Experiment: Electrolysis of Water

The students love this one! It is classic chemistry experiment that is an easy crowd pleaser.

First, students are introduced to the concept of decomposition, i.e. AB →  A + B. Electrolysis of water is a satisfying experiment because it visually demonstrates the chemical components of water.

Almost all students are able to tell you that the chemical formula for water is H2O. Most students can identify that in a water molecule, there are two hydrogen atoms for each oxygen atom. Taking it a step further, when we pass an electrical current through water in a voltameter, water decomposes into hydrogen ions (positive ions) and hydroxide ions (negative ions). The hydrogen ions move towards the cathode (negative electrode) and the hydroxide ions move towards the anode (positive electrode). Opposites attract!

Extension: We can say that a redox reaction has occurred at the cathode and anode. Electrons at the cathode are given to the hydrogen ions to form hydrogen gas (reduction reaction). Similarly, electrons at the anode are given to the hydroxide ions to form oxygen gas (oxidation reaction).

For my classes, the concept that I wanted the students to take away was the sheer elegance of the 2:1 hydrogen to oxygen ratio and recognise this experiment as a decomposition reaction. I did not go into detail about the movement of electrons at the anode and cathode. I emphasised more that there was strong attraction between the ions and the electrodes, which broke down the water molecules.

Test for oxygen gas

The test for oxygen gas is the glowing splint test. Collect oxygen gas from the top of the voltameter and put your thumb on it to make sure the gas doesn’t escape. A splint is lit and burned until the wood is bright orange/red. Wave the splint until the flames are out and stick it into the test tube of oxygen gas. The splint should relight because everything needed for combustion is present (oxygen + fuel).

I tried this for each of my two classes. The first was a failure, there was no rekindling. The second time I used more oxygen gas and the class saw sparks. Both times, I stuck the glowing splint into the test tube sideways. Suggestions for improvement are most welcome. What I will try next time is holding the glowing splint directly above the glass tube of the voltameter and releasing the oxygen gas directly.

Test for hydrogen gas

The pop test! Enough said. A lighted splint it held up near the opening of a test tube filled with hydrogen gas. Hydrogen reacts with oxygen to create a small explosion- pop!

This worked very well with both my classes. The girls were told to expect a small pop (so they wouldn’t be scared).

Overall, a very satisfying experiment. In the lesson, students also conducted a separate decomposition experiment: decomposing calcium carbonate into calcium oxide and carbon dioxide. In that experiment, students tested for the presence of carbon dioxide. The visual and practical nature of the experiments made understanding the concepts achievable and the tasks enjoyable.

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