You can use either water from out of the tap or bottled water, it doesn’t matter which. Warmer water has a lower viscosity and allows ions that conduct electricity to move more freely.
Sodium chloride (which is table salt) is an electrolyte which helps aid the conductivity of the water, as water by itself isn’t particularly conductive. By making the water more conductive, the current from the battery flows through more easily which results in the water being split into hydrogen and oxygen more effectively. You can also using baking soda (NaHCO3) instead of table salt. This will not produce chlorine gas.
These graphite shafts that are encased within the pencil are your electrodes and will conduct the electricity that comes from the battery. The graphite works really well as it doesn’t dissolve or get damaged in the water while you conduct the experiment. To increase the rate of electrolysis, make the graphite rough with the help of a blade or sandpaper. Consider using bare pencil leads from mechanical pencils to vastly increase the surface area. But handle them carefully as they are brittle.
The purpose of the cardboard is to suspend the pencil graphite in the water without allowing it to touch the walls of the glass. Because cardboard has no metallic properties to it, it can sit on top of your glass without affecting the outcome of the experiment.
Use a 6-volt battery but if you cannot find one, use a 9-volt battery. You can find this size of battery at pretty much any convenience store or supermarket.
Doing this completes the connection to the battery and allows the current to be transferred from the battery all the way through into the water.
For this experiment to work, the graphite of the pencils needs to not be touching the side of the glass so just double check that here and adjust the pencils if need be.
Once you connect the alligator clips to the battery and graphite, the current begins to flow immediately. There will be more bubbles coming from the hydrogen pencil because there is twice as much hydrogen as oxygen in each water molecule.
