Gar Jar Demos

I’ve recently been doing some demos showing how the pH of the period 3 oxides changes. The demos involved the combustion of various period 3 elements with oxygen, dissolving the oxide in water and adding universal indicator to show the pH. It’s very visual and always goes down well with the students. This made me think about all the different experiments I do using gas jars and the fume cupboard and I thought I would list them.

I generally demo sodium, magnesium, carbon and sulphur in a gas jar of oxygen. Sodium can be a bit erratic: sometimes it burns well, other times it all seems to have reacted before it gets into the gas jar. If you are going to use magnesium, close your eyes! It glows extremely brightly and the students will need some sort of eye protection to dim the glow. Carbon is a nice gentle reaction and my favourite is sulphur: it burns with a lovely purple flame. Make sure you carry this out in the fume cupboard, though, as sulphur dioxide is not very nice to breathe in.

Moving on from oxygen, do you do any carbon dioxide demos? A really good one is the displacement reaction that occurs when burning magnesium is put in a gas jar of oxygen. Students will predict that the burning magnesium goes out where as in fact, it carries on reacting, displacing the oxygen from carbon dioxide to form magnesium oxide and leaving behind flecks of soot – very visual and unexpected:

2Mg + CO₂ → 2MgO + C

Another great gas jar reaction is the reaction between chlorine and sodium to form sodium chloride. This is usually carried out by combusting a small piece of sodium on a brick and putting a gas jar of oxygen over it. Again, it is very visual but great to show how two harmful substances can produce a harmless substance (I still wouldn’t eat the salt though!)

If you are feeling daring you can also react bromine with potassium but please try this one yourself before you show it to your class!

Do you also carry out gas jar demos? If so, what do you do? I’d love to hear more about them so please feel to post your thoughts below.

NB: As with any demo you carry out, you the teacher are responsible for carrying out appropriate risk assessments. The aim of this blog post is to give you some ideas or inspiration – not to tell you how to do things.