This is an old rhyme taught to students in many chemistry classes so that they’ll remember the correct way to combine strong acid and water.
The worst laboratory accident I’ve ever witnessed happened when a student failed to follow this simple rule, though it was made far worse than it should have been by mistakes on the part of the school administration.
I was taking organic chemistry during the summer semester at my undergraduate college. The university administration, in its infinite wisdom, decided to cut costs that semester by doubling up the organic chemistry labs and by cutting back on air conditioning the science building.
Bear in mind that we were in West Texas. In July. It was ninety degrees outside, and there were thirty of us packed in a lab built for twenty with Bunsen burners going all the time. It must have been close to 115 degrees Fahrenheit in that lab.
So, we were wearing our safety goggles, yes. But almost everyone was wearing short-sleeved shirts, and many kids were wearing shorts.
One day, a student was at the crowded fume hood trying to mix hydrochloric acid and water. He made the mistake of pouring the water into the acid. He made a bigger mistake when, after the mixture started to spatter and smoke, he panicked and pushed the big reagent bottle away from him.
The huge bottle of hydrochloric acid fell over, sending an equally-huge splash of acid over a knot of about a dozen students nearby. A girl who was wearing shorts got acid all down the backs of her legs; she was in bandages for weeks afterward. I don’t know anyone in the room who didn’t find holes in their clothing from splashed acid afterward.
When things settled down, we learned an important sub-lesson: cotton protects you from acid much better than nylon or polyester. The people who were wearing cotton got holes eaten in their clothes but not their skin. The people who were wearing synthetic clothes that got soaked got burned.
Anyhow, I’m still amazed none of the injured kids sued the school. The next day, we were separated into two different labs, and the building was cooled to nearly arctic levels so that we could wear long pants and long-sleeved shirts without fear of getting heat stroke.