The labs we did today came from the Home Scientist CKO1A instruction manual which goes along with their chemistry kit. In Session X-1: Observe Electrolysis there are two labs, in the first one you set up two test tubes full of water (with some epsom salts) upside down in a beaker and then place the wire ends of the battery adapter into the tubes. Once you connect a battery, current starts to flow through the water breaking it down to create hydrogen and oxygen gases in the test tubes. We tried to set it up like the lab describes but the battery adapters I bought had short leads and we got pretty frustrated with it. Luckily I happened to have two electrolysis set ups (see photo below) that were loaned to us by another homeschool mom.
The black stand in the bottom of the beaker holds the test tubes in place and the screws are the electrodes, so all we had to do was connect the wires to the battery (after filling the tubes with water). The other advantage of using this set up is that the test tubes were very small so it didn’t take as long to fill up with gas (though it still took almost an hour). We skipped measuring the volume of the gas but did notice that one test tube filled up twice as fast as the other because you make 2 hydrogen molecules for every oxygen molecule produced. The blue arrows in the photo above point to the water level in the test tubes.
While this was bubbling away, I set up the second part of the lab which involved doing electrolysis with salt water. We took 50 ml of water and a tsp of salt, stirred until it was dissolved and then made electrodes out of Al foil. We took this lab outside since it produces chlorine gas. We also put a few drops of phenolphthalein pH indicator in the salt water. Phenolphthalein is clear in neutral solutions but turns a bright pink in basic solutions. I hadn’t done this before so I was as suprised as the students when I hooked up the battery and the solution turned pink starting at one electrode and making its way away across the beaker. The bubbling gas production was also quite vigorous. We weren’t prepared for how quickly this took place so I had the students repeat it themselves and we took lots of movies and pictures. If you leave it hooked up for more than a few seconds the Al foil starts to break apart. When the students did it, the reaction didn’t seem quite as vigorous and that’s because I used a regular teaspoon to put in the salt and they used the chemical spatula which gave them less salt. Here’s a video of the electrolysis of salt water.
The solution turns pink because sodium hydroxide (a base) is formed along with hydrogen gas at the foil electrode (cathode) where you first see the solution turn pink. Chlorine gas is formed at the anode but most of it dissolves into the water. Very cool little experiment.
I had the students watch Tyler Dewitt’s video on Electrochemistry before class.
He also has this great video which explains exactly what was happening in both labs we did.