Little Baby Acid Atoms


Five weeks into a new term as busy and brutal as any I can remember, I’ve finally got round to doing something which should probably feature more commonly in the day to day existence of a teacher…

I planned a lesson!

Yep, the complete lack of Burbling has been partly the result of exhaustion, partly a complete lack of time, but mainly just the fact that I’ve not done anything new. I’ve staggered into the lab, grabbed the relevant file from the shelf, and been privately grateful that once upon a time, I was actually able to plan lessons and design resources that are worth repeating.

So, what did I do?

Well, one of the features of our exciting new timetable is that Year 9s now have 3 lessons a week, split into a double and one single (previously, they had just had the one double). I think it’s an improvement. It means we can see them twice a week, so they haven’t forgotten everything in intervening 7 days. It means that if they miss a day for a trip or something else, it won’t be two weeks between lessons. And it means that the double can be entirely devoted to practical work, with supporting theory in the single.

We start the Edexcel iGCSE in Year 9 and launch into cells. I’ve Burbled before about the letters they write as angry babies, trying to persuade their mothers that weaning them onto raw onion with its thick cell walls and vacuoles full of sulphuric acid is a bad idea, and how much better the soft, thin cell walled starchy sweetness of banana would be… And I’ve described how we isolate cheek cells with saline rinses and centrifugation. Then we go on safari for protoctista…

Which, this week, led us to diffusion. In the past, I’ve done this….acid-bath-and-agar-blobs-noddy-noddy-version.

No preamble, no brain storming, just do the activity and interpret it.

It’s OK, but it’s formulaic – they all do the same thing – and the SA/Vol idea is actually a hard one to start with. I’ve never been entirely happy with it.

I have tried this… 1lt1-flipped-diffusion-in-jelly-blocks-1 but it’s still not quite the thing – a bit too strait-jacketed, a bit too directed.

But with the extra lesson, I tried this.

Starting in the single lesson, I bring out the block of agar with phenylthalein indicator to demo. Oooh, the excitement! It is it jelly? Can we eat it? Can we touch it?

We discuss the colour: pink? magenta? purple? pinkmagentapurple?

I describe the nature of agar, and carefully carve out a little 1cm cube.

What might this represent?

They very quickly suggest a cell – which pleases me – and I stress the point that yes, of course, cells are 3D. They remember this from the model apple cells they made in Year 7. I then introduce the hydrochloric acid and tell them about the indicator. I pour some acid into a boiling tube and pop in the cube, and pass it round.

Start with a description – what can they see?

This takes longer than you might think. But it forces them to look  – they’re fascinated – and suggestions come in. They get the idea that the cube is decolorising from the outside in. But visualising what’s actually going on is more difficult. I sketch a cube on the board – they laugh at my attempts to get the right shade of purple – surrounded by acid.

If we could really see the acid, what would it look like?

Little Baby Acid Atoms, says one. That’s OK. I can work with that.

I draw in lots of Little Baby Acid Atoms.

What are all these Little Baby Acid Atoms doing?


What state is the acid in? Yes, that’s right, it’s a liquid. So what are the Little Baby Acid Atoms doing?

A little more discussion and annotation and we have particles moving into the cube, decolorising as they go, from a high concentration to a low concentration, terms that they dimly recall because, hey, they’ve done this in Chemistry.

All of which probably sounds very similar to what biology teachers all over the world do. But it’s just the build up to the punch line…

So, next lesson, I say, in the double, I want these students to investigate the effect of temperature, these students to investigate the effect of cube size, and these students to investigate the effect of acid concentration.

The next lesson was joyous.

I arranged things so that they arrived to a lab with each pair of students having what they needed (no mass stampede to the back row to grab apparatus). They liked this – somehow made it seem a bit more grown up, and bit more Real Science. With roughly 3 pairs of students per treatment, they all took a slightly different approach – but it was emphatically their investigation.

In terms of experimental design, they pretty much knew exactly what to do – these are the students who were the first year to go through with my reworked KS3 SoW, with its emphasis on experimental investigation AND NOT TAKING NOTES – needing no reminders about controls or variables or measurements. Sure, they needed some tips on water baths and diluting stock acid, but they made all the critical decisions themselves – autonomy is so motivating! – and did a brilliant job.

In the next single period, each group reported their results back to the rest of the class, and there was our framework for diffusion.

I sometimes worry about whether I’m doing the right thing. This was a wonderfully heart-warming affirmation that maybe I’m onto something.

If I get to plan another lesson any time in the next year, I’ll be sure to share.



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