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My son and I were using a mini science experiment kit for creating batteries with copper, zinc, and lemonade, similar to the famous lemon battery project. My son's hypothesis was that the liquid had to taste like lemonade to make a battery, and so he hypothesized (I thought correctly) that lime juice would work, but water and milk would not.

However, when we conducted our tests, the battery successfully ran a digital clock with all three liquids. I washed the cups and copper and zinc sheets thoroughly between each test, but the non-acidic liquids continued to function as batteries.

Did I do something wrong, or is the liquid not required to be acidic to make a battery?

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    $\begingroup$ Milk is slightly acidic with pH of about 6.5 $\endgroup$
    – wuschi
    Oct 10, 2015 at 18:19
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    $\begingroup$ You can use a potato as well - which I don't think is acidic $\endgroup$
    – Gerhard
    Oct 10, 2015 at 20:01
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    $\begingroup$ You might try cooking oil or distilled (not filtered but distilled) water as liquids which should not support the battery. It takes a remarkably small current to run a digital clock, so even very poor electrolyte solutions can do the job. $\endgroup$
    – Jason Patterson
    Oct 11, 2015 at 1:56
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    $\begingroup$ For reference, there are a few scholarly sources about lemon batteries. This one is nice and to the point. $\endgroup$
    – Michael DM Dryden
    Nov 9, 2015 at 22:15

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Sadly, the lemon battery is a bit of a cheat: it's not the lemon that's providing the energy at all. The very first battery ever was very similar to the lemon cell, but used damp cloth, or cardboard soaked in salty water (might be worth giving it a go, I've never tried).

The energy really comes from the metal of the two electrodes which have had lots of energy piled into them (heat etc) when they were refined. When you wonder where the energy comes from, think of the guys in the refinery and their furnaces, some of that is "in" the metal, and you're getting it back.

The purpose of the solution (or briney felt discs) is to give one part of this reaction (the electrons) a good run for their money, going through the electrical circuit, as they can't travel through the electrolyte, while the other part, the ions, can cross the other way, through the solution. They meet up at the other end, and the reaction is complete and the zinc slowly corrodes away.

So what you're testing is the quality of electrolytes. Salts and acids in solution make good electrolytes: salty water, lemons, potatoes, coke. Milk acts as an electrolyte because it contains some ions from salts (which nutritionists helpfully/confusingly call electrolytes).

Pure water is pretty bad as an electrolyte. There's one of two options. First, it's not absolutely useless as one, and maybe the clock requires so little power that water is good enough. The other option is that tap water is hard, or otherwise has salts (minerals etc) in it which increase the ion concentration greatly. Probably a combination of those two factors. It might be worth trying distilled water (if you have some for the car, for example) which is probably much poorer.

More about the "first battery" is here: it's lots of these metal-electrolyte-other-metal cells end-to-end. The zinc is slowly being corroded by the copper the necessary combination of a. ions (which are big, heavy old things that amble through the electrolyte) and b. poor, zippy, lightweight electrons which can't, and have to go "the long way round" through the electrical circuit. https://en.wikipedia.org/wiki/Voltaic_pile

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  • $\begingroup$ (Cells and batteries are an active area of research with a lot of money behind them. Parts of this answer are simplified to apply to traditional and educational cells and batteries, and scoot over special cases, which is still a good starting point to learn about more exciting varieties). $\endgroup$
    – Dan Sheppard
    Jul 7, 2016 at 0:32
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What is necessary is a solution with a sufficient conductivity (not necessarily an acid - could be any molecule able in solution, to ionize or dissociate into charged species). I don't know the conductivity of milk but I'd expect it to be very low due to the high content of fat. Anyways, I'd inspect what kind of preservatives or additives they add to the milk that could explain what you observed. :)

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    $\begingroup$ Milk usually has not more than ca 3.5% fat - which means that 96.5% are l water. And there are plenty of salts and proteins in milk, which are charged and can conduct electricity. Which are all natural and no additives ;) $\endgroup$
    – Gerhard
    Oct 10, 2015 at 20:04
  • $\begingroup$ In the incredibly unlikely circumstances that this interests someone: the conductivity of milk is a good indicator of mastitis, or related problems, as the low conductivity of milk increases with blood contamination (blood is salty). $\endgroup$
    – Dan Sheppard
    Jul 7, 2016 at 0:45

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