Silane. I never want to smell silane ever again.

I made some accidentally last year, and will not repeat it. I had done the standard high-school demo where magnesium ribbon is burned in a crucible, mass gain measured, and the mole ratio of oxygen/magnesium calculated confirming that magnesium oxide is indeed MgO. I figured I could do this nice easy demo and if it worked out, turn it into a whole-class lab. And kids always love when magnesium burns, so it could be great.

Turns out that this reaction leaves gray stains on the crucible. So after school I tried to clean the crucible. After washing and scrubbing failed, I got out the 6 M hydrochloric, which usually works great for iron stains and such. I was pretty surprised to see a lot of bubbling, followed by puffs of flame and pops and the worst thing I had ever smelled. I mean, worse than anything I'd ever smelled doing bench orgo, which I actually spent many years doing. This smelled like death.

Turns out the gray stuff was probably Mg2Si, formed by reduction of SiO2 by the burning Mg. Upon acidification, off came SiH4.

I don't know what other teachers do: maybe keep a dedicated set of crucibles for this experiment and don't allow anyone to ever put acid in them? But I gave up plans to turn this into a class lab.