 How does a torch cell work? This is a normal torch cell, not rechargeable. This one is rechargeable. How is it possible to get electricity out of a lemon? This is electrochemistry. And it all comes down to the standard electropotential table. At first glance, this table looks quite intimidating, but there is sense behind it. Let me just talk to you very quickly about an approach to the teaching of chemistry in general. For any student studying chemistry, it's potentially a minefield because there are thousands of different chemicals and elements. And so potentially they've got to learn thousands and thousands of different chemical reactions in possible tasks. But really the way in which scientists go about studying chemistry is that they will look at the various reactions and try and look for patterns in what they see. And then once they get that pattern, they will try and formulate a general rule and then use that general rule to predict the behavior of other chemicals. So largely our job as a science teacher as a chemistry teacher is to try and help students to find these patterns of behavior in what they see. For example, if I were to bite this lemon, it would be very sour. And because I'm a chemist, I know, ah, sour, that means acidic. And immediately I'll tell you a whole lot of things about the behavior of this lemon and its juice. I know it'll turn litmus paper red. I know that it will, if I mix it with bicarbonate and soda, it will give off bubbles, carbon dioxide, et cetera, et cetera. I'll tell you that before I go and do it because there are various patterns that we've observed about sour substances. We call them acids. So it's looking for patterns that really is what chemistry teaching is all about. The periodic table up there is the biggest pattern of them all. It's a fantastic piece of work. And by simply knowing and understanding that table, you can predict thousands of behaviors and properties of different elements and chemicals and formulas and that sort of thing simply by using that pattern in the form of a table. This activity is based on exactly what we see here. The lemon is able to produce electricity because I've taken two different metals, copper and zinc, and I've put them into an electrolyte. And if ever you take two different metals in an electrolyte, you can generate electricity. I'll get a certain amount of volts out of it. This table has got various half reactions and next to it positive negative potentials. So if we take different metals from here, we should be able to get cells set up. So what I've done is I've collected a whole lot of metals together. Right, so we've got all our different metals. Those are electrodes. All we need is a bit of electrolyte. Electrolyte, well, I'm going to use some table salt. I've already put that table salt into some water. So I've got a salt solution here and we then pour that electrolyte, sodium chloride solution into the tissue paper. So that all those metals are lying in an electrolyte. Right, now we're going to connect these metals and see what kind of potential they develop. And for that we need a voltmeter. I've got a multimeter here. Now we know that for this standard electrode potential series, hydrogen is used as the reference point. It's given a value of zero volts. We can't use hydrogen here because it's a gas. It's very difficult to set up a hydrogen electrode. But we can choose one of the metals as our reference point and you measure the potential of every other one to that. So I think what I'm going to do is I'm going to choose lead, which is closest to hydrogen as my reference point, and see what kind of voltage we get out of that. Okay, I'm going to put the positive electrode on the lead and then against zinc, 460 millivolts. Gold, negative, 600, about 600 millivolts. So in other words, we're now going into negative territory. We then record this in a table. We've used the lead electrode as our reference and those are the different metals and these are the results that we got in volts. And that is very much what we see here. So it's very easy now for you to move your discussion from what you've found practically to this. These values of course are your standard electrode potential and we're not working under standard conditions here, clearly. So it's quite easy to explain why we haven't got the right values, but really all we're trying to show them is where did this table originate? Another very important part of your teaching is how do you make this come alive for the students? What sort of things can we talk about that will make it relevant in their lives? Well, the obvious one is galvanizing, where you galvanize iron to protect it against rust. So the iron is coated with a very thin layer of zinc. Another one that a lot of people don't know about, they call this tin. Well, tin is non-magnetic and there's my iron. It's magnetic. Actually, this isn't tin. This is actually iron that's been coated with tin. It's galvanized so that it stops the iron from rusting. So those tins that you're throwing away, you're actually throwing away iron. And finally, as always, I invite you to send in any comments, questions, comments you might have via the website in the comments section or on my email address and I will respond to you.