Lemon battery

Use a lemon... to light up a diode or power a buzzer!

Difficulty:
Danger:
Duration:
10 minutes
Lemon battery

Reagents

Safety

  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

The lemon juice near the magnesium strip is hissing. Is this normal?

Yes, that’s alright. Magnesium is an active metal, and it is reacting with the citric acid in the lemon juice. This reaction yields magnesium citrate and releases hydrogen gas, which is making the hissing sound.

I don't have a whole lemon. Will a piece be enough?

Don't worry; use the piece of the lemon you have. Just make sure that the magnesium and copper aren’t touching each other.

The buzzer isn’t buzzing. What should I do?

First, make sure you’ve connected the copper rod to the red crocodile clip wire and the magnesium strip to the black crocodile clip wire.

Second, check that the red crocodile clip wire is clamped to the buzzer’s red wire, and the black crocodile clip wire is clamped to the buzzer’s black wire.

Make sure that the crocodile clips are clamped to the metal, not the insulation.

Step-by-step instructions

Connect pieces of two metals to the buzzer.

electricity-v2_lemon_en_iks-s-01

Insert the metals into the pulpy section of the lemon and listen!

electricity-v2_lemon_en_iks-s-02

Disposal

Dispose of solid waste together with household garbage. Pour solutions down the sink. Wash with an excess of water.

Scientific description

If a lemon battery can power a buzzer, it might seem that with enough lemons you could power your entire house. But don't go buying a ton of lemons just yet! As it turns out, the lemon's role in a "lemon battery" is not as prominent as it may seem. It's the two pieces of different metals submerged in lemon juice that are producing electricity.

What's the secret? Any tangible object contains electrons , but metals are special—they allow some of their electrons to move freely inside–and even amongst–themselves. Moreover, electrons cling to different metals differently. Copper, for example, can easily lure some electrons away from magnesium through the wire , thus creating an electric current. Magnesium on its own, however, won't let its electrons leave. That's where the lemon comes in. Tiny magnesium particles with some electrons missing (magnesium ions Mg2+ ) can abandon the metal chunk for the lemon juice, leaving the spare electrons free to go from Mg to Cu. To accept these Mg2+ ions , the lemon juice has to get rid of some other + (positive) ions. Luckily, lemon juice happens to contain a lot of citric acid, which can yield H+ ions  as acids do. Some of these ions can take electrons from the copper  and turn into hydrogen molecules H2 . These form hydrogen gas bubbles and leave the solution. The electrons will thus flow through the wire, making the buzzer buzz, until the piece of magnesium dissolves completely.

What if you desperately needed electricity but didn’t have any copper Cu or magnesium Mg? No worries! Some other pairs of metals would work just fine. To choose a good pair, you can use the chart chemists call the "metal reactivity series." A metal in this series will give electrons to any of the metals to its right (just like magnesium Mg gave electrons to copper Cu), and the farther apart the metals are in the series, the better they will push electrons through the wire.

If, perchance, you're out of lemons too, you can use any juicy fruit, vegetable, or even any solution that contains a lot of ions. Salt water, soda, or juice would work just fine.

How about a cucumber aluminum-silver battery? Or a soda zinc-gold one? Invent your own battery using the stuff you have at home! You can even use two cells to power an LED, like in the "Daniell cell" experiment. Keep in mind that the more reactive metal will slowly dissolve in the process, making the fruit or liquid you use unsuitable for consumption.

Why does the buzzer work?

The buzzer works because there is an electric current flowing through the wires. But where is the current coming from? In fact, in putting both a magnesium plate and a copper rod into a lemon, we created a galvanic cell – a chemical source of electricity – powerful enough to make the buzzer squeak.

How does this cell work?

The working principle of the cell is based on the difference in reactivity between copper and magnesium. Magnesium is a rather reactive metal; its atoms readily get rid of two electrons to form magnesium ions Mg2+. As these magnesium atoms develop a shortage of electrons, the plate develops a positive charge.

Copper is less reactive than magnesium, so if these two metals are included in the same electric cell, the electrons will travel from magnesium to copper – through the buzzer. This shift is what makes the buzzer work. Since electrons are negatively-charged particles, the copper rod accumulates an excessive negative charge.

Neither metal is comfortable under such conditions, and here the lemon comes into play. Or, rather, not the lemon itself, but the lemon juice, which contains citric acid. In a solution, citric acid partially dissociates into citrate anions and hydrogen ions H+ (or protons). In other words, it acts as an electrolyte solution – a solution that can conduct electricity. These protons take the excess of electrons from the copper rod to form hydrogen molecules:

2H+ + 2e → H2

At the same time, positively-charged magnesium ions leave the magnesium plate and pass into the solution, which causes the magnesium to gradually dissolve:

Mg0 – 2e → Mg2+

The process will continue until the magnesium plate dissolves completely.

How does an electrolyte solution work?

An electrolyte is usually a substance which can split into ions when dissolved. The resulting solution is called an electrolyte solution. Incidentally, citric acid is not the only substance that can act as an electrolyte. The same can be said for sodium chloride or almost any other water-soluble salt. As both positive ions (called cations) and negative ions (called anions) are released when an electrolyte dissolves, they can help maintain the balance between the charges in the cell, compensating for an excess of negative or positive charge from the metal plates. Without this balance, the battery cannot function.

What other metals can be used in this experiment?

If we just look at the metal reactivity series, we will find that the most reactive metals are situated on the left side, while the less active ones are on the right.

Li → K → Ba → Ca → Mg → Al → Zn → Fe → Sn → Pb → HCu → Hg → Ag → Pt → Au

Since the electrons move from the more active metal to the less active one, the cell will work if there is a sufficient difference in the reactivity between the two metals in it. For instance, copper Cu and zinc Zn would be another pair of metals suitable for the experiment.