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  • Even really simple experiments can sometimes teach you interesting things

  • 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.

Even really simple experiments can sometimes teach you interesting things

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B2 中高級

酸和鹼(熱成像) - 視頻週期表 (Acid and Alkali (THERMAL IMAGING) - Periodic Table of Videos)

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    林宜悉 發佈於 2021 年 01 月 14 日
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