This is a unique variation on electrolysis (using electricity to take water apart) that will let you test to see which side of a battery is positive, and which side is negative.
You will need:
- a nine volt battery
- two pieces of copper wire about 6 inches long
- a potato
- a knife
Carefully cut the potato in half. Remove about an inch of insulation from each end of each wire. Attach one end of a wire to one side of the battery. Attach one end of the other wire to the other terminal on the battery. You can use a little tape to hold the wires in place.
Stick the free ends of the wires into the cut part of the potato, about an inch apart. Wait and watch. Soon, one of the wires will start to give the potato around it a greenish color. That is the wire connected to the negative terminal. The other wire may develop some bubbles around it, depending on how much moisture there is in your potato. That is attached to the positive side of the battery.
What is happening? The electric current is causing electrolysis of the water in the potato, taking the water apart into hydrogen and oxygen. The hydrogen is released around the positive wire, forming bubbles if the potato is moist enough. The negative wire is producing oxygen, which combines with the copper of the wire, forming a green copper oxide. That is what colors the potato green.