What Einstein Told His Barber: More Scientific Answers to Everyday Questions - Robert L. Wolke (2000)
Chapter 6. All Wet
It's the one substance that is indispensable to all living things.
It makes up more than half of our own body weights.
It is the most abundant chemical on Earth, with more than a billion billion tons of it covering 71 percent of the planet's surface and probably another billion tons in those little plastic bottles that everyone carries around these days.
When even a little bit of it is discovered on another planet, astronomers grow giddy with speculation about the existence of extraterrestrial life.
It is water, H2O, one of the simplest and most stable of all chemical compounds.
We normally think of water as a liquid, because that's what it is throughout our most comfortable range of living temperatures: between, say, 40 and 80 degrees Fahrenheit (4 and 27 degrees Celsius). But as you know, at any temperature below 32 degrees Fahrenheit (0 degrees Celsius) it prefers to exist as the solid that we call ice. And at any temperature above 212 degrees Fahrenheit (100 degrees Celsius) it prefers to exist in the form of vapor—an invisible gas, just like the nitrogen and oxygen gases in the air.
(Water vapor isn't steam. Steam is a cloud of tiny droplets of liquid water that are too small to fall out of the air.)
Water doesn't have to reach its boiling temperature in order to turn at least partially into vapor. Wherever there is water there is water vapor in the air around it. We sometimes call it humidity, and it has far-reaching consequences in many aspects of our lives, far beyond making us uncomfortable in the summertime.
In this chapter we'll look at some of the amusing things that water does when in its liquid form, such as making coffee stains, making the oceans both salty and blue and making your wet shower curtain slap you right in the … shower. We'll take a small detour through the Panama Canal on our way to the kitchen, where we'll play with some ice cubes and lollipops before going to the laundry to find out what's inside all those detergent bottles and boxes. Then we'll examine how water vapor affects cosmetics, clothes dryers and that old devil humidity.
And as usual, we'll explode a few misconceptions along the way, this time concerning the color of water, the flow of glass and whether warm air can really “hold moisture.”
The Watery Blues
Why is the ocean blue? Is it just a reflection of the sky?
No. That's a common belief that just doesn't hold water, so to speak. First of all, the ocean's surface isn't exactly what you'd call a mirror. And second, how come it's a much darker blue than the sky?
No, the world's oceans really and truly are blue—many different shades of blue (ask any sailor), depending on several factors, a few of which we'll talk about below.
But here's a surprise: Even crystal pure water—without the salt, silt and fish—is blue. That's in spite of the fact that almost every dictionary defines water as “a colorless, odorless liquid.” All you have to do is fill your bathtub and see for yourself that it isn't colorless.
So why is water blue? Because when daylight, which contains all colors of light mixed together hits the water and penetrates it, certain colors in the daylight are absorbed by the water molecules. The light that is reflected back from the bathtub and reaches your eye after passing through the water is then diminished in those particular colors, so it has a different color composition from the original daylight.
Fill your bathtub and look at the water. You'll see that it's a pale blue color. (I assume that your bathtub is white.) The only reason that you don't see blue in a glass of water is that you're not looking at enough water. The color builds up, or accumulates, as you look into or through thicker and thicker layers of it. If the windowpanes in your house were ten times as thick as they are, you'd see that the “colorless” glass is really green.
Specifically, water molecules show a slight preference for absorbing the orange and red portions of the sun's light. Light that is diminished in orange and red looks to us as if it has too much blue, compared with what we call “white light.” So the water appears blue.
But an ocean is a much more complicated kettle of fish than just H2O. In addition to the obvious salts and minerals, it contains plankton, for one thing: tiny plants (phytoplankton) and animals (zooplankton) that are too small to settle out and that float perpetually about until decomposed by bacteria or eaten by anything bigger than they are. (It's a cruel world.)
Seawater also contains a lot of miscellaneous dissolved organic matter that scientists call by its German name, gelbstoff. Loosely translated, it means “yellow crap,” because that's what it looks like when it's dry.
When daylight enters seawater, the phytoplankton absorbs mostly blue light plus a little red, while the gelbstoff absorbs mostly blue light. These absorptions shift the balance of the remaining light from the pale blue of pure water to a deeper, purplish blue. That's why the oceans are darker than the water in your bathtub, which I certainly hope is devoid of gelbstoff.
Unfortunately, the many faces that the seas present to us in different weather and in different parts of the world are not that easy to explain. For one thing, it's not just the absorption of light that gives seawater its color, it's also the scattering of light. Certain colors of light are scattered by microscopic particles of matter in the water. When a photon of light hits one of those particles, which might be anything from a single molecule on up, it can ricochet off in another direction. This changes the distribution of colors that reaches our eyes.
It is just such a scattering of light from air molecules that makes the sky blue, because air molecules scatter blue light more than other colors. Some scientists have tried to explain the oceans' blueness as being due entirely to the same kind of scattering, but they've apparently never peered into their bathtubs.
Phytoplankton is an especially good scatterer of green and yellow light, so in general the more phytoplankton there is, the more greenish the water will be. That's what is largely responsible for the beautiful greenish turquoise color of the waters surrounding the Caribbean and South Pacific islands. The tropical climate and abundant sunlight there create lush breeding grounds for plankton.
All That Salt and No Popcorn
Why are the oceans salty?
When you say “salty,” you're undoubtedly thinking of sodium chloride, common table salt. But to a chemist, a salt is any member of a large class of chemicals, and there are dozens of them in the oceans.
To put the word “salt” in perspective, please indulge me in a one-paragraph chemistry lesson.
A “molecule” of salt (it's not really a molecule in the strict sense, but I won't tell anybody if you don't) consists of a positively charged part and a negatively charged part that, being oppositely charged, attract each other. The positive and negative parts are called ions. In the case of sodium chloride the positive ion is a charged atom of sodium and the negative ion is a charged atom of chlorine. But a salt's positive ion can be a charged atom of any metal, and there are some eighty-five known metals. Also, there are many negative ions besides chloride, so you can see that there is a very large number of possible salts.
End of chemistry lesson.
