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.

[MUFFLED THUMP]

[ELECTRICAL CRACKLING]

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