local internet provider. They often get in old broken modems and throw them away. They are more than happy to provide me with the 9 volt power source. This saves me from having to use 9 volt batteries that get expensive. Next, I stopped by the local hobby shop to explore the remote control plane section. They sell these very thin and strong graphite/carbon rods used in building the wings of these planes. Cutting these up into 1 inch pieces makes nice electrodes that do not react with anything over time. I then strip some of the insulation off of the wires and purchase some heat shrink wrap at the same hobby store. I simply twist the bare wire on the carbon electrode add the shrink wrap with a little heat from a hair dryer and the wires are now connected to the electrodes. Next, it is time to "dumpster dive" into my recyclying bin at school and get some water bottles. I cut them in half. I then take a hot nail and poke a hole in the lid to fit the electrodes and add hot glue to helps stop leaks. The lid is screwed on and then carefully placed in a ring stand. The other half is used as a cheap beaker. Students are provided with .1 M sodium sulfate solution, told to pour it in the inverted top with the electrodes and plug it in.

  Bubbles started and so did the questions....Are there the same amount of bubbles on both sides? What are the bubbles? How do we capture them? Some students thought they were air or electricity. Mike Geyer came up with great teaching strategy...make a model of everything in the beaker. I convinced the students that the sodium sulfate was just a catalyst so let's look at the other items...water. We made models of water. One class then started asking...what happens if we took the models apart? Sure enough....we got a 2 to 1 ratio of hydrogen to oxygen...more bubbles on one side than the other. One group got some cheap plastic pipets and without me telling them started to capture the bubbles. When we placed a lit match to the one pipet...sure enough...there was a small popping sound indicating hydrogen.

     It did not work this well in every class but at least it was a start and provides ideas for the next time around. Many people have had issues with getting a 2:1 ratio of gases using sodium sulfate solution and have suggested that 1 M NaOH works well. I recently read this on the Modeling listserve and have not tried the different solution yet....I would like to suggest this might be a great experiment for students to tackle......Let me know what you think...

For more detailed information on building your own Hoffman apparatus, I suggest you follow the link to the Foundations of Energy website for lessons created by high school teachers and ACS. The resources there are available partly because of a grant from BP.



Students who demonstrate understanding can plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.

*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.


Students who demonstrate understanding can plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles. 

Assessment Boundary:

Assessment does not include Raoult’s law calculations of vapor pressure.


Emphasis is on understanding the strengths of forces between particles, not on naming specific intermolecular forces (such as dipole-dipole). Examples of particles could include ions, atoms, molecules, and networked materials (such as graphite). Examples of bulk properties of substances could include the melting point and boiling point, vapor pressure, and surface tension.

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Comments 1

Michael Caldwell | Wed, 03/01/2017 - 23:45

Here's a cool version of a Microscale Hoffman I found in an old textbook from RSC; my chem students made a bunch of these as a little project.  They are made with disposable pipettes, pencil graphite (from mechanical pencils) and glued with a hot glue gun.  Blu-tac sealed the bottom of the graphite electrode so it didn't leak everywhere!  We used 9V batteries to run them and it almost perfectly shows the 2:1 molar ratio when electrolysing water.