Electrochemistry, Science teaching
In many general chemistry texts (1), the electrochemistry chapter is introduced with a picture of a “lemon cell”, leads of Zn and Cu piercing a lemon (or sometimes a grapefruit). A voltmeter connected across them shows a potential of about 1 V. (It has been shown how several fruits connected in series can be used to run a calculator .) The texts then discuss oxidation–reduction, half-cells, and the rest of what we teach freshmen about electrochemistry, and the lemon is never mentioned again.
It would appear to be questionable scientific pedagogy to present something, even something as neat as a lemon cell, without explaining how it works. One risks having a thoughtful student ask for an explanation or, worse, having the student assume that the lemon cell illustrates what follows in the text. Generally, this includes the statement that an electrochemical cell requires separation of the oxidation and reduction half-cells, as illustrated by the Daniell cell (Zn2+/Zn and Cu2+/Cu half-cells connected by a salt bridge), presentation of the electrochemical series (half-cell reduction potentials), and use of the Nernst equation. In the lemon cell, there is only one solution, which contains neither Zn2+ nor Cu2+, and the measured potential is not the difference of the Zn2+/Zn and Cu2+/Cu reduction potentials. Here, I present some measurements on the lemon cell (or an equivalent system) and interpret them to explain what is actually going on. These experiments could be easily performed by students in the freshman chemistry laboratory.
Goodisman, Jerry, "Observations on Lemon Cells" (2001). Chemistry Faculty Scholarship. 2.