The process by which we separate the elements hydrogen and oxygen from water is called electrolysis. The word "lysis" means to dissolve or break apart, so the word "electrolysis" literally means to break something apart (in this case water) using electricity. It is difficult to break up water into it’s elements lots of energy is needed to do that. Through a chemical means though, we can separate the compound of water into the elements that combine to make it. Electrolysis is very easy - all you have to do is arrange for electricity to pass through some water between to electrodes placed in the water. It’s as simple as that!
CAUTION! HEALTH & SAFETY: The reaction neutralizes the solution, which can be disposed of by pouring it down the drain.
A battery or solar panel with a voltage greater than 1.5 volts - 9 volt batteries work well. Two pieces of electrical wire about a foot long. It’s convenient, but not necessary, if the wires have alligator clips at each end.
Two number 2 pencils
small piece of cardboard
electrical or masking tape.
1 teaspoon Epsom Salt
1 tablespoon red cabbage juice
1. Remove the erasers and their metal sleeves from both pencils, and sharpen both ends of both pencils.
2. Fill the glass with warm water and add 1 teaspoon of Epsom salt and 1 tablespoon of red cabbage juice.
3. Stir the mixture thoroughly.
4. Pour some of the water mixture into the petri dish until it is ¾ of the way full. 5. Attach wires to the electrodes on the solar cell or battery, and the other ends to the tips of the pencils, as shown in the diagram below. It is important to make good contact with the graphite (lead) in the pencils. Secure the wires with tape.
6. Punch small holes in the cardboard, and push the pencils through the holes, as shown in the diagram below.
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7. Place the exposed tips of the pencils in the water, such that the tips are fully submerged but are not touching the bottom, and adjust the cardboard to hold the pencils. 8. Wait for a minute or so: Small bubbles should soon form on the tips of the pencils. Hydrogen bubbles will form on one tip (associated with the negative battery terminal - the cathode) and oxygen from the other.
Attach the wires to the positive and negative poles of the battery or solar cell.
Make good contact with the graphite in the pencils.
Where’s the chemistry?
The chemical equation for electrolysis is:
energy (electricity) + 2 H2O -> O2 + 2 H2 .
At the cathode (the negative electrode), there is a negative charge created by the battery. This means that there is an electrical pressure to push electrons into the water at this end. At the anode (the positive electrode), there is a positive charge, so that electrode would like to absorb electrons. But the water isn't a very good conductor. Instead, in order for there to be a flow of charge all the way around the circuit, water molecules near the cathode are split up into a positively charged hydrogen ion, which is symbolized as H+ in the diagram above (this is just the hydrogen atom without its electron, i.e. the nucleus of the hydrogen atom, which is just a single proton), and a negatively charged "hydroxide" ion, symbolized OH-:
H2O -> H+ + OH- .
You might have expected that H2O would break up into an H and an OH (the same atoms but with neutral charges) instead, but this doesn't happen because the oxygen atom more strongly attracts the electron from the H - it steals it (we say the oxygen atom is more "electronegative" than hydrogen). This theft allows the resulting hydroxide ion to have a completely filled outer shell, making it more stable.
But the H+, which is just a naked proton, is now free to pick up an electron (symbolized e-)...
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