* Citric acid is a weak organic acid with the molecular formula C6H8O7, which means a molecule of the acid contains six carbon atoms, eight hydrogen atoms, and seven oxygen atoms. When it is dissolved in water, it partially ionizes to yield three H+ ions (hydrogen atoms which are missing their electron) and a C6H5O7(3-) ion (the 3- means that it has three extra electrons.) The resulting solution is called an "electrolyte." Chemical Reaction
When two dissimilar metals are put in an electrolyte, a chemical reaction called "oxidation-reduction" occurs. Because different metals have different levels of attraction for their electrons, one of the metals will lose electrons, a process which chemists call oxidation, and the other gains electrons, which is called reduction. The metal being oxidized is called the anode, and the one being reduced is the cathode. The entire arrangement of anode, electrolyte, cathode and conductor is called a galvanic cell. If the metals are then connected outside of the solution by an electrical conductor like a wire, a current of electrons will flow in the wire from the anode to the cathode. Inside the solution, positively charged ions from the oxidizing metal will migrate to the other metal, where they will again receive electrons and plate that metal. Lemon Cell
While there are many electrolytes which will cause this reaction, citric acid is frequently used as a demonstration because it occurs naturally in citrus fruits. The two metals often used are zinc, as in a galvanized nail, for the anode and copper, as in a penny, for the cathode. A lemon can be used as both the container and the source of citric acid. If a slit is made in one end of the lemon, the penny can be inserted about half-way. A galvanized nail is pushed into the lemon close to the other end. This is not a battery that will power anything useful, but a sensitive voltmeter will indicate that it does in fact produce electricity.