The main metal ions in seawater are sodium, magnesium, calcium and potassium, while the main negative ions are chloride, sulfate, bicarbonate and bromide. Your question, then, is how all this stuff got into the oceans in the first place. The short answer is that it was washed out of the land by rain-water, which then flowed as rivers to the seas.
Seawater is continually being recycled. Each year, the top meter (3 feet) or so of the oceans evaporates into the air, moves around in various weather systems and falls back onto the oceans and land as rain and snow. Of this precipitation, 76 percent falls on the oceans and 24 percent falls on the continents. The water that lands on the continents flows down in streams and rivers, eventually returning to the seas. In the process of washing down, these waters pick up anything that will dissolve, mostly the salts that exist in the soils, rocks and minerals.
Any chemist will tell you that sodium salts dissolve more readily in water than do salts of potassium, magnesium, calcium or most other metals. More than any others, then, it's the sodium salts that dissolve and wash down into the oceans. There are approximately equal amounts of sodium and potassium in the soils, rocks and minerals, yet there is twenty-eight times more sodium than potassium in seawater.
All of these dissolved salts make up 3.47 percent of seawater, by weight. Only six elements make up more than 99 percent of those salts: chlorine, sodium, sulfur (in the form of sulfates), magnesium, calcium and potassium, in decreasing order.
Another source of sea salts is volcanic eruptions, both on land and under the sea, which spew out enormous amounts of solids and gases. Among the prominent volcanic gases are chlorine and sulfur dioxide, which may account for the fact that chlorine is the most abundant element in seawater, making up 55 percent of the salts' weight, while sulfates are second only to chlorides as the negative-ion portions of the salts.
Putting all this together, sodium and chlorine make up 86 percent of the salts in the oceans. So if you want to say that the oceans are salty because of sodium chloride, nobody will give you much of an argument.
You Didn't Ask, but …
Why are the oceans salty, but not the streams, rivers and lakes?
Rainwater washes down from the land into the streams, rivers and lakes, carrying dissolved salts just as it does when it washes into the oceans. But the difference is that the oceans are much older than the other waters—4 or 5 billion years old, compared with mere millions. Over those billions of years the oceans have been recycling their water—evaporating water that rains out on the land and flows back, returning each time with a fresh load of salts. These cycles have continually increased the load of salt in the oceans.
Where the Devil Is Sea Level?
I understand that the Panama Canal has locks because the Atlantic and Pacific Oceans aren't at the same level. Then what do we mean when we talk about elevations above “sea level”? Which sea?
Whoa! That's not why the Panama Canal has locks. The locks are there for the purpose of lifting ships over the hump of land known as the Isthmus of Panama. To get over that hump, the ships have to be floated upward 85 feet (26 meters) above the entrance ocean and then lowered back down into the exit ocean on the other side. That goes for both directions.
Why didn't they just dig a flat ditch from one ocean to the other, a so-called sea-level canal? Mostly because it would have meant excavating a tremendous amount of dirt at tremendous expense. But also, there would have been torrents of water gushing through a sea-level canal. That's not because of any permanent difference in level of the two oceans, however; their average sea levels are just about the same. It's because of the tides.
At the Pacific end of the canal the tides can rise and fall as much as 28 feet (5.5 meters), whereas at the Atlantic end the tides vary by only about 2 feet (60 centimeters). So there would be periodic surges of water through the canal from the Pacific to the Atlantic—that is, from east to west.
Do you think I got that backward? Isn't the Pacific Ocean at the western end of the canal? Nope. Because of the way the Isthmus of Panama snakes around, the Pacific entrance to the canal is 27 miles (43 kilometers) east of the Atlantic entrance. Check a map.
A ship passing through the Panama Canal from the Pacific Ocean to the Atlantic Ocean sails from east to west. (It's actually southeast to northwest, if you must quibble.)
Now back to what we mean by “sea level.”
Obviously, because of the tides, we can talk only about the average, or mean, level of any ocean at any location. While the mean levels of the Atlantic and Pacific Oceans may be approximately the same at the Panama Canal, for example, that doesn't mean that all the oceans in the world have settled down to a common level. You might expect them to be that way, because if you look at a globe you'll see that they're all connected; Earth's oceans are one gigantic swimming pool with chunks of land scattered about. But even when you average out the tides, there are reasons why the oceans differ in their levels.
For one thing, because gravitational effects are bigger for bigger masses, the bigger oceans will be raised into higher tides by the moon's gravitational pull. (Only the biggest lakes have tides.) Weather patterns also affect the sea levels. When the air pressure over an ocean is low, the water will actually expand. Moreover, prevailing westerly winds can actually make the water pile up somewhat toward the east. And finally, differences in ocean depth can have a gravitational effect on the water's level, because the deeper an ocean is, the more tightly its waters are packed down by gravity, and the lower its surface level will be.
These are all small effects, but when working on such huge bodies of water, they can make significant differences in the sea level at different locations in the world. Since they are all connected, the waters do try, of course, to seek a common level, but they are just too slow to keep up with all of these changing conditions.
So what is mean sea level? It's a carefully compiled average, measured over a period of nineteen years at many tidal stages in many places around the world. Whenever you hear that something is so many feet or meters above “sea level,” or that the atmospheric pressure at “sea level” is so many inches or millimeters of mercury, it's understood that they're talking about mean sea level: a worldwide, long-term average.
Why-ing Over Spilt Coffee
When spilled coffee dries on my kitchen counter, it forms a brown ring, with almost nothing inside. Why does all the coffee go to the edges to dry?
For years, people have observed this phenomenon without giving it a second—or even a first—thought. Hundreds of less-than-fastidious, coffee-sipping scientists have probably glanced at the ring, mumbled something about surface tension and told their lab assistants to clean it up.
But it wasn't until 1997 that six scientists at the University of Chicago pondered this earthshaking question and published their results in the prestigious international scientific journal Nature for the benefit of all mankind—or at least for those slobs among us who don't wipe up their spills before they dry.
Here's what they concluded after producing reams of mathematical calculations, undoubtedly supported by lots of caffeine.
