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  • HF - hydrofluoric acid is probably the most feared chemical compound that there is.

  • The reason it's so feared is that is that it is very corrosive. It will burn through human skin.

  • Even quite a small exposure on your skin can cause a heart attack.

  • And I really wanted--and I still want--to do an experiment with HF.

  • ... or rather, I want Neil to do an experiment for me with HF.

  • I want to see what happens if we put a lightbulb--an electric lightbulb--the old, not energy saving sort

  • into a beaker of HF. The reason why it's interesting is because HF can dissolve glass.

  • Glass, the chemist's friend: we use it for test tubes, bottles, Kipp's Apparatus

  • but HF attacks glass. Chemically, it breaks the silicon-oxygen bonds and forms silicon-fluorine bonds

  • and the compound that dissolves away in the liquid HF.

  • It was really a bit disappointing. The lightbulb sat there glowing, peacefully in the HF

  • The HF has to be in a special plastic beaker that is cloudy, so you can't see through the side.

  • And it just sat there. And poor Brady had to stand with the camera: watching, watching, watching...

  • in case something happened. Because the reaction produces some heat, that's the reaction of glass with HF.

  • And also because the lightbulb--an electric lightbulb is energy inefficient--is also producing heat.

  • The water warmed up. And just like when you have a hot bath in a cold bathroom the walls starting misting up.

  • Of course the top didn't mist up because there was a large spotlight which was producing heat on the top

  • so, the top was warmer. So fortunately we could still see what was going on.

  • And then, rather like fishing where there's a a long, boring period and then everything happens quickly.

  • Suddenly, the lightbulb shattered.



  • But it didn't shatter in the way you would expect. It was not like firing a bullet or smashing it with a hammer

  • It neatly cracked all the way around the surface exactly where the glass was at the surface

  • between the liquid and the air. Which was something that we hadn't expected.

  • And it produced a really quite neat cut.

  • Then, as soon as that happened, the bottom part--which was very thin because a lot of it had dissolved--slowly

  • sank away. The acid rushed in, dissolved the filament, and so, the light went out in a puff.

  • And then, the electrodes--which are much thicker so they didn't dissolve so fast and which were still live--

  • started sparking a bit because of some electrolytic reaction. Exactly what's happening, I'm not sure

  • and because we were using alternating current it would be quite complicated because it was switching

  • between positive and negative. And then Neil had to start clearing up. Now the clear up procedure is pretty

  • simple chemistry. You add sodium carbonate and you make sodium fluoride. In principle, you could

  • have used it in toothpaste but it's not the of right grade for using with humans.

  • So, he added sodium carbonate. CO2 was released--lots of bubbling--and then it's safe.

  • What I think it illustrates, yet again, that every time you do an experiment, it will often go in the way you plan

  • but you learn things because there's always an unexpected aspect.

  • And, it was a nice experiment because it went well. It didn't explode, but, it gave a really nice result

HF - hydrofluoric acid is probably the most feared chemical compound that there is.

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