Painting a line in the road

When the going gets tough in class, I might tell my students the story about the man painting the line in the middle of the road. It’s an old joke but it expresses how it feels to be a chemistry teacher sometimes. Let’s call the man Stan.
The foreman checking Stan’s work comes to speak to him. He wants to know why he painted 200m in the first hour, only 50m the second hour and a measly 20m in the third hour. “Well,” says Stan, “I’m getting further and further away from my paint pot.”
I thought about telling this story today but thought better of it. We were measuring the Enthalpy of Neutralisation with some Year 10 students. They added the acid to the alkali, diligently measuring the temperature rise. Results in hand we set about doing the calculation. We went back to review how we worked out the energy released. Not too bad, as we had covered it only the last lesson. Then we needed to work out the number of moles reacting. Let’s go back and remind ourselves how we work out the number of moles of acid in 50ml of 2.0M HCl (not the units we used in class of course). Units of volume need to be reviewed as well of course as we need to convert to dm3. No, we don’t add the number of moles of alkali to this do we, because we look at the equation and see that they react 1:1 and so the number of moles is the same. We spend a little time looking at the equation. With all this done, we just about have time to put the numbers into the equations and work out the answer. What does it all mean? Sadly this is completely lost in the process of reviewing the principles we needed to solve the problem in the first place.
Is Chemistry a difficult subject? It is if you don’t carry your paint pot with you every step of the way. Should I expect my Year 10s to be completely fluent in the language of chemistry and to understand and recall everything they have done so far? I don’t think so, yet. Perhaps if we keep going back they will eventually get it. However, by the time we return to where we started the objective is sometimes lost.

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