When a coffee puddle finds itself on a flat, level surface, it tends to spread out in all directions. In any given direction, the liquid will stop spreading when it hits a barrier, any slight irregularity in the surface that it can't cross, such as a microscopic ditch. Depending on where the barriers happen to be, the puddle will take on a certain shape: longer in this direction, shorter in that, like an amoeba.
As evaporation takes place, the puddle will start to dry first where it's thinnest: at the edges. That would have the effect of making the puddle shrink, pulling its edges back, but it can't do that because they're stuck in the ditches. So as water evaporates from the edges, it has to be replenished from somewhere, and the only place it can come from is the interior of the puddle.
Thus, there's a movement of water from the interior of the puddle to the edges, where it evaporates. That flow of water carries along with it the microscopic brown particles that give coffee its color. The brown particles then find themselves stranded at the edges when the puddle finally runs out of water.
First, clean your kitchen counter; no grease films allowed. If your countertop is light-colored, spill about a quarter-teaspoon (a milliliter) of coffee—black, no sugar—on it and let it dry overnight. You'll see the brown ring. If your countertop is dark, the effect is much better if you use salt water. Dissolve about half a teaspoon (a few grams) of table salt in half a cup (250 milliliters) of water and make a few quarter-teaspoon (milliliter) puddles on the counter. When they're dry, you'll see white rings of salt. The salt crystals are coarser than the coffee particles, so the rings will be more irregular.
When I'm taking a shower, why does the curtain sneak up and slap me on the leg—or somewhere?
You're lucky you asked that question today, because today is bargain day. I'm going to give you four answers for the price of one. It's not because I'm feeling generous, but because I can't make up my own mind about which one to believe.
To my knowledge, the National Science Foundation has not yet funded a comprehensive university research project designed to solve this perplexing problem, so scientists have been left to debate their theories over coffee and beer. Here are four contending solutions to the great shower curtain mystery. Ya pays yer money and ya takes yer choice.
(1) Hot air rising. The story goes that the air inside the shower is heated by the water and, as everybody knows, hot air rises. (But if you don't know why hot air rises.) If the hot air in the shower is rising, then cold air has to rush in to replace it at the bottom, and in the process it blows the curtain inward. This is a nice, simple, appealing and wrong explanation. Just try it with cold water instead of hot, and you'll see the curtain move inward just as much. (You don't actually have to be in the cold shower; you can do the experiment while standing outside.)
(2) Electrostatic charge. When water streams out of a narrow opening such as the hole in a showerhead, it can pick up a static electric charge. It's not too different from scuffing your feet on a carpet, whereupon some electrons are scraped off your shoes onto the carpet and you develop a positive charge. Electrons can also be scraped off—or onto—the water by the showerhead, depending on what it's made of.
But if the water's molecules were to pick up, say, a negative charge on their way out of the showerhead by picking up some negative electrons, those extra electrons would repel some electrons from the surface of the shower curtain, because similar charges repel each other. That would leave the curtain's surface with a deficiency of negative charge, and its inherent positive charges would dominate. The negative water and the positive curtain would then attract each other, as is the wont of opposite charges, and the curtain would move toward the water.
This isn't quite as far-fetched as it may sound. (Wait'll you see some of the other explanations.) Induced electrostatic charge, which is what the phenomenon is called, does happen and is well-known. Have you ever opened a carton packed with those styrene foam packing peanuts, especially when some of them are broken into small fragments? Just try to keep those maddening motes from jumping around or sticking to your hands as you try to brush them away. It's because of induced electrostatic charges.
On a dry winter day, put a few small fragments of styrene foam packing peanuts on a table. (If you haven't received a package lately, you can use torn-up scraps of lightweight paper.) Now walk across a nearby carpet while scuffing your feet to acquire a body charge. Go quickly to the table and try to touch the plastic peanuts. Even before you touch them, they will jump up to meet your hand. Your body's static charge induced an opposite charge in the plastic and the resulting attraction of opposite charges was enough to make them leap up toward you.
Whether or not induced electrostatic attraction is strong enough to move a shower curtain, however, is up for grabs.
(3) Bernoulli's Principle. The water is carrying along some entrained air, making an air current near the inside surface of the curtain. According to Mr. Bernoulli, the faster a gas moves across a surface, the lower its pressure against that surface. Since there is no speeding airstream on the outside of the curtain, the air pressure on the inside is lower and the curtain moves inward.
(4) The Coanda Effect. Fluids have a tendency to stick closely to a curved surface over which they are flowing. This phenomenon is known as the Coanda Effect in honor of Henri Coanda (1886–1972), a Romanian aeronautical engineer who first called attention to it.
Hold a drinking glass horizontally in the stream of water from a slowly running faucet, so that the stream falls onto one side of the glass. Notice that when the water gets to the bottom it doesn't fall straight down. It sticks to the glass and follows its curved surface beyond the bottom before falling off.
In the shower, if the curtain is already curved inward somewhat, perhaps from one of the other effects, the water flowing over its surface may pull it farther inward because of Coanda stickiness.
Figuring out exactly why a flowing fluid sticks to a curved surface took Coanda and other aerodynamic engineers more than twenty years. Here's the ultimate explanation.
The molecules of a fluid exhibit some stickiness toward one another; what some molecules are doing affects their neighbors because they are sort of tied to one another. (Techspeak: Fluids have a certain viscosity.) If one layer of a flowing fluid's molecules should have some adherence to a surface over which it is flowing, the rest of the molecules will be dragged down to the surface along with them to some extent, and the fluid as a whole will tend to stick more than we might expect.
In the case of the water on the glass, the first layer of water molecules wants to stick because water wets glass. (It doesn't wet wax, for example.) The second layer wants to stick to the first layer, so it is also weakly attached to the glass. The third layer sticks to the glass through the first two layers and so on, with each successive layer sticking less strongly than the preceding layer. Many other layers are dragged along for as long as the stickiness exceeds the pull of gravity, and then the water finally falls off the glass, having gone farther around the curve than we would have expected.
