Bubble, bubble, toil and trouble…

When I’m starting off with a new class, and encouraging them to “have a go, because, after all, what’s the worst thing that can happen?”, I like to tell them about when I myself was in Year 10. My best subject of that year was History, and we had been studying the Renaissance and the Voyages of Discovery. As the end of topic test approached, I was vying with my best friend, Chris Mortimore, for top spot in the class (this was back in the days when class results were pinned to the classroom door for all to see). Lots of the available marks were for key dates, who did what to whom, or when did how to what, and so on…. I learned them all. Or nearly all.

When the final results were read out, Chris had beaten me to first place by one mark, because he had learned one date that I had not – 1517, the year when Martin Luther nailed his theses to the church door in Wittenberg and kicked off the Reformation. Nearly 40 years later, the only date I can remember from that year is the one I got wrong.

So my message is that getting things wrong is a really powerful way of learning. Don’t be scared of making a mistake – it will help you remember, it will help you learn.

It can also be a barrel load of fun, as my Year 10s found last week.

The context was Respiration, exploring this idea that living things need to release energy from food, and that while the reaction is chemically identical to combustion, the actual process must be slower and more controlled and take place at remarkably low temperatures, something made possible by the wonderful enzymes they learned all about in Year 9. I show them some yeast in a warm water bath, bubbling merrily away after the addition of some sugar. Look, I say happily, here’s an example of a living organism respiring sugar, using enzymes to do so, and producing carbon dioxide as a waste product.

They nod with varying degrees of enthusiasm and understanding.

But hang on, I say, how do you know that any of that is true?

They stop nodding and look puzzled. What do I mean?

Why take my word for it? I’ve just told you a whole load of stuff about what’s going on in that boiling tube – you’re just going to accept it on faith? What kind of scientists do you think you are?!?!?

They continue to look puzzled. Where is this going?

Go on, I say, design some experiments and test at least 3 of the statements I’ve made about respiration in yeast.

Both of my classes, Set 1 and Set 2, found this wonderfully difficult. Firstly, they struggled to grasp what I actually meant. I tell them to go back to the statement:

here’s an example of a living organism respiring sugar, using enzymes to do so, and producing carbon dioxide as a waste product

and think about how they could test the veracity of otherwise of what it says.

OK, they say, we could test the gas to see if it really is carbon dioxide….

Right! That’s the idea! Off you go!

Even on this easy starter for ten, there are some hilarious moments. The demo I showed them had a delivery tube passing bubbles through a test tube of tap water. Many of the groups let the gas bubble through the tap water and then attempted to collect the gas with a syringe, and then squirt the contents of the syringe through some lime water. When I suggest that they just move the delivery tube to a test tube full of lime water, they look at each other, and then laugh in mutual recognition of their own inability to see the simple and obvious. No matter – they won’t forget this experimental detail ever again.

I was also hugely amused by the group who tried to establish optimum temperatures of 37’C by heating the tube directly in the bunsen….

And then they really struggled. What else could they do? I tell them not to over-complicate. They have a very simple experimental set up – they only need to tweak the contents of the tube to test the other assumptions.

But what other assumptions????

And then, slowly, group by group, they started to see it.

What if we left out the sugar? What indeed! Try it!

They try it and, lo and behold, there is no bubbling. Seems that sugar is, indeed, vital for whatever is taking place in the boiling tube.

What next?

Well, there were, of course, some clues in the apparatus and reagents I had provided. Why the HCl and NaOH? Well, what effect would those chemicals have on conditions inside your boiling tube? Oh, pH change. Oh, enzymes. Oh…. oh!

Bingo, now they’re re-running the experiment but at high and/or low pH. And waddayaknow, the bubbling stops, or slows significantly. It seems reasonable to infer that some enzymes have been denatured.

The groups who had earlier tried to establish 37’C conditions in the blue flame of a roaring bunsen now realise that it gives another test – boiled yeast will be dead yeast. And dead yeast, it turns out, doesn’t bubble either.

What seems obvious to an experience scientist, or experienced teacher, was a really difficult thinking exercise for some very bright Year 10 students. But hard thinking is memorable thinking and without doing a single second of revision, or copied a single note from the board, the vast majority will have understood and remembered all the important points of the lesson.


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