The only time the school chemist, other than putting the plug in the mains, has to use AC electricity is during the study of the conductivity of solutions, a topic long gone from the UK A-level syllabus. Physicists talk of electrical resistance and the chemist of electrical conductivity – great cooperation there.
Conductivity was very difficult material for students to understand with terms such as specific conductivity, molar conductivity and “molar conductivity at infinite dilution” and then there was the arithmetic. However, a conductometric titration was quite straightforward to carry out and gave direct evidence for changes in the concentration of ions in solution. It was an alternative to indicator based, potentiometric pH, and those awful thermometric titrations. It is the only way of titrating a weak acid with a weak alkali. Conductivity is also the only way of measuring the chemical purity of water in the lab (not pH as carbon dioxide makes the water acidic). My purified water has a pH of 5 and a conductivity less than 5 uS/cm. My tap water has a pH of 7.5 and conductivity greater than 650 uS/cm. (This does upset the information in textbooks!)
There was a technical difficulty. Conductivity relies upon the electrodes having a constant surface area, which means no electrolysis (gases adhering to the surface or solids appearing would alter surface area) should take place. One can avoid this problem using an alternating current. Our CLEAPSS low voltage supply in the lab had two AC sockets. Time to use them for the first time since they were bought several years ago to see if there was any evidence for the change in conductivity during a chemical reaction using a microscale approach of carrying reactions in "puddles". "Puddles" is a name given to this technique by Bruce Mattson at Creighten Univeristy. I and my collegues have really taken to it now. The puddle is formed on a propylene plastic sheet, using a plastic folder from the office suppliers.
Here is evidence for the ionic version of the chemcial equation. As hydrogen ions are added to hydroxide ions, water is formed, to all intents a molecular liquid so ions are removed and conductivity drops in value. Once all the hydroxide ions are removed, addition of acid increases the conductivity.
And then looking at the number of drops required, there was evidence for the stoichiometry of the reaction
Similar effects were seen in precipitates. Ions are removed in the formation of precipitates. See the Figure below where 0.1M potassium iodide solution is added to 00.05M silver nitrate solution.
As I am over 70 years old now with no prospect of being a made a visiting professor at an exotic University, I can use the word “pedagogy” without being accused of jumping on a bandwagon. Chemistry is difficult because the subject is holistic. Not only do we have to link the corners as shown in the picture, we also have to link the 3 main branches, often separated in our syllabuses, physical, inorganic and organic which are all linked. I remember seeing pennies drop as A-level revision covered the links. Dare I say it not quite the same in physics. Optics and Forces appeared as complete different topics at school level with no connections at all, so you can pass an exam by leaving out difficult area (to me) such as AC current and magnetic fields etc and concentrate on other sections of the syllabus.
Conductivity and electrolysis
Conductivity is now off our syllabus but electrolysis the MACRO event with metals, gases etc produced, is not. Do the wonderful Macro events of chemistry blind our students in understanding what is happening at the nano-levels and how equations come together thus making the undersanding so diffiult?
I am always amazed by chemistry (yes seeing the conductivity fall was a "buzz" moment). These extremely robust carbon-fibre rods have made all this work so much more accessible.