The attraction that air molecules have for one another is a lot smaller than in the case of water (Techspeak: The viscosity of air is a lot less than that of water), so it will stick to the shower curtain's surface a lot less, but the effect is still there. Both the water and its entrained air probably contribute to the attraction of the shower curtain.
That is, if you believe that the Coanda Effect is the true cause of your flapping, slapping curtain. Me, I favor the electrostatic explanation.
Unless I use one of those fabric-softening dryer sheets, all my clothes come out of the dryer full of static electricity, sticking to one another. What do fabric softeners have to do with static electricity?
Not much, except that the stuff in the dryer sheet happens to be good at both jobs. You can obtain the static-elimination function all by itself as a liquid in a spray can, so you can de-static your clothing even while you're wearing it without your having to—DO NOT TRY THIS AT HOME!—climb into the dryer.
The main ingredient in both types of products is a surfactant, a chemical that is made of what might be called bisexual molecules; they are attracted to both oil and water. Most other chemicals show a strong preference for one or the other.
For example, common salt (sodium chloride) is made of electrically charged atoms (Techspeak: ions), and charged atoms like to mix into—dissolve in—water because water molecules have electric charges that attract them. But salt won't have anything to do with fats and oils because their molecules don't have any attractive charged parts. Just try to dissolve some salt in olive oil and see how far you get.
Surfactants, however, are peculiar in that one end of each molecule is a fatty material that is attracted by oils, while the other end is charged and is attracted by water. Soap and detergent molecules are surfactants; their oil-loving ends latch on to oily dirt and drag it into the water by means of their water-loving ends. Or looking at it the other way, their water-loving ends drag water into oily places that it wouldn't ordinarily invade, thereby making the water wetter.
Now let's impregnate a paper sheet with a soapy-feeling surfactant chemical and throw it into the dryer along with our wet clothes. As they tumble, the clothes rub against the sheet and become coated with surfactant. The rather hefty fatty ends of the surfactant molecules impart a slippery, waxy feel to the clothes, “softening” them.
Then when the clothes begin to dry, their friction against one another rubs off some electrons and static electricity begins to build up. The charges can't build up as long as the clothes are wet because water conducts electricity well enough to conduct the rubbed-off electrons back to where they came from. When the water is gone, the charged ends of the surfactant molecules take over, conducting the charges away and killing any “static cling” that might result.
Deprived of their static cling, socks find themselves unable to bond with their partners and may suffer a severe separation-anxiety syndrome. In fact, a sock may become so depressed and emotionally unraveled that it will slink away through the vent tube in search of psychiatric help. That's why you will sometimes find a sock missing when you put away your laundry. I know you have wondered about that.
There are three kinds of surfactants whose names you will see as ingredients on the labels of dryer sheets, clothes softener liquids, antistatic sprays and synthetic detergents (see the following). They may be listed as cationic(CAT-eye-ON-ic), anionic (AN-eye-ON-ic) or nonionic (NON-eye-ON-ic). The charged ends of the molecules can be either positively charged (cationic) or negatively charged (anionic). The nonionic surfactant molecules aren't charged at all, so they may be good at clothes softening but are of no use for killing static cling.
A widely used cationic surfactant is dimethyl ditallow ammonium chloride, and a common nonionic surfactant is polyethylene glycol monostearate. Laundry detergents (see the following) commonly contain the anionic surfactant sodium alkylbenzenesulfonate.
As if you cared, right? But now you can have fun decoding the fine-print ingredient lists on all those product labels. Run right down to the laundry and check them out.
Every laundry detergent claims to be “New,” “Improved,” “Unique” and better than the others. Aren't they all just soap?
No, those detergents aren't soap, although soap is a detergent. The word “detergent” simply means a cleansing substance, from the Latin detergere, to wipe off.
After more than two thousand years of using soap, which is easy to make by boiling up wood ashes with animal fat (don't you wonder how that discovery was made?), humans finally created synthetic detergents, which in many cases work even better than soap. Today we reserve the word “detergent” exclusively for those artificial chemical concoctions that take up so many acres of shelf space in our supermarkets.
All detergents, including soap, are surfactants, chemical compounds that have the knack of bringing oil and water together. Most dirt adheres to our skins, clothing, dishes and cars by means of a sticky, oily film. Coax the oily film into the water and you have succeeded in removing the “glue” that stuck the dirt to the objects.
But all those colorful bottles and boxes on the store shelves may contain a mad scientist's laboratoryful of other chemicals besides surfactants. Otherwise, how could the manufacturers keep claiming that their products are any different from or better than all the others?
Here is a list of what may be hiding in your laundry products, household cleaners, soaps, window cleaners, dishwashing detergents and the like, in addition to surfactants. And don't forget the most expensive ingredient of all: advertising. Lots and lots of advertising.
Acids and alkalis: Acids help to remove mineral buildup, while alkalis attack fatty and oily soils. Examples: acetic acid, citric acid, ammonia.
Antimicrobial agents: Kill disease microorganisms. Examples: pine oil, tricloban, triclosan.
Antiredeposition agents: Once you get the dirt off, you have to keep it from going right back to where it came from. Examples: carboxymethyl cellulose, polyethylene glycol, sodium silicate.
Bleaches: Remove stains and “whiten and brighten” your clothes. Examples: sodium hypochlorite (chlorine bleach), sodium perborate (“color-safe” bleach).
Builders: Counteract hard water, which interferes with the surfactant's performance. Examples: sodium carbonate (washing soda), sodium tripolyphosphate. The latter is one of the notorious phosphates in detergents. If phosphates get into the sewers and then into streams and lakes, they can harm the environment by disrupting the ecological balance.
The phosphates make algae grow in profusion, and when the waters can't sustain any more algae they die off, which provides a feast for bacteria, which use up oxygen in the water and kill the fish, which makes even more dead bodies for the bacteria to feed on, etc. Because of this, phosphates have been largely eliminated from detergents.
Corrosion inhibitors: Protect the metal parts of your washing machine or kitchen utensils. Example: sodium silicate.
