Tag Archives: electrons

Electrolysis Made Easy(ish)

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Some STEM Learning trainee teachers, looking very keen!

Back in November last year (was it really that long ago??) I wrote a blog post about water, in which I described a simple at-home version of electrolysis. I didn’t think much of it at the time, beyond the fact that it was oddly exciting to do this experiment—that usually involves power-packs and wires and all sorts of other laboratory stuff—with just a 9V battery, a tic tac box and some drawing pins.

Then, hey, what do you know, someone actually read my ramblings! Not only that, read them and thought: let’s try this. And so it was that Louise Herbert, from STEM Learning (that’s their Twitter, here’s their website), contacted me last month and asked if I’d mind if they used the Chronicle Flask as a source for a STEM learning course on practical work.

Of course not, I said, and please send me some pictures!

And they did, and you can see them scattered through this post. But let’s have a quick look at the chemistry…

Electrolysis is the process of splitting up compounds with electricity. Specifically, ionic compounds: the positively-charged ion in the compound travels to the negative electrode, and the negatively-charged ion moves to the positive electrode.

Water is a covalent compound with the formula H2O, but it does split into ions.

Only… wait a minute… water isn’t ionic, is it? So… why does it work on water? Er. Well. Water does split up into ions, a bit. Not very much under standard conditions, but a bit, so that water does contain very small amounts of OH– and H+ ions. (In fact, I can tell you exactly how many H+ ions there are at room temperature, it’s 1×10-7 mol dm-3, and, in an astonishing chemistry plot twist, that 7 you see there is why pure water has a pH of, yep, 7.)

So, in theory you can electrolyse water, because it contains ions. And I’ve more than once waved my hands and left it at that, particularly up to GCSE level (age 16 in the U.K.) because, although it’s a bit of a questionable explanation, (more in a minute), electrolysis is tricky and sometimes there’s something to be said for not pushing students so far that their brains start to dribble out of their ears. (As the saying goes, “all models are wrong, but some are useful.”)

Chemists write half equations to show what the electrons are doing in these sorts of reactions and, in very simple terms, we can imagine that at the positive electrode (also called the anode) the OH– ions lose electrons to form oxygen and water, like so:

4OH– —> 2H2O + O2 + 4e–

And conversely, at the negative electrode (also called the cathode), the H+ ions gain electrons to form hydrogen gas, like so:

2H+ + 2e– —> H2

These equations balance in terms of species and charges. They make the point that negative ions move to the anode and positive ions move to the cathode. They match our observation that oxygen and hydrogen gases form. Fine.

Except that the experiment, like this, doesn’t work very well (not with simple equipment, anyway), because pure water is a poor electrical conductor. Yes, popular media holds that a toaster in the bath is certain death due to electrocution, but this is because bathwater isn’t pure water. It’s all the salts in the water, from sweat or bath products or… whatever… that do the conducting.

My original experiment, using water containing a small amount of sodium hydrogen carbonate.

To make the process work, we can throw in a bit of acid (source of H+ ions) or alkali (source of OH– ions), which improves the conductivity, and et voilà, hydrogen gas forms at the cathode and oxygen gas forms at the anode. Lovely. When I set up my original 9V battery experiment, I added baking soda (sodium hydrogencarbonate), and it worked beautifully.

But now, we start to run into trouble with those equations. Because if you, say, throw an excess of H+ ions into water, they “mop up” most of the available OH– ions:

H+ + OH– —> H2O

…so where are we going to get 4OH– from for the anode half equation? It’s a similar, if slightly less extreme, problem if you add excess alkali: now there’s very little H+.

Um. So. The simple half equations are… a bit of a fib (even, very probably, if you use a pH neutral source of ions such as sodium sulfate, as the STEM Learning team did — see below).

What’s the truth? When there’s plenty of H+ present, what’s almost certainly happening at the anode is water splitting into oxygen and more hydrogen ions:
2H2O —>  + O2 + 4H+ + 4e–

while the cathode reaction is the same as before:
2H+ + 2e– —> H2

Simple enough, really, but means we use the “negative ions are going to the positive electrode” thing, which is tricky for GCSE students, who haven’t yet encountered standard electrode potentials, to get their heads around, and this is why (I think) textbooks often go with the OH–-reacts-at-the-anode explanation.

