字幕表 動画を再生する 英語字幕をプリント After this, whenever he was lecturing the reaction, he would hold up his hand and say, "I had a hand in that reaction." So this time, I've got potassium dichromate, some sulfuric acid, and then finally on top of this mixture, which is in- dissolved in water, I'm going to add some diethyl ether. Diethyl ether is an organic compound which does not mix with water. So if I add it carefully, it should float on the surface. So if you look now, you can see, there's a colorless layer here of ether, with the dichromate underneath. So now, I put some hydrogen peroxide into this small beaker, and we'll add a little and see what happens. So watch carefully and with luck something will change. So... What you can see is that there's quite a violent reaction, and the ether layer has gone bright blue. And the reason it's gone bright blue is because we have formed a chromium compound with the formula chromium-O₅. Which, unusually for an inorganic compound, will dissolve in an organic solvent. And so the hydrogen peroxide has oxidized the chromium. It was chromium-O₄⁻², it's now chromium-O₅. A quite dangerous compound. And it's important with this blue compound, it's very intensely colored, so there isn't very much here... but one must be very careful not to isolate this material. There is a famous story of a chem- a chemistry professor in Cambridge, whose student made a whole test tube full of this solid compound. And the professor was shaking the test tube, when it went off with an enormous bang, and removed 2 of his fingers. And after this, whenever he was lecturing the reaction he would hold up his hand and say, "I had a hand in that reaction." And all the students got very frightened. Slowly, what is happening, is that the hydrogen peroxide is mixing and reacting with it. But in a long tube like this, you don't get very good mixing. If I was to stir it with a rod or something like that, I don't have one here, it would all react at once. But this is quite nice to do it like this, because then you see the before and after color at the same time. And you can see the reaction is taking place here, and the oxygen, O₂, which is released by the reaction, is coming up. And you're left with some the, um, chromium in solution here, and it's a, um, greenish color for chromium 2. So kind of like the reaction point is like a wave that's moving down the... Yes. Huh. But, that's nothing to do with the chemistry, that's because the mixing is so bad in this sort of vessel. That's why chemical engineers have to design proper-shaped vessels so that reactions go in a controlled manner. You saw in this case it wasn't very controlled to begin with, it all splurted out of the top. Which looks quite spectacular, but is not very good practice. We're trying again, this time I thought to put water. I put warm water in here so the reaction may be faster, and I'll put in most of the acid, as well. [Nerf gun firing] [clinking] And... then a bit of permanganate... and now we've got enough peroxide still left. You can see it's decolorising straight away.