Enzymes: Enzymes are natural chemicals that speed up natural chemical reactions. In laundry products they speed up the destruction of specific kinds of stains, such as grass. Examples: protease, cellulase.
Fabric softening agents: Soften fabrics and control static electricity. Example: quaternary ammonium compounds.
Fragrances: Cover up the smells of all the other ingredients and make you think your laundry is “fresh,” whatever that means.
Optical brighteners: Make clothes look brighter by converting yellow light or invisible ultraviolet light into bluish or whitish light. Example: stilbene disulfonates.
Preservatives: Protect the product from oxidation, discoloration and bacterial attack. Examples: butylated hydroxytoluene, EDTA.
Solvents: Keep all the ingredients dissolved in liquid products. Examples: ethyl alcohol, propylene glycol.
Suds control agents: Control the amount of suds or make the suds keep their “heads.” Examples: alkanolamides and—guess what?—soap.
Life in the laundry isn't as simple as when all we had to do was boil up a nice kettle of goat fat and ashes.
My son's teacher told the class that glass is really a very thick liquid, and that given enough time you could see it flow under the influence of gravity. Really?
That's a commonly quoted “amazing fact” that is simply not true.
Liquids do get somewhat thicker as they are cooled, and because all glass was once a liquid while it was hot and being formed into shape, some people like to think that it gets thicker and thicker as it “supercooled,” until it gets so thick that it acts like a solid. Well, the truth is that it is a solid.
If glass flows, its motion apparently requires more than four thousand years to become detectable, because that's how long glass has been around and nobody has yet come up with any convincing evidence of its motion. That's one hole in the “supercooled liquid” theory. But the bigger, absolutely gaping hole is that glass is not a supercooled liquid, despite the fact that quite a few textbooks and encyclopedias say it is.
The “supercooled liquid” fable has been around at least since I was in school and accepted everything my teachers said as gospel. But science and I have marched many a mile since then, and there is no longer any excuse for perpetuating the myth.
If you've ever observed glassblowing or the process of molding glass into shapes, you know that when it's hot enough the glass certainly does flow like a very thick (viscous) liquid. But as it cools down, we observe no sudden transition from liquid to solid, as we do, for example, when water cools down and turns into solid ice. That has led many well-meaning scientists to conclude that the glass must still be a liquid even at room temperature, because it didn't turn abruptly rigid. Moreover, the argument goes, solids are usually crystalline, meaning that their atoms or molecules occupy precise geometric positions with respect to one another, and glass's molecules don't. Examples of crystalline solids are ice, table sugar, salt or almost any mineral you can think of. If a typical solid's atoms and molecules weren't fixed in place, they could slip and slide over and around one another; in other words, they would flow like a liquid.
But a substance's molecules don't have to be in the form of a highly organized crystal in order for it to be a solid. There are such things as amorphous solids (from the Greek, meaning “without form”), in which the molecules are indeed fixed in place, but in a more or less random arrangement. That's the case with glass. It's a solid, all right; it's just not a crystalline one. (Techspeak: Its structure does not exhibit long-range order.) When glass is cooled from the molten state, its molecules can't find a repeatable, orderly arrangement into which to fit themselves. It's the same with many other amorphous solids such as plastics and lollipops. Translucent lollipops are sugar (sucrose) in an amorphous, glassy form, as distinguished from its crystalline form in the sugar bowl.
Melt some sugar over very low heat in a small pan. If some sugar crystals stay on top without melting, stir them in with a fork. When it is all melted, but before it gets too dark, pour it out onto a cool, flat surface such as a tile countertop or the bottom of a metal frying pan. The sugar molecules will cool so fast that they don't have time to arrange themselves into crystals and will wind up in a glassy form. After it cools, you can eat your caramel-candy glass.
By the way, you can forget the word “crystal” that glassware manufacturers apply to their better-quality merchandise; scientifically speaking, it's just plain wrong. A “crystal chandelier” or “crystal goblet” is made of glass that is as amorphous as any other. It's just a particularly clear and brilliant quality of glass, usually containing lead oxide.
Okay, now it's time to address the urban legend that simply will not die: that the windowpanes in several-hundred-year-old buildings are thicker at the bottom, having flowed down somewhat over the years as any good supercooled liquid should. If you examine old cathedral windows that still have their original glass in them, you're sure to find many that are indeed thicker at the bottom. The trouble is that nobody has ever done measurements on enough panes to determine if significantly more of them are thicker at the bottom than at the top, middle or sides.
But even if a significant fraction of old panes were found to be thickest at the bottom, it wouldn't prove that they had flowed. Early window glass was made by methods that were quite crude compared with our modern plate glass processes, and uneven thicknesses were tolerated as being preferable to more serious defects such as bubbles and scratches. Now if you were a workman assembling windows from panes of uneven thickness, wouldn't you be inclined to set them in with their thickest parts at the bottom?
You Didn't Ask, but …
There must be some relatively high temperature at which glass does begin to flow. What is that temperature?
Glass experts talk about a “transition temperature” at which rigid glass does indeed become slightly plastic. For ordinary window glass, the transition temperature is about 1000 degrees Fahrenheit (550 degrees Celsius). Everyone must agree that the windowpanes in old buildings never got that hot.
If water freezes at exactly zero degrees Celsius, and ice melts at exactly zero degrees Celsius, what would happen to a bowl of ice and water at exactly zero degrees Celsius?
Absolutely nothing, as far as you would be able to tell. The ice and the water would dwell in peaceful coexistence. But down at the molecular level, a chaotic dance would be going on.
Zero degrees Celsius (32 degrees Fahrenheit) is indeed both the freezing point of liquid water and the melting point of solid ice. You are undoubtedly picturing a poor little zero-degree water molecule that can't make up its mind whether to flow or float, to be liquid or solid. Well, that's really a good way to look at it, because the individual molecules do indeed get to make decisions, in a manner of speaking.