Likewise, in the presence of excess alkali, the half equations are probably:

Anode: 4OH– —> 2H2O + O2 + 4e–
Cathode: 2H2O + 2e– —> H2 + OH–

This time there is plenty of OH–, but very little H+, so it’s the cathode half equation that’s different.

Taking a break from equations for a moment, there are some practical issues with this experiment. One is the drawing pins. Chemists usually use graphite or platinum electrodes in electrolysis experiments because they’re inert. But good quality samples of both are also (a) more difficult and more expensive to get hold of and (b) trickier to push through a tic tac box. (There are examples of people doing electrolysis with pencil “leads” online, such as this one — but the graphite in pencils is mixed with other compounds, notably clay, and it’s prone to cracks, so I imagine this works less often and less well than these photos suggest.)

A different version of the experiment…

Drawing pins, on the other hand, are made of metal, and will contain at least one of zinc, copper or iron, all of which could get involved in chemical reactions during the experiment.

When I did mine, I thought I was probably seeing iron(III) hydroxide forming, based, mainly, on the brownish precipitate which looked fairly typical of that compound. One of Louise’s team suggested there might be a zinc displacement reaction occurring, which would make sense if the drawing pins are galvanized. Zinc hydroxide is quite insoluble, so you’d expect a white precipitate. Either way, the formation of a solid around the anode quickly starts to interfere with the production of oxygen gas, so you want to make your observations quickly and you probably won’t collect enough oxygen to carry out a reliable gas test.

In one of their experiments the STEM Learning team added bromothymol blue indicator (Edit: no, they didn’t, oops, see below) to the water and used sodium sulfate as (a pH neutral) source of ions. Bromothymol blue is sensitive to slight pH changes around pH 7: it’s yellow below pH 6 and blue above pH 7.6. If you look closely at the photo you can see that the solution around the anode (on the right in the photo above, I think *squint*) does look slightly yellow-ish green, suggesting a slightly lower pH… but… there’s not much in it. This could make sense. The balanced-for-H+ half equations would suggest that, actually, there’s H+ sloshing around both electrodes (being formed at one, used up at the other), but we’re forming more around the anode, so we’d expect it to have the slightly lower pH.

The blue colour does, unfortunately, look a bit like copper sulfate solution, which might be confusing for students who struggle to keep these experiments straight in their heads at the best of times. One to save for A level classes, perhaps.

(After I published this, Louise clarified that the experiment in the photo is, in fact, copper sulfate. Ooops. Yes, folks, it looks like copper sulfate because it is copper sulfate. But I thought I’d leave the paragraph above for now since it’s still an interesting discussion!)

The other practical issue is that you need a lot of tic tac boxes, which means that someone has to eat a lot of tic tacs. There might be worse problems to have. I daresay “your homework is to eat a box of tic tacs and bring me the empty box” would actually be quite popular.

So, there we are. There’s a lot of potential (haha, sorry) here: you could easily put together multiple class sets of this for a few pounds—the biggest cost is going to be a bulk order of 9V batteries, which you can buy for less than £1 each—and it uses small quantities of innocuous chemicals, so it’s pretty safe. Students could even have their own experiment and not have to work in groups of threes or more, battling with dodgy wires and trippy power-packs (we’ve all been there).

Why not give it a try? And if you do, send me photos!

Like the Chronicle Flask’s Facebook page for regular updates, or follow @chronicleflask on Twitter. Content is © Kat Day 2019 (photos courtesy of STEM Learning UK and Louise Herbert). You may share or link to anything here, but you must reference this site if you do. If you enjoy reading my blog, please consider buying me a coffee through Ko-fi using the button below.
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Any old ions?

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The other day, thanks to an expedition to the swimming pool, I found myself drying my hair twice in one day. As I did so it occurred to me that the process was a lot faster at home with my old, battered hair dryer that it had been in the pool changing rooms.

I pondered variables (what can I say, I’m a scientist). I hadn’t washed my hair at the pool, I’d just rinsed it (hence the need to wash it again later) which meant no shampoo and, critically, no conditioner. Does conditioner’s presumably slightly hydrophobic nature help your hair to shrug off extra moisture? This didn’t seem like it ought to make that much difference. It seemed much more likely that the difference was simply due to the hair dryer itself.

hair-dryer

My battered old hair dryer.

And this got me thinking about the nature of my much-loved, rather battered, bright red hair dryer.

