Saltwater Pentacell

1. Cut the stranded copper wire into five sections of 4 inches (10 cm) each. Strip 2 inches (5 cm) of insulation off one end of each of the five pieces, and then strip 1 inch (2.5 cm) of insulation off the other end of each piece. This will leave a 1-inch (2.5 cm) sleeve of insulation holding the bundle of fine wires together. Twist the strands at the 1-inch (2.5 cm) end of each piece tightly together. Then separate the strands of each 2-inch (5 cm) end so the loose strands look something like a broom (see photo below). These are your copper electrodes.
   
2. Cut five pieces of aluminum foil, each about 4 x 4 inches (10 x 10 cm) square. Fold each piece in half, and then in half again, parallel to the first fold, so that the foil square ends up four layers thick, with final dimensions approximately 1 x 4 inches (2.5 x 10 cm). These are your aluminum electrodes.
3. Add the salt to the water and stir. This is the electrolyte solution—a liquid that can conduct electricity.
4. Fill each plastic cup about three-quarters full of the electrolyte solution. Then put one aluminum electrode and one copper electrode in each cup. The broomlike end of the copper electrode should be in the solution.
5. Each cup and its electrodes make up one saltwater cell. Connect the cells in series by clipping alligator-clip leads from the copper electrode of one cup to the aluminum electrode of the next cup, and so on, until all five cells are connected (see photo above). As you attach each alligator clip to an electrode, you can simultaneously clip the electrode to the top of the cup to hold it in place, as shown in the photo below. At the end of the series, the aluminum electrode in the first cup and the copper electrode in the fifth cup should be left unconnected.
   
6. Adjust the two electrodes inside each cup as necessary to make sure that they don’t touch each other.

Use alligator-clip leads to connect the aluminum electrode in the first cup to one leg of the LED, and the copper electrode in the fifth cup to the other leg.

Did the LED light up? Sometimes you have to look directly into the end of an LED to tell if it’s on. If you’re in doubt, darken the room or cup your hands around the LED to block the ambient light.

If the LED isn’t lit, reverse the legs. (A diode—in this case a light-emitting diode, or LED—allows current to flow only in one direction. If it’s connected “backwards,” it won’t light.)

If the LED still doesn’t light, try adding 1/2 teaspoon (2.5 mL) of vinegar to each cup and stirring. The acidity of water varies from place to place, and if your water is not acidic enough, the vinegar may make a difference. If your water is already acidic enough, you probably won’t need to use the vinegar.

After the LED is lit with five cells, try using four cells. If it lights, then try using three cells. What is the smallest number of cells that will do the job?

Each cup, with its electrodes and electrolyte solution, is a simple electrochemical cell. The two electrodes are made of dissimilar materials (in this case, two different metals) with different chemical activities. A tug-of-war for electrons occurs between the two electrodes, resulting in a potential difference, or voltage. In the cells you’ve made, aluminum is the more active metal—atoms of aluminum lose their electrons more easily than do atoms of copper. The potential difference causes electrons lost by the atoms in the aluminum electrode to travel through the LED to the copper electrode, and this flow of electrons is the electric current that lights the LED.

If this flow of electrons continued, and nothing else happened, then fairly quickly there would be a buildup of electrons on the copper electrode and a shortage of electrons on the aluminum electrode.

Because electrons have a negative charge, this would result in the copper electrode becoming negatively charged and the aluminum electrode becoming positively charged. Additional electrons that tried to move from the aluminum to the copper would be repelled by the copper and attracted back to the aluminum, and electron flow would stop.

This is where the saltwater electrolyte solution comes into play. Table salt is sodium chloride, and when it’s dissolved in water, it forms positive sodium ions and negative chloride ions. The positive sodium ions are attracted to the negative copper electrode, where they participate in neutralizing the extra negative charge through chemical reactions. Likewise, the negative chloride ions are attracted to the positive aluminum electrode, where they participate in neutralizing the extra positive charge. Therefore, there’s a constant flow of charge from one electrode through the LED to the other electrode, and then through the electrolyte solution, forming a complete circuit.

The five cells make up a battery when they’re connected in series. (A battery is two or more electric cells that are joined together.) The five-cell battery has five times the voltage of each individual cell.

It takes a minimum voltage to light an LED. If you don’t have enough cells, you won’t provide the necessary voltage.

In any electrochemical cell, the greater the difference in the activity of the two materials making up the electrodes, the greater the strength (voltage) of the cell. Since chemical reactions are taking place at the electrodes, the larger the area of the electrodes, the greater the number of electrons that can be pulled per second, and the larger the current (measured in amperes, or amps).

The big idea here is the difference in the abilities of two materials to lose and gain electrons. This same idea is at the heart of the wide variety of batteries used for everything from flashlights to cell phones. The materials, size, and shape of these batteries may differ from those of this saltwater pentacell, but the general principle remains the same.

A solid choice?

Why does this Snack use stranded wire with the wires spread apart? Try substituting a piece of 18- or 20-gauge solid copper wire. Do you get the same results?

Metals are not created equal

Try using other metals for electrodes. Can you find metals that will allow you to light the LED using fewer cells? Galvanized nails can be used for zinc, regular iron nails for iron, old silverware for silver, and brass hardware for brass. (A commonly available non-metallic material that can also act as an electrode is carbon pencil lead.)

Measure it

If you have an electrical meter available, try making quantitative measurements of voltage and current for different combinations of metals.

Make a buzz

Try substituting a 1- to 3-volt piezoelectric buzzer for the LED.

We were introduced to this Snack by Art Morrill.

Shakashiri, Bassam. Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 4. Madison: University of Wisconsin Press, 1992. See pages. 91–95 for a general discussion of batteries.