Henry's Law for Dummy Chemists

Henry's Law

There's a type of comedy (if you want to call it comedy) that calls for embarrassing the heck out of people and then laughing at them. It works even better if you can get them to embarrass themselves... not unlike the self-flagellation that so many J-school grads carry out when they try to translate science-y stuff into something the hoi polloi can comprehend. If you need an example, we have one for you: check out how Karize Uy tried to answer the question, "What Is Henry's Law?" for WiseGEEK.com.

Before we go any farther, here's what Henry's law actually says:

As long as temperature is held constant, the amount of a gas that will dissolve in a liquid is proportional to the partial pressure of the gas in equilibrium with the liquid. 

Here's Karize's translation of that definition:

"..the mass of a dissolving gas within a certain amount of liquid is equally proportional to the pressure exerted on the gas..."

As if that didn't make enough sense, she reworded her reword: "...the more pressure there is, the more gas can dissolve and blend into the liquid," All of that strongly suggests that Uy is so unfamiliar with chemistry that she doesn't understand words like "dissolve," "concentration," and "proportional"; as well as the phrase "partial pressure." All of that raises the question,  what is this "communications" grad doing writing about chemistry? We rather liked this bit of padding:

"In formulaic terms, the gas law can be summed up this way: p = khc, where “p” represents the gas’s partial pressure, and the c is the solute’s concentration. The kh is the constant variable, depending on which gas is analyzed. The formula can also be expressed in other inverse forms, such as kh,pc = p/c or kh,cp = c/p."

First, Karize, that's not what "formulaic" means. Second, The construction "constant variable" is an oxymoron: a value is either a constant or a variable, but not both (FWIW, k_h is Henry's constant). Third, we have no idea WTF "kh,pc = p/c" is supposed to mean. Thumbs down to Uy and her editor, Lauren Fritsky.

The remainder of Uy's post is boilerplate about what Henry's law means in the real world, concentrating on the bends for divers and carbonation of soft drinks. Even that is... questionable, resulting in such claims as,

"Henry's law explains why soft drinks don't taste as good when exposed to air for long periods of time..." which Karize claims is because "...much carbon [has] been released."

We're pretty sure she means carbon dioxide... Of divers, Uy says,

Henry’s law can also be experienced by divers who usually feel heavier the more they descend into deeper waters. This is because the nitrogen is absorbed more by the bodily tissues because of the increased pressure underwater. When divers swim up, they experience a lighter feeling because of the release of gases, a very similar occurrence of the carbons fizzing out of the soda."

Again, we're pretty sure "heavier" or "lighter" has squat to do with Henry's law; not to mention that this is a truly ignorant attempt to describe decompression illness (aka the bends). It's just the failure of our Dumbass of the Day (and of her editor) to grasp even fairly simple scientific principles. Sigh.

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