I’m going to ‘fess up here. I didn’t buy this hair dryer because I’d carefully researched its specifications and features and decided it was the best model for the job. No, if I’m honest I bought it because it was red and all the others were boring black and silver. Rational eh?

However, I do remember something about this particular hair dryer, and you can just about see a reference to this feature on the photo. Namely, it apparently contains an “ionic generator”. The little dial that you can see in the middle of the photo (set to orange, which if I recall correctly means, ‘maximum ions’) apparently adjusts the ion levels.

At the time I did try to find out exactly what the technology might be. I recall it was difficult – there didn’t seem to be much information out there – and since to be honest I didn’t care that much so long as it dried my hair, and it wasn’t particularly expensive (and it was red, RED!) I bought it anyway.

It only takes a quick glance at Amazon to see that the idea has not gone away. There are lots of ‘ionic’ hair dryers on the market, making claims such as, “Ionic conditioning with 90 per cent more ions“, “Heat-balancing ionic technology for condition and shine and a frizz-free finish“, “stylish dryer with ionic technology–seals in moisture to the hair cuticle for increased shine and silky, glossy hair” and the simple “4X More Ions“.

Hm.

Ok, well first of all what are ions? Whereas most people have a faint idea what atoms and molecules are, far fewer are confident to describe ions – despite the fact that they are firmly a part of the compulsory GCSE Science syllabus and were, of course, also included in  O-level Chemistry before that. Exactly why this should be is tricky to explain. Possibly it’s simply because ‘atoms’ and ‘molecules’ do occasionally crop up in everyday speech, whereas ions are that bit more obscure. Possibly it’s because children learn about atoms in the most basic terms quite early on, and come back to the idea regularly, but ions only turn up relatively briefly (unless, of course, you choose to study A-level Chemistry). There may be an element (hoho) of confusion over the fact that element 26 is called ‘iron’, which in most English accents sounds the same as ‘ion’. And just to really confound everyone, there are such things as iron ions.

But I think the most likely is that ions are a bit tricky to understand.

I’ll have a go.

Ions are charged particles.

There, that was easy, wasn’t it?

What do you mean, what does ‘charged’ mean? It means they have either a positive or negative charge.

What do you mean, ‘what does that mean’?

Oh all right. All right. Back to basics.

helium atom

A helium atom containing a tiny nucleus made up of two protons and two neutrons (red and blue), surrounded by an ‘electron cloud’. 1 fm = 0.0000000000010 millimetres.

First of all we need to understand a bit about atoms. Atoms are made up of two parts. There is the nucleus, which is made up of protons and neutrons (except for hydrogen’s nucleus, which is just a proton) and then, whizzing around that, are electrons. Electrons are quite fiddly things that behave frankly very oddly. In particular, they don’t actually drift around atoms in stately orbits as shown in most diagrams. In fact, they are sort of there and sort of not-there at the same time, and chemists talk about an ‘electron cloud’ as a result. An electron cloud need not contain lots of electrons (this depends on the size of the atom) – it just describes an area where you might find one or more electrons.

Anyway, that’s all a bit complicated and for our purposes it doesn’t really matter – all we need to know is that there’s a nucleus in the middle and electrons around it.

Electrons have a negative charge, protons have a positive charge, and neutrons have no charge. It’s quite difficult to rigorously define what I mean by ‘charge’ without getting into some tricky maths and physics. If you are ok with the idea of negative numbers (who hasn’t had an overdraft at some point or another?) then think of it like this: electrons are -1 and protons are +1 (and neutrons are 0). If you have one proton and one electron, the overall ‘balance’ is zero – their charges cancel each other out. In the case of helium, there are two protons and two electrons. This neat bit of balancing is no accident: it’s the case for all atoms. Carbon has 6 protons and 6 electrons. Oxygen has 8 protons and 8 electrons. Calcium has 20 protons and 20 electrons, and so on.

If the electrons and protons aren’t balanced for some reason (usually as a result of a chemical reaction) then the thing that you were calling an atom a moment ago stops being an atom and becomes, wait for it….

An ion!

