One of the first experiments we undertook enabled us to observe the process of hydrogen electrolysis to separate the elements in water .. hydrogen and oxygen ... into their gaseous state. The setup was very simple and the kids were ecstatic with what they observed. We had to look closely ... as the bubbles were small ... but we indeed could see the accumulation of oxygen gas (O2) and hydrogen gas (H2) on the electrodes (we used two pencil leads, a polymer carbon, not the element we know of as Lead).
As we all know, the chemical formula for water is H2O, meaning there are twice as many hydrogen molecules as oxygen molecules. When electricity is introduced to water, the molecules split with hydrogen gas (H2) appearing at the cathode. Since there are more hydrogen molecules than oxygen molecules in water, the electrode that the hydrogen goes to will have more bubbles.
[ Admin Note: I was able to find two diagrams that illustrate this process - however, you may wish to flip one image horizontally so that the cathodes are on the same side of the image (as they are shown, the images are criss-crossed). ]
The image shown below is of Buddy's journal entry. As they are young yet, I didn't discuss with them the process of reduction or oxidation. Our focus was simply to understand that by passing a current of electricity through the water, we could separate the atoms of water to get a pure element.
I have been reading aloud Kathleen Krull's Marie Curie (Giants of Science) and through much of the book, it discusses that Marie and her husband Pierre worked tirelessly to isolate radium from a complex compound called Pitchblende. This simple electrolysis experiment gave the kiddos a small window through which they could understand this process.
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