Most people know that water covers the majority of the surface of the earth. With a chemical formula everyone can recognize, H2O, water contains the elements hydrogen and oxygen. But what happens when you try to break these elements apart? This process is actually easier than you might think, and can be the perfect quick lab activity to get students thinking about some of the basics of chemistry in the back to school season.
Water is a relatively stable compound and won’t be split apart easily, we need some sort of energy input to fuel this reaction.
Breaking water into hydrogen and oxygen can be done a few ways, but one of the easiest and most common is through a process called electrolysis. Electrolysis uses electricity to provide the energy needed to split water molecules. Some of our more advanced students out there might remember a few other ways of breaking up water (like hydrolysis) which utilize energy in chemical bonds of other nearby molecules. Why not heat up the water to provide this energy I hear you ask. Well water will boil and turn into a gas before it breaks apart (decomposes). It will eventually decompose into hydrogen and oxygen if you keep heating up the gas to a balmy 3,000 degrees C.
Why does electricity work but a flame wouldn’t? This begins to get into some electrochemistry. The oxygen in water is covalently bound to hydrogen meaning they’re sharing electrons. Breaking these bonds would mean electrons would have to go to either the hydrogen or the oxygen creating ions. Electricity is simply the flow of electrons from an outside source. These electrons will provide the energy required to break the covalent bonds and stabilize ion intermediates in order for water to split into hydrogen and oxygen.
Once split apart, hydrogen and oxygen have lots of energy. Both hydrogen and oxygen are even used as rocket fuel when they’re cooled down to their liquid states! When burned together, they release tons of energy as heat along with water vapor as a byproduct. This is why you can sometimes see a rainbow after some rocket launches.
It might seem like tons of energy is needed to break water apart, but this is deceiving. The reduction potential for the electrolysis of water (basically how much electricity is needed for electrolysis) is between 1.2 and 1.5 volts. That means you can easily watch electrolysis happen with a 9V battery. There’s a great wikihow describing how to do this at home but we’ll summarize it below.
Take a plastic cup and fill it with water. Add about a spoonful of salt (although I find baking soda works better) and stir until it all dissolves. Then take two push pins and punch them through the bottom of the cup so that the pointy end is coming out into the water. Finally, take your 9V battery and attach the positive and negative ends to the ends of the push pins. If all went as planned, you should see bubbles coming out of the push pins in the water. These bubbles will be oxygen and hydrogen. But how do you know which push pin is producing hydrogen and which is producing oxygen? There are a few ways of testing for this. The first is to carefully hold a match near to where the bubbles are coming from. The string of bubbles that makes small popping sounds is hydrogen. Another test is to see how much gas is being produced. Hydrogen will be produced faster than oxygen since in H2O there are twice as many hydrogen atoms compared to oxygen.
Cleanup is easy. Simply disconnect the battery and wash out the remaining solution with plenty of water or neutralize with a mild acid like vinegar. This is because one of the byproducts is lye, a base, and this can cause some issues with older pipes.
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