Oxygen atoms have 8 protons and 8 electrons, but oxygen ions (properly called oxide ions) have 8 protons and 10 electrons. Which means they have a bit more minus than plus. They are, if you like, a bit overdrawn. If you add it up, you find the number works out as -2. And so we say that oxide ions have a charge of -2, and chemists (who are lazy) write this as O2-. Which is not, we must be careful here, the same thing as O2. That means two oxygen atoms joined together, to make an oxygen molecule. What do you mean it’s confusing?

One more example then. Calcium atoms have 20 protons and 20 electrons, but calcium ions have 20 protons and 18 electrons. Add that up and you get +2. We say that calcium ions have a charge of +2, and write Ca2+(and there’s no such thing as Ca2, so that’s one less thing to worry about).

Where have we got to? Ions are charged particles, and that means that they either have a positive charge or a negative charge. These charges are typically between 1 and 3, positive or negative.

mineral water label

Fizzy mineral water, chock full of lovely ions.

Ions are very important, because they form during chemical reactions and many everyday substances are made up of ions. For example table salt, sodium chloride, is made up of Cl– ions and Na+ ions.

Tap water, and indeed bottled water, are full of ions. Tap water has chloride ions (chlorinated water is a jolly good thing, assuming you don’t want typhoid, and is definitely not harmful regardless of what your nearest quack might try and tell you). It might also have fluoride ions, which are also very good for your general health (again, there’s lots of nonsense spread about this). Both tap and mineral water usually contain some sodium ions and some calcium ions. The ion balance does affect the taste – the more sodium there is the more salty the water tastes, for example – but that’s about it really. The ions don’t give the water any special properties except, perhaps, the ability to conduct electricity (which pure water, as in just H2O, actually does really badly).

Having explained ions, let’s get back to hair dryers for a moment. Ionic hair dryers claim to produce streams of negatively-charged ions. They usually claim to use something like the mineral tourmaline to do this, but despite much searching I struggled to find out much about how this was supposed to work or, most crucially, what the negative ions actually are. Negative ions are not a thing in and of themselves. They must be ions formed from atoms, so which element? Or elements?

After much hunting I eventually came upon an interesting piece written by Andrew Alden, at about.com. He explains that tourmaline has an interesting trick called pyroelectricity, which means that it does become charged when heated. The ancient Greeks even knew about this: in 314 BC Theoprastus noticed that tourmaline (called lyngourion at the time) attracted sawdust and bits of straw when heated.

Ed Trollope, from Things We Don’t Know, helpfully explained this pyroelectric effect as follows: the crystal structure of tourmaline becomes polarised (in other words the charges already in the structure become unevenly distributed) if you change its temperature. This results in a voltage across the crystal, which in turn leads to a small current being generated.

But I’m still unclear what the ions, if they exist, actually are. And the problem is that if you search for this, the first umpteen links are all pure and utter nonsense. Tourmaline is a complicated mineral that contains a whole host of different metal ions as well as oxygen, OH– (hydroxide) and fluorine. Does it produce oxide ions, O2-? These are very reactive and wouldn’t hang around for any useful length of time. And if they did they would surely be harmful. Presumably they would cause the production of ozone (definitely not a good thing). Are the manufacturers using the word ‘ions’ when they actually mean ‘electrons’? They are not the same thing of course, but perhaps ‘ions’ seemed like a friendlier word.

Electrons would reduce static in your hair, but then static is short-lived anyway. Would it speed up drying time? None of the explanations I’ve actually seen for this including, most memorably, “The negative ions break down water molecules to one-fifth of their size” (errrr, what?), provide a really satisfactory, scientific explanation as to why it should. Or why it should ‘seal water into the hair’, whatever that means. It is feasible that the reduction of static could help keep the hair strands separate, which might help, but surely brushing or even running your fingers through your hair would have a much bigger effect. What I’m also not clear on is whether the tourmaline in your hair dryer carries on producing streams of ions/electrons indefinitely, or whether it becomes degraded over time. Which you would expect, if the charged particles are coming from the tourmaline itself. Is my battered old hair dryer really doing anything at all anymore, if it ever did?

It’s all very unsatisfactory. My best guess? Ionic hair dryers do reduce static build-up in hair, which would leave it smoother immediately after drying. Conditioner and styling products will also help with this mind you, and will probably have a more significant effect. The rest, I strongly suspect, is pure woo. And my hair dryer dries my hair faster simply because it runs hotter and with a faster airflow than the cheap, basic models in the swimming pool changing rooms.

But if you know better, I’d genuinely love to hear from you.