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Alternative electrolysis: Petri dish and pencils

An alternative way to set up the electrolysis of sodium chloride is to use much smaller quantities with pencils and a petri dish.
9.7
The previous experiment used commercially-bought equipment with electrodes and power packs. And although it’s effective, it’s quite difficult to get that as a student set of equipment. So what we’ve got here is a very low cost way of doing the same experiment. So we’re going to use brine solution again with universal indicator, but this time for the electrodes, we’re going to use some ordinary pencils, which we’ve sharpened at both ends. And we’re also going to use a battery pack instead. This battery pack has four D-Cell batteries joined together in series, and we can use that instead of our power pack to give us the current to drive the electrolysis.
46.8
We’ve got a glass or plastic Petri dish, and we can pour brine solution with universal indicator into our Petri dish. And then attach electrodes to one end of our sharpened pencil and rest the other into our solution.
67.3
Then, make the final connection at this end to turn it on, and then we can start to look at what is happening in our solution. So, again, at the positive electrode, we’ve got some bleaching occurring where we have the chlorine produced. And at this electrode or the negative electrode, we’ve got the hydrogen being produced, leaving the hydroxide behind, which then gives us our purple colour in the universe indicator. What students could also do is they could reverse the terminals around and see they would have a change going on as well. And it also encourages students to think about the contents a pencil has because they’re very conductive because they contain high amounts of carbon.
104.8
This reaction is much safer because we are producing much lower amounts of chlorine. And if we want to, we could actually cover it over as well with the lid of the Petri dish, which would actually contain the chlorine and prevent students from breathing it in. At the end of the experiment, if they unplug the lead, swirl their solutions together which will help neutralise, then they can take out the pencils, give them a rinse, and then the solution itself can be poured down the drain.
An alternative way to set up the electrolysis of sodium chloride is to use much smaller quantities with pencils and a petri dish. Once again, there should be two gases produced and two colour changes in the solution. To ensure that you can see the colour changes more clearly, gently move the pencils or tap the petri dish to disturb the solution.
The pencils need to be sharpened at both ends. Although standard HB pencils work well, 2B or greater have more graphite in them and so are better conductors.

Equipment list

  • Pencils
  • Petri dishes
  • Battery pack (two or three D cells linked to give 3 or 4.5V)
  • Leads and crocodile clips
  • White paper or tile to stand dish on

Comment

We’ll ask you to pick one of these practicals to look at in depth in step 1.13, but for each make a few notes about the advantages and disadvantages for each one. Share your notes about this practical here.
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Teaching Practical Science: Chemistry

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