Chad Hustings blogged this past school year about building his own Hoffman apparatus for each group of students. I have been using a Hoffman apparatus that had been purchased by my district before I began teaching there over 20 years ago to demonstrate electrolysis of water, but providing each student group with the ability to perform an electrolysis themselves is a powerful activity. I have used a different version of a homemade Hoffman apparatus previously, but after reading Chad's blog post, I decided to use a version closer to his. To save time, I chose to use batteries rather than building power sources from old parts like he did. I encourage you to read Chad's post because I have decided not to duplicate his explanation or references here. I hope you will find the worksheet and teacher notes I am posting to be useful follow-up materials to his original post.
If the Hoffman apparatus is built ahead of time (this takes about 5 minutes for each one if the teacher builds them), then the activity and discussion should take less than a 45 minute period.
- empty/clean 16 – 20 oz. sports drink bottle with label removed
- box cutter/scalpel
- 9-volt battery
- two metal thumbtacks
- 50-100 mL 0.1 M sodium sulfate
- clay/Play dough or some other material to hold the 9-volt battery in place
- two wide stem disposable beral pipettes or small test tubes to collect the gases
- 100 mL graduated cylinder. (You may choose to use Epsom salts to replace the sodium sulfate.)
Part A - Building the Hoffman apparatus:
I found it easiest to build the apparatus before having students use them. This avoids having them use the scalpel/box cutter. It is surprisingly easy to push the thumbtacks into the bottle cap. I did try using plastic cups instead of bottles, but the sport drink caps are sturdy and can be reused many times while the cups won't hold up. If re-using, you will likely need to replace the thumbtacks.
- Remove the cap from an empty and clean plastic sports drink bottle. Using the top of the lid, line it up with a 9-volt battery to see where the two battery contacts will be centered. Place the lid top-side up and gently push two metal thumbtacks into the lid so that they will line up with the battery contacts. The thumbtacks should not touch.
- Using a box cutter or scalpel, cut the bottle in half. The top part will look like a funnel. Screw the lid back on the bottle.
Set up the Hoffman apparatus as described below and observe.
Step 1. Using clay or paper towel as a holder, place the 9-volt battery into the center of the bottom of the container.
Step 2. Add about 50-100 mL of 0.1 M sodium sulfate (depending on size of the bottle) to the top portion of the apparatus.
Step 3. Place the top portion of the apparatus into the bottom portion (using it as a holder) so that the thumbtacks line up with the battery contacts.
Building a Hoffman apparatus for each group of students and setting out the other materials.
I put this activity together and tried it with my own students after reading Chad's blog. I used the ACS Energy Foundations link as a resource. You may be interested in more of their material, especially their middle school curriculum that uses the Hoffman apparatus to teach about electrons and covalent bonding.