Let's consider first what goes on when liquid molecules freeze. There are some rather strong attractions between water molecules that tend to make them stick together. (In Techspeak, they are called hydrogen bonds and dipole-dipole attractions.) In liquid water, the molecules are moving fast enough that the attractions can't really take hold. But as water—or anything else for that matter—cools, its molecules move more and more slowly. Zero degrees Celsius happens to be the temperature at which the molecules are moving just slowly enough that they can grab hold of one another with their attractive forces and settle down into the unique fixed positions that characterize ice. Ice's molecules are tied rigidly in place; they can't go swimming around the way liquid molecules do.
Now let's put an ice cube into liquid water. Some of the ice molecules at the surface of the ice will break their attachments to their fellows and join their freely swimming brethren. In other words, they will melt. Meanwhile, some of the liquid molecules near the ice's surface may be moving more slowly than the capture speed (they're not all moving at the same speed), and they will freeze onto the ice. So both melting and freezing can be taking place simultaneously, some molecules going one way and some going the other.
As long as the water is slightly warmer than zero degrees Celsius, there will be more melting going on than freezing, because there won't be enough slow water molecules to be captured onto the ice. Conversely, if the water's temperature is slightly lower than zero degrees, there will be more freezing going on than melting, because there will be more slow water molecules to be captured. At exactly zero degrees, there will be just as many ice molecules melting as there are liquid molecules freezing. Millions of tiny molecules are going each way, but from our relatively gigantic human perspective, we see absolutely nothing going on. The ice and water just sit there—until, of course, they begin to warm up, and then melting takes over.
In a sense, then, zero degrees Celsius is neither the “melting point” nor the “freezing point” of water. It's the temperature at which melting and freezing are happening equally. Scientists call this exact-balance point an equilibriumpoint. They would say that at zero degrees Celsius, ice and liquid water are “in equilibrium.”
Equilibrium is a very important concept in chemistry because there are many situations in which, down at the molecular level, two opposing processes are going on at equal rates, so that up here at the human level we see no apparent change.
For more, look up “equilibrium” in the index of any chemistry book. But I warn you: There may be lots of equations.
Melting and freezing are so closely interrelated that just by touching an ice cube you can kick some water molecules from liquid to solid.
Wet your fingers and touch the ice cubes in your freezer. The cubes may stick to your fingers so tightly that you can lift them up. The ice cooled the water on your fingers down to its own temperature, which is obviously below the freezing point. When the water on your fingers froze, it grabbed on to the ridges of your fingerprints and held on to them while simultaneously fusing itself onto the ice cubes, thereby “gluing” your fingers to the cubes.
Help! We're Trapped in an Ice Cube!
Why are my ice cubes cloudier in the middle than at the edges?
The cloudiness is a mass of tiny air bubbles—air that was dissolved in the water and expelled when the water froze. You can see the individual bubbles through a magnifying glass.
There is always some air dissolved in any water that has been exposed to … well, the air. For this, the world's fish are truly grateful. They are particularly grateful for the fact that, even though air is only about 21 percent oxygen, oxygen dissolves in water twice as readily as the other 79 percent, which is mostly nitrogen.
When water freezes, the loosely moving water molecules settle down into rigid positions. In doing so, they squeeze out the dissolved air molecules, because there is simply no room for them. When the water begins to freeze, the outer portions freeze first because they are in the best position to have the heat sucked out of them. As the dissolved air molecules are squeezed out, they become trapped within the encroaching casing of ice. The air molecules are forced closer and closer together as the growing wall of freezing water closes in on them. Eventually, they are packed so close together that they congregate into bubbles. And there they remain, trapped when the interior water finally freezes.
Help! I'm Breathing!
My girlfriend is worried that if the humidity gets to be 100 percent, we'll be breathing pure water and drown. Of course, that's silly, but I can't explain why.
Ask your girlfriend, “A hundred percent of what?”
Chicken Littles who fear drowning in air are forgetting that “humidity” is purely relative. Everybody goes around talking about “the humidity” as if it's something absolute, but it's really the relative humidity that they're talking about—relative to some maximum, but still small, amount of water vapor in the air. And mind you, that's vapor, not liquid. Even when the relative humidity gets to be 100 percent at room temperature (we'll see that humidity varies with temperature), there is only one water vapor molecule in the air for every forty or fifty air molecules.
“Vapor” is a funny word. All it means is “gas”—the form of matter in which the molecules are flying freely around with huge spaces between them. Any substance can be transformed into a gas if we heat it hot enough to drive its molecules completely away from one another. It's only when the gas in question recently arose from a liquid that we refer to it as a vapor. We call the oxygen in the air a gas because we've (most of us) never seen it as a liquid. But we don't generally refer to gaseous water as a gas because we know that it came from liquid water. We call it “water vapor.”
Why does water choose to go into the air as vapor, anyway? At every temperature, water finds a unique balance point between its tendency to exist in the form of liquid and its tendency to exist in the form of vapor. At warmer temperatures, the balance favors vapor, because the molecules are moving faster and can escape more easily. So the higher the temperature, the higher the tendency for water to be in the form of vapor, rather than liquid.
If you put some water of a certain temperature into a closed box, it will fill the box with the amount of vapor that is characteristic of its temperature, and then stop. It stops when there are as many liquid molecules leaving the liquid each second as there are vapor molecules hitting the liquid's surface and sticking. When these two rates are equal, there is no further net change. (Techspeak: The liquid and the vapor are in equilibrium;.)
A lot of people, including some scientists, would like to say that the air in the box is now saturated with water vapor, as if the air were a wet rag, holding as much water as it can. But that's a misleading way to look at it. We'll put it another way: The amount of vapor in the box is 100 percent of the maximum amount that there can be at that temperature. In other words, the relative humidity is 100 percent. If there were only half that amount of water vapor, we would say that the relative humidity is 50 percent, and so on.
If we lived in a closed box with some liquid water in it, the relative humidity would always be 100 percent—100 percent of the maximum amount of vapor for that water temperature. But of course, we don't. We live in a constantly shifting sea of winds bearing warm air, cold air, high- and low-pressure air and everything else that the weather can devise to blow water vapor around from place to place. That's why the relative humidity isn't always 100 percent, even when it's raining, or even over the ocean.
