When Life Gives You Lemons, Make a Crude Electrochemical Battery

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Not quite the saying that we’re all familiar with, but it does add some flavor to it.

Here’s another Tuesday night project that I wound up immersed in: the lemon battery. I’ve seen this project posted in a multitude of places (along with the notorious potato battery), but I wasn’t quite sure how it worked. I knew that fruits are great conductors of electricity…but how? Naturally, when I encounter something I don’t quite understand, I obsess over it until I actually do.

[By the way, it takes a very special partner in crime to simply say, “Okay, honey,” when one spontaneously declares, “I want to build a lemon battery!” outside of any sensible context. Joe is clearly a winsome catch. And lab partner, to boot.]

Constructing the battery

Making the battery is pretty simple. First, squeeze the lemon to release the citric acid from the pulp — the juicier it is on the inside, the better. Then insert something made of mostly copper and something made of mostly zinc on opposite ends (I used a penny and a galvanized screw). Why these elements in particular? For one, they’re fairly easy to find in household items. Secondly, well, we’ll get to that in a little bit.

Hook up each end to some alligator clips if you have them, and connect those to a voltmeter or multimeter. (You can also do without the alligator clips and merely touch the tips of the voltmeter/multimeter wires to the copper and zinc ends.) Then, ta-da! You should see a charge! But…why?

What’s happening here?

The answer is an oxidation reduction reaction, or redox for short. Don’t be discouraged by how many syllables there are in that term!

As zinc enters citric acid (C6H8O7), it dissolves as positively charged ions (Zn2+); this is because it sheds the two electrons in its outer shell.

[Note: Because of the way electron valence shells are organized, zinc has 2 valence electrons in its outer shell, but it wants either 0 or 8 total to be more stable. So, it chooses the easier route — to shed 2 rather than gain 6.]

Typically, these shedded electrons will bond with hydrogen ions floating around in the citric acid to form H2, a gas that ends up bubbling off of the copper electrode.  This reaction is called oxidation (the giving of electrons):

Zn → Zn2+ + 2e-

2H++ 2e- → H2 (stable & gaseous)

Copper, which has one electron in its outer shell, will also give away its valence electron. However, because it has a greater potential for taking electrons, it will attract free electrons in the citric acid. The electrons in the citric acid lost to the copper are made up for by moving electrons from the zinc through the external wire, creating a current. This itself is called reduction (the taking of electrons).

Remember when I asked why copper and why zinc? This is why. We want one side to be more positive (cathode) and the other to be more negative (anode), just like in a real battery!

Since we’re dealing with electricity, we only care about electrons and how they move. As far as we’re concerned, electrons are negatively charged and are attracted to positive charge (opposites attract, no?). It’s this flow from negative toward positive that creates electricity.

As Bill Nye would say, “It’s not magic — it’s science!”

Next time life gives you lemons, don’t skip the lemonade, but don’t hesitate to think about batteries or electrons either!

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