Will high school chemistry teachers one day deplete the world of alkali metals?

[Rogin the Armchair fan]

BBC bitesize for kids today had someone doing the same trick that my first year chemistry teacher did, of showing the reaction when you drop lithium and sodium into water. There can't be many of us who haven't seen that. But this must be repeated tens of thousands of times, every year, around the globe. How long before it all runs out?

[Guy Profumo]

Obviously potassium already has…

[Southport Zeb]

I suspect that demand for lithium is more likely driven by the battery industry rather than the few kilos used annually by chemistry teachers.

[Rogin the Armchair fan]

Yes, I'm sure, but I'm talking about supply. There's only so much of it.

[Bizarre Löw Triangle]

Yes, I'm sure, but I'm talking about supply. There's only so much of it.

Lithium exists in nature in the form of salts. you need to electrolyse it to get the metallic element. Although we mostly get lithium from bolivian salt pans, it's also present in sea water (there's around 180 billion tonnes in the world's oceans) and that's where most of the lithium from science experiments will eventually end up.

It's not yet economically viable to extract lithium from sea water (compared to mining bolivian salt flats) but the lithium used in chemistry experiments isn't really going anywhere.

[Kowalski]

I'm not sure how many schools do this kind of experiment in front of kids now. There's the obvious safeguarding issue - it usually destroys the equipment being used e.g. when I did my GCSEs the teacher inadvertently destroyed a smart looking pyrex bowl - and some schools have removed their fume cabinets.

[Lang Spoon]

From age 12 on we didn't just watch the teacher, we got to "experiment" ourselves with potassium in water, magnesium strips on the Bunsen burners, poisoning each other with salt and vinegar crisps tasting copper sulphate. Does none of this happen anymore?

[Janik]

Yes, I'm sure, but I'm talking about supply. There's only so much of it.

This implies a fundamental misunderstanding of what is going on when lithium reacts with water. Which is that it is a very different process from burning a fuel.

The key difference is a fuel is a compound, and lithium is an element. When a fuel is burned it gets broken down and therefore used up. At the end of the process the fuel no longer exists. But when an element reacts that doesn't happen as beyond radioactive decay, it can't. Atoms are immutable (by chemical processes at least) - Lithium atoms remain Lithium atoms throughout a reaction. So where has the Lithium 'gone', then? Well, nowhere - it's still there in the water bath in exactly the same quantity as was thrown in. It's just now no longer a block of metallic lithium but as Lithium Hydroxide salt dissolved in the water. If you dry out the bath, you will get the salt out of solution into a little pile of LiOH crystals, and if you then electrolyse those you can get all your lithium back exactly as you had it before the experiment was conducted.

This doesn't happen of course, because it is energy inefficient to do so. What actually occurs is the water with some small about of LiOH salt dissolved in it gets tipped down the sink. Where it joins watercourses and eventually flows out to join, as BLT notes, the many tons of the stuff already in the sea. But none of this changes the supply of Li atoms available on the earth one iota.

Why is it inefficient to extract the Lithium back from the LiOH? Because doing so requires a large energy input. That is obvious from the large energy output when the reaction went the other way and the salt was formed, i.e. all the light, sound and heat involved in the crowd-pleasing demonstration is energy being given off. Though thoroughly reversible, the reaction does require at least as much energy to be put back in to drive it the other way. If it didn't demand that it would be violating the 2nd law of thermodynamics.

[Janik]

So in answer to the thread title - no, they never will. Not even if they continue to do their demonstrations for evermore.

[Various Artist]

Standing ovation for that contribution, Janik.

[Lang Spoon]

I hope Rogin is sleeping better tonight.

[3 Colours Red]

I know someone who lost the sight in one eye after stealing the school supply of sodium and then spitting on it.