CLEAPSS devised a simple small electrical conductivity indicator. The resistor in the circuit limits the current to protect the Light Emitting Diode (LED). The carbon fibre-rod electrodes are very strong unlike graphite.
Its use in secondary education though appears limited to teachers because that concept of electrical conduction applying to metals and non-metals carries on the students to 16 and beyond. The addition is that molten salts and aqueous salt solutions conduct electricity.
In one of our specifications for chemistry for students to 16, the word conductivity comes up 7 times always in relation to metals and non-metals. The word “electrolysis” appears 27 times. The macro event of seeing metals and gas bubbling at electrodes, has over taken the nano interpretation. Electrolysis (chemical reactions at the positive and negative electrodes) occurs to enable the conductivity electrical current to pass through a liquid (no wires).
If the electrodes are placed in ethanol or cyclohexane, there is no light coming from the red LED on the conductivity indicator. If it is placed in pure water, there is no light but if placed in tap water, the light comes on. This happens with salt water but not water with refined sugar (sucrose) or glucose.
In the photos below, the red LED is not showing when the electrodes are in pure water. When solid potassium manganate(VII) is added to water between the 2 electrodes and the potassium manganate dissolves, the LED lights up BUT the purple colour moves to the (red) positive electrode.
Could it be that what we thought was 1 purple particle in potassium manganate(VII) when potassium manganate(VII) dissolves in water, is possibly 2 or even more electrically charged particles, one of which is purple negatively charged and attracted to the positive electrode?
Here is a more a more dramatic demonstration. It does use a very toxic copper chromate. A risk analysis on the safety requires material is kept to the minimum, which is possible using a microscale technique. Chromates are banned from use in schools of many countries on toxic and environmental grounds. “As the dose makes the poison”, this is so minimalist, and the educational important so important, I really think that it should be carried as a demonstration at least with the result projected onto the screen.
One drop of 0.1M copper sulfate solution (blue) is added to 1 drop of 0.1M potassium chromate solution (yellow). A Brown solid appears. Now add 2 to 3 drops of 2M ammonia and a green solution (blue and yellow combined colours) appears.
About 2 drops of this green solution are placed on filter paper and the electrodes applied for 60 seconds. The blue copper particle is attracted to the negative electrode and yellow chromate particle is attracted to the positive electrode. (Chemists will realise that I am emphasising the blue colour by using ammonia solution which forms the deeper blue Cu(NH3)4 dipostive ion.)
Going back to the potassium manganate(VII) particle, what was a purple particle appears to be comprised of a purple negative particle and colourless positive particle.
What we thought was a blue copper sulfate(blue) and potassium chromate(yellow) particle is more complicated as shown in the diagrams below.
When it comes to salt, which students know is a colourless solid, dissolves in water (as in sea water) and also conducts an electric current, we have to say there is not one particles of sodium chloride but 2 particles, one negative and one positive. (Purists will say that the copper, sulfate and chromate ions carry two charges. Yes but one thing at time, otherwise we shall have cognitive overload.
Students can carry out an investigation on any salt. Some will just light up the LED and bubbles appear around the electrodes. Others will start to produce metals at the positive electrode and halogens around the positive electrodes. This video shows the addition of a tiny sample of tin chloride, running at 4 times the normal speed.