Lemon battery

Make a sound or light up a diode... with a lemon!

Difficulty:
Danger:
Duration:
10 minutes
Experiment's video preview
Chemical formula

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

Juice is hissing around the magnesium plate. Is it normal?

Everything is alright. Magnesium is an active metal, and it reacts with citric acid. The reaction yields magnesium citrate and releases hydrogen.

Step-by-step instructions

Take two pieces of metal with differing activity: copper Cu and magnesium Mg.

electricity-v2_lemon-v2_en_iks_dif-buzzer-01

Connect the two metals with wire.

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Put the metals into your electrolyte solution— lemon juice.

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Disposal

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

Scientific description

Why does the buzzer work?

The buzzer works because there is an electric current flowing through the wires. But where does the current come from? In fact, when we put both a magnesium plate and a copper plate into a lemon, we create a galvanic cell – a chemical source of electricity. It is powerful enough to make the buzzer squeak.

How does the 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+. The magnesium atoms thus develop a deficit of electrons, and 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, an excessive negative charge accumulates on the copper rod.

Neither metal is comfortable under such conditions, and here the lemon helps them. Or, rather, not the lemon itself, but the lemon juice, which contains citric acid. In a solution, citric acid partly dissociates into citrate anions and hydrogen ions H+ or protons. In other words, it acts as an electrolyte solution – a solution which can conduct electricity. These protons then take the excessive electrons from the copper plate and form hydrogen molecules:

2H+ + 2e → H2

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

Mg0 – 2e → Mg2+

The process will continue till the magnesium plate is dissolved completely.

How does an electrolyte solution work?

An electrolyte is usually a substance which can split into ions when it dissolves. The resulting solution is called an electrolyte solution. Citric acid is not the only substance which can act as an electrolyte – sodium chloride or almost any other water-soluble salt can do it too. As both positive (cations) and negative (anions) ions are formed when an electrolyte dissolves, they can help maintain the balance between the charges in the cell, eliminating excessive 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 to the left, while the less active ones are to 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 example, copper Cu and zinc Zn would be another pair of metals suitable for the experiment.

That's interesting!

What is diode, and how does it work?

Diode is a tiny device that can conduct electricity (in one direction) and sometimes turn this energy into useful work. In our case, we will deal with a light-emitting diode (LED): it glows when passing through electrical current.

The base of all modern diodes is a semiconductor. The latter is a special material with quite poor electroconductivity that can, however, be increased – for instance, by heating. By the way, what is electroconductivity? It is ability of a material to conduct electrical current.

Interestingly, unlike a regular conductor, any diode contains two “types” of semiconductors. Even the word diode (from the Greek δίς) indicates that it comprises two elements: an anode and a cathode.

An anode in a diode is made of a semiconductor that contains so-called holes. They are the void regions that can be filled with electrons. These holes may be imagined as empty shelves designed specifically for electrons. Moreover, these “shelves” may to a certain degree freely move throughout the anode. A cathode in a diode is also made of a semiconductor. However, this second semiconductor is different: it contain excess of electrons that, again, can almost freely move throughout the cathode.

Notably, this construction of a diode allows electrons to freely pass through a diode in one direction, but prohibits their movement in the opposite direction. When electrons move from a cathode toward an anode, then on the border between these parts “free” electrons in the cathode meet with electron vacancies (“shelves”) in the anode. There, electrons gladly occupy these vacancies, allowing the current to move further. You can see this process in the video below.

Now, let’s imagine that electrons have to move in the opposite direction. They have to leave their cozy shelves and move into the material that has no shelves at all! Obviously, this way is no gain for electrons, and thus the current doesn’t flow in that direction.

Therefore, any diode may work as some sort of a check valve for electricity: it passes through a diode in one direction and cannot pass through in the opposite direction. This unique property provided for the use of diodes in electronics. Any computer, smartphone, laptop of a tablet has a processor containing millions of microscopic diode-like devices called transistors.

Light-emitting diodes, in turn, are employed in lighting and indicating. To make a diode that produces light, they thoroughly select its semiconductor components. In a certain combination of selected semiconductors, transfer of electrons from a cathode to vacancies in an anode is accompanied by emission of a photon, i.e. a portion of light. For various semiconductors, glow color is different. An important feature of diodes, in comparison with other electrical sources of light, is their safety and high efficiency, or degree of converting electrical current into light.