Frighten your timid friend with this fact: In a steam bath or wet sauna the relative humidity is 100 percent and then some. First of all, the temperature is deliberately kept high to get as much water vapor into the air as possible. But in addition to that maximum amount of water vapor, there are actually tiny droplets of liquid water suspended in the air. We call it steam or fog. In a steam room, you're actually breathing liquid water. But nobody ever drowned from breathing fog or steam at a reasonable temperature because there is still plenty of air in between the suspended droplets.
(Caution: Steam can be dangerously hot, depending on how it has been produced and at what pressure. The steam in a steam bath is “cool steam” and is no hotter than the air in the room.)
You Didn't Ask, but …
What is the “dew point” that weather reporters are always talking about?
Meteorologists love to tell us the dew-point temperature, even though few of us know what it is and even fewer of us care. But as long as I'm this deep into water vapor, I might as well explain that too.
The dew point, or dew-point temperature, is the temperature below which the liquid-vapor balance of water shifts to favor the liquid side of the scales. That is, condensation wins out over evaporation.
If the temperature is above the dew point, liquid water will continue to change into vapor until it is all evaporated; wet things will dry. But if the temperature is below the dew point, the balance shifts in favor of liquid, and vapor will tend to condense. When that happens up in the atmosphere, the vapor condenses into masses of microscopic droplets of liquid water that are too small to fall and stay suspended in the air. We call these masses of tiny water droplets clouds.
A more earthbound example: If the ground gets any colder overnight than the dew-point temperature, water vapor in the air will condense out onto the grass and leaves as drops of liquid dew. That's important to farmers because dew is, after all, free water for the crops. Also, there are ecosystems in the world where it almost never rains and where small animals depend on dew for their water supply.
I wish that people wouldn't use the word moisture to mean water vapor. “Moist” means slightly wet or damp, and moisture is the water —liquid water—that makes an object moist. Yet people use “moisture” to mean water vapor, which is water in its gaseous form. Air can't hold moisture, meaning liquid water; it can only be said to “hold” water vapor, but even that isn't really correct.
Cosmetic advertisements, especially for “moisturizing” skin creams and lotions, just love to use the word “moisture” when they mean “water.” Although moisture is nothing but water, that's considered too common a word for such elegant products. So the next time you're in a fancy restaurant, be sure to ask the waiter for a glass of moisture.
And by the way, what do cosmetic “moisturizers” do, anyway? Do they add moisture, or water, or whatever you want to call it? No. If that were true, why couldn't you just put water on your dry skin? “Moisturizing” lotions and cosmetics coat your skin with a concoction of oils and other water-blocking chemicals, so that your skin's supply of natural water stays in instead of evaporating. It sounds paradoxical, but the cosmetic's oil produces water.
Fire Makes Drier
Why does a hair dryer have to both heat and blow?
This is one of those questions that seems so natural that we forget to ask them. But that's what I'm here for: to make you wonder about things you take for granted, and then to replace your complacency with the smugness of knowing.
The water in your hair or clothes must first be converted from liquid into vapor before it can be spirited off by a stream of air. Blowing away liquid water isn't easy, as you can tell by the hurricane-force winds they have to use to dry your car at the car wash. Warming the liquid water in your hair or clothes—and that's what the warm air does—speeds up the water's molecules, so that more of them can fly off into the air. (Techspeak: Warmer water has a higher vapor pressure;.) The heat therefore speeds up evaporation of the water, and once it has evaporated into the form of vapor it can be swept away by the blowing air.
But how much vapor can the air-heated water produce? How fast can it evaporate?
Liquid water molecules can keep flying off and becoming vapor molecules only until the space above the liquid is so crowded with vapor molecules that just as many are bouncing back into the water as are flying out of it. (Techspeak: until the liquid and vapor are in equilibrium.) That's where the blowing comes in. The moving air from the dryer blows away some of those water vapor molecules so that they can't go back into the liquid. This “makes room” for more, and the evaporation continues.
That's why clothes and hair dryers do both heating and blowing. One without the other wouldn't do the job nearly as well. What if your hair dryer's blower blew out, so that it only heated your hair, or if its heater blew out and it blew cold air?
If there is already a lot of water vapor in the air—for example, if the bathroom is already steamed up and humid from your shower—the water in your hair won't be able to evaporate as fast. It will require a much longer heating and blowing time to dry your tresses to that silky, sexy, slow-motion slinkiness that they show on TV.
To Have, but Not to Hold
Why is it that warm air can hold more moisture than cold air can? That's why it's more humid in the summer, isn't it?
No. It's usually more humid in the summer because there's more water vapor available.
I don't mean that the oceans, lakes and rivers somehow expand in the heat. (Well, maybe a tad.) More precipitation? Perhaps. But it's not the amount of water itself; the humidity can be quite low over the middle of the ocean. What counts is how much water is being converted to vapor (or gas). It is more humid in the summer because the water supplies—the oceans, lakes, rivers and rains—are warmer, and water has a greater proclivity for making vapor when its temperature is higher.
Notice that I have said nothing at all about the air or its ability to “hold water.” Humidity is the water vapor that comes from water, whether there is any air present or not. When we say, “It's humid today,” we assume that the “it” in question is the air because, after all, what else is there? But the air plays no role whatsoever in humidity; like Mount Everest, it is simply “there.” It is a bystander.
Think of it this way: We happen to be immersed in a sea of air, just as fish are immersed in a sea of water. If somebody suddenly dumps a load of red ink into the ocean a fish might say, “My, but it's red today.” But “it” isn't the water itself; “it” is the ink that has been mixed into the water. Well, humidity is water that happens to be mixed into the air.
Nevertheless, you'll hear scientists and meteorologists explain humidity and other weather phenomena by talking about the “amount of moisture that the air can hold” and saying that warm air can “hold more moisture” than cold air can. That's a mistaken and misleading notion. The air isn't holding on to water vapor; it has no such holding power.
Here's why. Air and water vapor are both gases, and in gases the spaces between the molecules are so vast that any two gases can mix in any proportions without either one “knowing”—or controlling—how much of the other is there. All the air can do is accept the water vapor—whatever amount the water chooses to give off according to its temperature. It is purely water's decision as to how eagerly it wants to be in the form of vapor instead of liquid.
