This experiment shows that an ionic salt will conduct electricity when molten
but not when solid. It also shows the products of electrolysis
at the positive electrode and the negative electrode.
The salt chosen is zinc chloride
because of its low melting point of 535 K or 262 C.
We no longer use lead bromide due to its toxicity
and for the fact that it does have a tendency to
decompose on heating without electrolysis.
First of all we set up the apparatus.
For this you'll need a heat proof mat, a tripod, a pipe clay triangle
and a crucible.
Ensure the crucible sits firmly in the pipe clay triangle and doesn't fall through.
Next we set up the electrode assembly.
For this you'll need two graphite rods about 15 centimetres in length
the longer the better and a bung or holder.
Slide the rods into the bung and make sure the bung is as high up the rods
as possible to avoid heating when we turn on the Bunsen burner.
This is quite a fiddly procedure and we recommend that it's done before the demonstration starts.
Now set up the electric circuit.
For this you'll need a power pack and ammeter set up in series with leads to the electrode assembly.
Switch the power pack on and set a voltage of about 12 volts.
Test the electric circuits to show you have a current.
It is possible for the graphite rods to crack where they pass through the rubber bung,
suspect this if the circuit does not work and there is no obvious cause.
Once you're happy that the circuit is complete, position the electrodes into the crucible.
Make sure they go as far down the crucible as possible but not touching the bottom.
Now you need to fill up the crucible with zinc chloride.
You should be aware that as the zinc chloride begins to melt
it will shrink in volume so do make sure you've got enough in the crucible.
Once you're happy with the set up you can begin heating using a moderate flame on your Bunsen.
As the Zinc Chloride melts, the ammeter will give a reading.
Bubbles of chlorine will be seen at the positive electrode
We can test for chlorine using moist indicator paper, which goes red
and then should bleach.
An alternative test is moist starch potassium iodide paper,
which you can see clearly goes black.
The electrolysis may be carried out in a well ventilated open laboratory.
The amount of chlorine generated in 15 minutes of electrolysis
is small enough to be acceptable in an average sized, well ventilated lab.
Electrolyse the molten salt for about 15 minutes, adjusting the current to about half an amp.
During this time there will be little to see
and it is best to get the class to go back to their places to do another activity.
You could allow small groups of students to come up
and see if they can see the bubbles of gas at the positive electrode
and maybe smell the bleachy smell.
After fifteen minutes, you need to turn the power pack off,
remove the Bunsen and take the electrode assembly out of the crucible.
Allow the crucible to cool for some time.
Once the crucible is cool you can remove it from the pipe clay triangle
and place it into a beaker of water to dissolve any unused zinc chloride.
You can filter the contents of the beaker to reveal the metallic zinc on your filter paper
This will need to be dried and then can be tested using the circuit tester.
Since the chlorine was seen at the positive electrode, it seems likely that the zinc was produced at the negative electrode.