and unexpected things, especially if you look at the temperatures.

What we're going to do is to show you two different experiments.

The first one is pouring sulphuric acid into water. Brady: Cool, now see what it does.

Prof. Poliakoff: Our thermal imaging camera cannot penetrate water,

and it cannot penetrate glass, so we've done the experiment in a plastic beaker

so you can see through the beaker, and see the inside surface

of the water.

Now the acid is much denser than water, so it sinks to the bottom.

But then, the water begins to dilute the acid, and the acid

interacts with the water and generates heat.

The interaction is the water molecules

interacting with the H+ ions and the (SO4)2- ions.

So the cations and the anions from the acid.

And since the acid we're pouring in already has some water,

there will be a degree of ionisation, and when you dilute it

it will ionise more, and there's more water that can solvate,

that's "interact with each of the ions."

So, if you watch, the first thing that's interesting

is that the acid layer gets really hot.

But the next thing that's interesting is that you know that hot things rise,

but the acid layer stays hot, and the water on top stays relatively cold.

And the reason for this is that the acid is far denser than the water,

So even with the heating, it's not light enough to convect up,

and so what you have to do is to stir the acid with a rod, and then it starts mixing things up.

I was quite surprised by the lack of convection, but it really demonstrates

how important it is to stir a reaction mixture.

If you didn't stir it, and were doing some reaction, each time you poured in the acid

you'd pour it in slightly differently, and each time you might get a slightly different result.

So if you want to make a really reproducible experiment,

you need to stir it hard, all the time.

The obvious next step, and our second experiment,

is to see what happens if you dissolve an alkali metal hydroxide in water.

These hydroxides are solids at room temperature, and they're sold as pellets

rather like small beads.

And when you tip these beads into water they sink to the bottom,

and then the water begins to dissolve the pellets.

Now you've got to understand that the solid consists of a metal ion

metal +, and OH-.

And there's a whole array of these, not just two ions,

and the ions, because the metal + is attracted to OH-, require energy to separate them.

And that energy has to be put in.

However, because these ions are small, they interact with the water,

and the water molecules interacting with both the positive ions and the negative ions give out heat.

In the case of alkali metal hydroxides, the energy that you get

from the water interacting with the ions is more than the energy than you need

to pull those ions apart

So overall, dissolving hydroxides generates heat.

So if we look, those pellets at the bottom of the beaker start glowing quite hot

And the solution just above it starts to glow hot.

But just like the acid, that solution is very dense,

far denser than ordinary water, so it doesn't shoot up towards the surface,

but that concentrated layer gets hotter and hotter.

And you have to be quite careful because you can generate a lot of heat without realising it.

Just as before, if you stir it you can get that heat to go up into the bulk of the solution.

But unlike the acid, because the pellets are solid they stay at the bottom of the beaker.

So you keep on generating the heat at the bottom of the beaker,

but very slowly you can see the heat spreading.

If I'd been asked beforehand what would happen with the temperature in these two,

and how they would behave, if I'd thought a long time I might have guessed,

but even to me it was quite a surprise to see this in reality.

The process that's generating the heat is fundamentally the same in both cases.

This is water molecules interacting with positive and negative ions.

The way they interact is very slightly different,

because in one case you have a metal ion that is positive,

in the other case you have an H+ ion, a proton, which is very, very much smaller.

Because H+ doesn't have any electrons round it,

so the quantum mechanics of the interaction is slightly different.

But on a more global scale, the mechanism is pretty similar.

So what the take home message is, is that both acids and alkalis

dissolve in water, generating heat, and very dense solutions.

Brady: Special thanks to Google's Making and Science Team for helping out with this series of thermal imaging videos.

To see more films they've helped make, with various creators across YouTube,

check out the link on the screen or in the video description.

And likewise, there are more links if you'd like to see films made by us here at Periodic Videos

over the years, including videos on each and every element on the Periodic Table.