We all have experiences with chemicals that stick in our memory. Experiences where we have witnessed just what chemical potential really means. Proton or electron transfer can be downright frightening sometimes. Rude and abrupt phase changes or angry exotherms. Sometimes nature rages back at our feeble attempts to take the dragon out for a walk on a short leash.
I can name many exciting materials, but I think that chlorosulfonic acid is one of the more exciting and obnoxious substances that isn’t explosive or neurotoxic. What are your favorites?
Lamentations on Chemistry 
My favorite is CS2 at the time of a liquid to vapor phase change via a run-away exothermic reaction that vomited the contents of the tank all over me.
DMSO–It catches fire rather unexpectedly.
Ah yes chlorosulfonic acid. We use it in synthesizing sulfanilamide in the sophomore lab. Should see what happens to the syringe needles after one section is done.
For raw beauty – the addition of aluminum metal to a round bottom flask filled with bromine.
For gaffaw factor – when the safety officer told a lab neighbor that his high pressure ethylene reactor had a lot of potential energy.
How interesting that DMSO catches fire. I have never had that experience. What were the circumstances?
Bromine and Al. Sounds like fun to me. I believe it is called “combustion”.
Nothing puckers the sphincter muscles like hanging around high pressure reaction vessels.
hah! Heated to reflux in a temperamental mantle. Probably pentane (which I’ve also experienced burning) and/or ether (which, fortunately, I haven’t) should be higher on the “fire!” list, but I’ve had DMSO catch fire more than any other solvent.
Run away oil bath.
I have not had it happen to me but I have had to put others’ out. It helps to plug the oil bath in a variac!
I’ve actually seen home made oil baths with coiled nichrome wire glowing red in the bubbling mineral oil- plugged straight into the wall. I was never lucky enough to see one in flames. \;-)
I have seen LAH/Ether in flames in my hood back in my undergrad research days. I was trying to kill it in a 1 L beaker with…gulp… water. Luckily, I had a transite square to set on it.