Now I suppose you want to know why warm water produces more water vapor than cold water does, right? That's science for you: Every answer generates more questions.
Water—like all liquids—has a certain tendency for its molecules to leave the surface of the liquid and fly off into the air. That's because all the molecules are moving with various speeds, and there will always be some of them at the surface of the liquid that have enough energy to go flying right off as vapor. Because molecules move faster on the average at higher temperatures than at lower temperatures, there will be more potential escapees from warm water than from cold. For example, at 86 degrees Fahrenheit (30 degrees Celsius) water produces seven times more vapor molecules in a given space than water at 32 degrees Fahrenheit (zero degrees Celsius).
There is always a sort of tug-of-war going on among the molecules of a liquid. Their speed wants them to fly off as vapor, but their attraction to their fellow molecules wants them to stay in the liquid. At every temperature, water must strike a balance between these two tendencies (Techspeak: The water attempts to come to equilibrium between these two states). At lower temperatures, the liquid tends to win out; at higher temperatures, the vapor gets the edge because of the higher molecular speeds. (The ultimate limit is when the liquid boils and turns completely to vapor.)
At a given temperature, every liquid has its own preferred balance point between vapor and liquid, because its molecules have their own degree of stickiness toward one another. A liquid whose molecules are stuck tightly together will not form vapor easily, so its balance point will tend to favor the liquid form over the vapor form. Gasoline's molecules, on the other hand, don't stick to one another very much at all, so their balance point favors the vapor and gasoline evaporates (vaporizes) much faster than water.
The tendency of liquid molecules to escape and fly off as vapor is called the vapor pressure of the liquid. In Techspeak, we would say that gasoline has a higher vapor pressure than water, and that warm water has a higher vapor pressure than cold water.
Let's say we're in a closed box containing some water. The water would soon strike a balance between liquid and vapor, according to the temperature. (The liquid and vapor would be in equilibrium.) If our box suddenly got colder, the water would have to strike a new balance (find a new equilibrium balance between liquid and vapor) based on that new, lower temperature. The new balance would be in the direction of less vapor and more liquid, so some of the vapor would have to condense out and become liquid. There would be rain or dew in our box.
Others may claim that it rained because “there was more water vapor in the cold air than it could hold.” But I never even said there was air in the box, did I? It rained solely because the water shifted its liquid-vapor balance all by itself. “It” would be just as humid or just as dry in the box if there were no air in it at all, but some other gas with a different reputed “holding power.”
I Can't See Where I'm Going!
When the windshield of my car gets all fogged up, how can I clear it most quickly?
Your car is a box of homemade weather, produced by your air intakes, your heater, your air conditioner and your passengers. But sometimes, because of your passengers' irrepressible habit of breathing, the car fills up with a lot of water vapor and some of it condenses onto the cold windshield, fogging it up. What do you do?
When it's very humid in your car and the wind-shield fogs up with condensed moisture on the inside, turn on the air conditioner, no matter how cold it may be outside. (You can always turn on the heater, even while the AC is on.) Direct the conditioned air onto the windshield and it will clear up in a jiffy.
What happened was that the air conditioner took in the water vapor (along with the air that it's mixed with) and cooled it down to a lower temperature at which the water would much rather be liquid. It condensed into liquid at the AC, where it was thrown away outside the car. There was then not enough water vapor in the car for the temperature, and the liquid on the windshield restored the balance by turning itself into vapor. Voilà! A dry windshield.
But what about the rear window? When it fogs up, there's no way to blow air-conditioned air onto it; the cool air all comes out of ductwork up front, where the driver needs it, and there is no blower for the rear window. So what did those clever car designers do? They embedded heater wires in the rear glass. Instead of blowing dried-out, cool air on it, you just heat the glass. That raises the glass's temperature above the point at which water prefers to be liquid, so it turns to vapor and the fog disappears.
Odd, isn't it? To defog the windshield you cool the air, but to defog the rear window you heat the glass, and the end result is the same. Why don't the car manuals ever explain this to you? How many people are driving around with fogged-up glass, not having the foggiest notion of what to do about it?
Now how about your bathroom mirror back home? After your shower, it's fogged up worse than any windshield ever got in the steamiest jungle, and just when you want to shave or put on your makeup. I'll bet you have neither an air conditioner in your bathroom nor heating wires embedded in the mirror, so you can't use either of the car window tricks on it. But you probably have a hair dryer handy. Just sweep across the mirror with it, as if you were painting the glass with hot air. The dryer's air will heat the condensed water on the glass enough that it will prefer to be vapor rather than liquid and it will evaporate, just as it does on your electrically heated rear car window.
The Smell of Rain
A farmer neighbor tells me that he can smell when it's going to rain. Is he joshin’ me?
Probably not. It's not the rain itself that he smells, but just about everything else. Almost everything smells a little stronger when it's about to rain.
Stormy weather is usually preceded by a drop in atmospheric pressure, or what the TV weather people like to call “barometric pressure.” (Is that what you feel when you're struck by a falling barometer?) That is, before it rains the pressure exerted by the air drops, and it presses down less heavily on the countryside.
Meanwhile, all the trees, grass, flowers, crops and, yes, even livestock are emitting their characteristic odors. Odors are tiny amounts of vapors emitted by substances, and we smell them when the vapor molecules happen to migrate through the air to our noses. When the air pressure is low and isn't pressing down so hard, it allows more of these vapors to escape into the air, and everything smells a little bit stronger.
Also, when the rain-bearing low-pressure front moves in, it is accompanied by a wind that carries along distant smells that are not ordinarily detected.
Of course, a farmer gets to be pretty good at reading weather clues, so he might be cheating a bit by consulting the sky, the winds, and even his arthritis.
By the way, doctors used to believe that people with arthritic joints could feel rain coming on because there are tiny gas bubbles in the joints, and when the air pressure decreases, those bubbles expand and cause internal joint pressure. Nice theory, but I understand that it's no longer in vogue.