SciFri 1.21.05

Ice skating depends on ice's response to pressure, which allows liquid water to exist below 32 degrees F. The pressure on the blades melts a little ice, and so a skater moves in a groove filled with water--the optimal situation for skating.

How well do you know H2O?

By Deane Morrison

Published on January 21, 2005

Too much rain. Too little snow. A tsunami. Melting ice caps.

Water dominates our lives and, until Friday, the lack of its chilly form--snow--has been the talk of the season in the Twin Cities. But how well do we know H2O (that's two parts hydrogen and one part oxygen)? Water has unusual properties that have allowed it to take such a central place in our planet's history, and contemplating them may help while away the days waiting for more snow--or spring.

For many scientists, water's most astounding trait is how it behaves when heated or cooled near its freezing point. Most substances expand and become less dense when heated; this is the principle behind the hot air balloon. Water, however, does things differently. Anyone who's inadvertently left a bottle of white wine in the freezer, hoping to chill it quickly, knows that water expands as it freezes. But did you know that water starts expanding as soon as it drops to 40 degrees F? Water is most dense at 40 degrees--not at 32 degrees, its freezing point. Therefore, a pool of water at 40 degrees will expand whether it is heated or cooled.

As it turns out, water's obstinate refusal to contract as it turns solid is lucky for life on Earth. Suppose water contracted as it froze; that would make ice denser than liquid water. Every winter, ice would sink to the bottom of lakes, putting it beyond the reach of the summer sun's warming rays. What sun did get into lakes would heat the surface water, which would sit on top.

"Lakes would be one to three feet of water on top, and the rest would be ice," says physics professor John Broadhurst.

Steam is an invisible gas. As it leaves a teakettle and hits the relatively cold air in your kitchen, it rapidly expands, cools, and condenses into water vapor. If you look sideways at the spout, you'll see a small clear space between the water vapor cloud and the spout--that's the steam.

Ice accumulating on the bottoms of lakes would make life hard for fish, plants, and other organisms. Also, with warm water parked at the top, it would be hard for lakes to circulate from top to bottom. Many lakes "turn over" in spring and fall, a process that circulates nutrients to all levels. In the fall, surface water that cools to 40 degrees becomes dense and sinks, allowing water from below to rise. In spring, the just-melted surface water warms toward the 40-degree mark; this also makes it denser, and so it sinks. The winds help stir up the water, and all life in the lakes benefits.

If ice didn't form on the surfaces of lakes, we may never have invented one of the most entertaining winter sports. Ice skating depends on ice's response to pressure. If water is subjected to pressure, its boiling point rises and its freezing point drops. In other words, pressure can allow liquid water to exist below 32 degrees. Skates, says Broadhurst, are a device for putting the weight of a skater onto a very small area--the thin bottom of the blades. The pressure melts a little ice, and so a skater moves in a groove filled with water--the optimal situation for skating.

"In a skating competition, the ice must be 26 degrees," says Broadhurst, the father of two daughters who skated competitively. "Above 26 degrees, the groove is deeper, the skate experiences more friction with the sides of the groove, and the skater is slowed down. Below 26 degrees, the groove is shallow and the skater can't land jumps."

The blades of toddlers' skates are thinner than the blades of bigger sizes. The thinning is necessary because with such a small amount of weight, the area of the blade must be reduced to get the right amount of pressure, says Broadhurst. He warns young skaters against borrowing an older sibling's big skates and stuffing them with socks. Because skates built for bigger people have thicker blades, they distribute the weight over a larger area. If you're too small for the size of your skates, this lowers the pressure on the ice, resulting in insufficient meltwater in the groove for smooth skating.

Another unusual property of water is its high heat capacity, or the amount of heat that must be applied to produce a given rise in temperature. Water takes longer to boil than, say, oil does. On the other hand, once water heats up, it conducts heat very well, which is why it's never a good idea to grab something out of the oven with a wet oven mitt. And if you're boiling water in a teakettle, that cloud shooting out the spout isn't steam; it's water vapor. Steam is an invisible gas, and it exists at temperatures at or above 212 degrees. As it hits the relatively cold air in your kitchen, it rapidly expands, cools, and condenses into water vapor. If you look sideways at the spout, you'll see a small clear space between the water vapor cloud and the spout--that's the steam.

Living things can't exist without this rather peculiar molecule. In a molecule of water, the oxygen carries a slight negative charge and the hydrogens carry a slight positive charge, so hydrogen is attracted to oxygen. Now suppose there's a molecule of fat sitting in one of your body's liver cells, which also has lots of water. To get energy from the fat, your body must strip hydrogen atoms away from carbon atoms in the fat--that is the process for generating energy. The only reason the body can succeed in doing this, says Broadhurst, is because the hydrogen atoms bound to the carbon atoms have been "loosened" by their pull toward the oxygen atoms in water. Imagine you're the hydrogen atom holding hands with the carbon atom and something is trying to break your grip. It's much easier to do that if another person, the oxygen atom, is tugging at your other arm.

"Water relaxes substances," Broadhurst explains. Water's ability to encourage molecules to break apart and bond together allows us to digest food and build things up, like muscle and bone.

The electrical charges in the water molecule also exert their influence in large and obvious ways. It's why you can remove a lot of dirt from clothes just by washing in plain water. With water around, the dirt is as likely to be electrically attracted to water as to the fibers in that old pair of jeans. The result: dirtier water, cleaner jeans.