Let’s say someone offers you a lot of money to pour a vial of acid on your face. They won’t tell you what kind, but they will reassure you that it’s a “weak acid.” Should you do it?

The answer, of course, is no. Even if they show you the money up front, it’s going to to be hard to enjoy your cash with no face. A weak acid can take your face right off your skull, and then probably take your skull off as well.

Hydrofluoric acid, for example, can dissolve a glass vial. It’s incredibly corrosive, but it’s still a weak acid. “Weak” and “strong” aren’t the same as “dilute” and “concentrated.” As it turns out, “weak” and “strong” have a specific meanings in chemistry. Acids are often known as “proton donors.” When an acid hits water, a hydrogen nucleus, otherwise known as a proton, breaks off from the acid molecule and swims around in the water. By definition, a strong acid will dissociate completely, shedding all its hydrogen atoms, when it is mixed with water.

Hydrochloric acid is a strong acid, even when it isn’t concentrated. Put it in water and it will change from HCl to H+ and Cl. This makes it a strong acid if only a single molecule goes into a gallon of water. Hydrofluoric acid, on the other hand, is incredibly reactive because fluorine is reactive, but no matter how concentrated it is, when placed in water it doesn’t split completely from HF into H+ and F, so it’s a weak acid.

Image:André Künzelmann