During last week’s Year 8 lesson one of the boys (let’s call him Joe) said when he put his arm on his desk it was a bit itchy and irritated. The desk was covered in water and I suspected it was acidic. I tested the wet bench with blue litmus paper which turned red. I then sprinkled solid sodium carbonate onto the bench to neutralise the acid present (this produced copious fizzing). Another boy (let’s call him Sam) was watching what I did and asked me how I knew what to do. Here are my deductions and the conversation with Sam….
The desk was covered in water and the boys had been heating iron and sulfur to make iron sulfide in the classical lesson looking at properties of elements and compounds. As part of the lesson, one factor being considered was whether the substance floated or sank. I suspected that Joe hadn’t fully understood how to do this for the compound he had formed in the ignition tube and had simply dunked it into the beaker which he then knocked over. As he had been heating sulfur, the chances were that oxides of sulfur had been formed and then dissolved in the water when he dunked it, forming an acidic solution. This was confirmed by the litmus paper and solid sodium carbonate from the chemical decontamination kit was a suitable adsorbent and neutralising compound which could then be swept up from the bench.
- Sam: “How did you know to do that madam?”
- Me: “Well Joe had been heating sulfur and I thought he had probably dunked his ignition tube into the beaker of water.”
- Sam: “Well how did you know to get the litmus paper?”
- Me: “What do we know about heating elements in air, from earlier in the year?”
- Sam: “They make oxides”
- Me: “Yes, and sulfur is a non-metal… What kind of oxides do non-metals form?”
- Sam: “I don’t know what you mean?”
- Me: “would you expect a non-metal oxide to be acidic or alkaline?”
- Sam: “acidic”
- Me: “So what did I need to see if the water on the bench was acidic?”
- Sam:”the litmus paper”
- Me: “and because it turned red I knew it was acidic”
- Sam: “but how did you know to get the powder?”
- Me: “Well I wanted to get rid of the acid, what could I have done?”
- Sam: “You could have added more water to it!”
- Me: “I could, but the water wouldn’t ‘get rid’ of the acidity. I needed to use something to neutralise it…”
- Sam: “An alkali, like sodium hydroxide”
- Me: “But the only sodium hydroxide I have is a solution which would make a big mess and it might make the bench alkaline which would be just as bad. The powder is calcium carbonate. What happens when acid reacts with a metal carbonate?”
- Sam: “salt + water!”
- Me: “Yes, but we saw bubbles, what was in the bubbles?”
- Sam: “a gas…”
- Me: “which gas?”
- Sam: ……..
- Me: the powder was calcium carbonate, which gas name could come from that name..?”
- Sam: “carbon dioxide!”
So a simple accident in the lab actually provided us with a nice opportunity to summarise a lot of the chemistry we had studied this academic year (acids and alkalis, indicators, neutralisation, salt formation and the acid/base properties of oxides).
Now the boys have decided I should write exam questions like the following…
“Joe has spilled some solution from a beaker onto the bench. Dr Turner thinks it might be acidic. Suggest how Dr Turner could test the bench to see if it is acidic, and which substance from the chemical decontamination kit should be used to make the bench neutral again.”