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A book of chemistry experiments for children suggests the following setup:

enter image description here Source: Adventures with Atoms and Molecules (Mebane and Rybolt)

Apparently, you begin with plain water in the cup; the light does not illuminate because there are no ions. Then, you sprinkle in salt water and the light comes on. As a result:

Salt is made of sodium and chlorine ions. The sodium ions have a positive charge. The chlorine ions have a negative charge. When you add salt to water, you are adding positive and negative ions to the water ... The flashlight does not come on until you add salt to the water. You see small bubbles at the negative paper clip. These are hydrogen gas. The hydrogen gas comes from hydrogen ions found in the water. When two positive hydrogen atoms pick up electrons, they combine to form a hydrogen gas molecule. Each hydrogen gas molecule is made of two hydrogen atoms.

How much current is necessary to make the hydrogen bubbles appear? We don't have a lantern battery, but I have some "D" cells. Would 4 "D" cells in series be able to supply the necessary current to make the bubbles, or do we truly need a lantern battery?

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  • \$\begingroup\$ Very little. Once your voltage is above a certain threshold electrolysis will happen. I just tried with tap water (which has a little bit of salts in it) and with 6v I get about 1mA for thin copper wire about one inch apart. In general the bigger the surface of the electrodes and the farther apart they are, the higher the current. Your problem is how much current you need to light the lamp. \$\endgroup\$ Commented Apr 21, 2023 at 0:34
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    \$\begingroup\$ Oops, correction: the bigger the electrodes and the closer they are, the higher the current. The volume of solution acts as a resistor. You could also use an LED, if you respect the polarities. You need distilled/deionized water and a very clean container because even traces of impurities can let a small current enough to see the LED starting to glow. With distilled water in the same glass I had salt water in, I get 60 uA that make a faint red glow appear. With tap water and a pinch of salt I get 5mA and a good red glow. (By comparison, in the same salt water without the LED I get 10 mA). \$\endgroup\$ Commented Apr 21, 2023 at 1:40
  • \$\begingroup\$ The OP's diagram does not give specs for the lamp, but it looks like an incandescent. In that case, it will have a fairly low resistance at first, but when it gets enough voltage to glow, the resistance increases. A 2-3 volt flashlight bulb might need 100 mA or more to reach full brightness. A 6V bulb would not leave any voltage across the electrodes at full brightness, so it will probably stabilize at 3-4 volts where it will just emit an orange-yellow glow. The LED is a good choice, but it might need a 220 ohm series resistor in case the salt water is very conductive, or if electrodes touch. \$\endgroup\$
    – PStechPaul
    Commented Apr 21, 2023 at 1:50

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The 4 D cells will be plenty... to light the lamp you only need enough current to light the lamp because it's a series circuit. To make bubbles you can use less current depending on how patient you are.

The advantage of the lantern battery is that making a reliable electrical connection is easier.

It will probably take quite a high salt concentration to get a good glow from the lamp.

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  • \$\begingroup\$ You write, 'depending on how patient you are.' Would a lantern battery make the bubbles appear more quickly? \$\endgroup\$
    – nuggethead
    Commented Apr 21, 2023 at 1:59
  • \$\begingroup\$ not significanlty faster, but much smaller batteries could be used if you are patient, \$\endgroup\$ Commented Apr 21, 2023 at 2:31
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    \$\begingroup\$ I don't think the size of the battery would matter. If the electrodes are as small as paper clips and the container is a cup, the current will be limited by the resistance of the volume of solution in the path of the ions. Even button cell batteries can give enough current to produce all the possible bubbles at a given voltage. I can see bubbles very clearly with just 1mA of current. \$\endgroup\$ Commented Apr 21, 2023 at 3:44
  • \$\begingroup\$ Tried this today, and 4 AA cells were plenty to make the bubbles. The bulb glows very dimly and it does take a very high salt concentration \$\endgroup\$
    – nuggethead
    Commented Apr 30, 2023 at 20:32

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