Sometimes everyday observations contain nice illustrations of scientific principles. In this Science Update, you'll hear how a quirk of dish drying shows off a fundamental idea behind the chemistry of water.
Why dishes dry differently. I'm Bob Hirshon and this is Science Update.
Today's Why Is It question comes from Lou Bergeron of San Francisco, who made an observation while doing the dishes. He noticed that a plastic cup takes a lot longer to dry than a glass does, and he wants to know why.
Well, Lou, we consulted our favorite chemistry expert, Joseph Merola of Virginia Tech. He says most plastic cups are made of polyacrylate, a material consisting of water-repellent hydrocarbons.
What happens is that if you put water into a cup like that, and you dump it out, and you wait for it to dry, you'll notice that the water beads up very well on a surface like one of those plastic cups.
Glass, on the other hand, attracts water. Its surface has lots of hydrogen and oxygen—just like the H and O in H2O. So, the water spreads out thin in a sheet.
And evaporation goes better—since evaporation comes from water molecules at the surface of the water jumping off into the gas phase—the more surface area you have, the better the drying will be.
If you've got a science question, we'll dish up an answer. Call us at 1-800-why-isit. If we use it on the show you'll get a free Science Update mug. For the American Association for the Advancement of Science, I'm Bob Hirshon.
Making Sense of the Research
The first thing that's important to understand here is that water molecules are attracted to one another. Not so strongly that an ice cube would leap out of your soda, slide down the hall, and splash into the nearest toilet, but strongly enough that tiny bits of water that are very close together tend to clump up and stick to each other. That's called cohesion.
The attraction between a water molecule and some other kind of surface is called adhesion. And it's different for every surface. As Merola explains, most plastic cups aren't very adhesive to water. In fact, they tend to repel water molecules.
Now, if you fill a plastic cup with lots of water, there's not much the water can do about that. It's held in the glass under its own weight. But if you wash the glass and leave just a thin film of water behind, all those water molecules will try and stick to each other while touching as little of the plastic as possible. And mathematically, the best possible shape the water drops can make are little beads. (Imagine if you were cooking ground beef in a pan—if you rolled the meat up into a ball, very little of it would actually be touching the hot surface.)
Glass, on the other hand, is strongly adhesive to water, because the chemistry is pretty similar (both have hydrogen-oxygen combinations). As a result, water spreads out and sticks to glass just about as easily as it sticks to itself. And because the molecules spread out so thinly, they evaporate faster. (During evaporation, water molecules have to escape directly into the air—and if the water's beaded, the molecules inside the bead have to wait for the surface molecules to evaporate first.)
Incidentally, not all plastics are repellent to water. The kind of material used in disposable diapers, for example, is incredibly adhesive; that's why they absorb so much.
Now try and answer these questions:
- What is the difference between cohesion and adhesion?
- How do these principles help explain why water beads on plastic, but not on glass?
- Can you think of other substances that are strongly or weakly adhesive to water?
- Why do you think water doesn't form giant beads in a river or a swimming pool?
The University of Arizona's Biology Project offers this online tutorial on the chemistry of water.
The Chemistry of Water, from the National Science Foundation, takes a look at one of our most famliar, yet complicated, substances.