2004-10-01 / Columnists

Drawing On Science


There’s an old cowboy song that begins, “All day I searched a barren waste without the taste of water—cool water. Old Dan and I with throats burned dry and souls that cry for water—cool, clear, water.” Water is the most important substance for life on Earth. We drink it, bathe in it, shower with it, swim in it, sail on it, move within it, flush with it, and—OK, you get the idea. Since there’s evidence of water on mars, comets, and a moon of Jupiter, water might also be as important to other life forms in the universe. In any case back here on our planet we experience water in all of its phases: solid, liquid, and gas (water vapor).

So what is water? Well basically it’s a compound of two parts hydrogen to one part oxygen. Its chemical formula is H2O and its chemical name is dihydrogen oxide (next time you go to a restaurant ask for a glass of dihydrogen oxide and see what happens). At 0oC (32oF) and below it is a solid (ice). Between 0oC and 100oC (212oF) it is a liquid and at 100oC and higher it is a gas. It is one of very few substances that, as a solid, floats on its own liquid. This is because the density (=mass of a substance in a given volume) of solid water (ice) is less than the density of liquid water. Why? Well in almost all solids the molecules that make them up are packed very close together. The solid is very dense. As the temperature rises the molecules move away from each other and the density decreases. As a result the volume increases. So the liquid phase of a substance is less dense than the substance’s solid phase and it has a larger volume than the solid phase. Placing a substance’s (dense) solid in its own (less dense) liquid will result in the solid sinking. Of course the gaseous phase is the least dense and has the largest volume since the molecules are really far apart from each other.

Solid water below 0oC behaves the same way when its temperature rises. Its molecules begin a slight movement away from each other. But when the temperature reaches 0oC and melting begins water acts differently. The molecules now begin to move closer to each other rather than away. The volume decreases . Once all of the ice is melted the liquid water’s volume still decreases because the molecules are still moving closer together. When the liquid water reaches 4 oC water is at its maximum density and least volume. As the temperature passes 4oC the molecules then behave ‘normally’ again. They begin to move away from each other and the water becomes less dense. But liquid water is never less dense than solid water no matter what its temperature is. So being less dense than liquid water, ice will float. Think about how happy fish are when the body of water in which they are living freezes over. The ice just sits on the top instead of sinking and crushing them.

Water is a part of every living thing (about 90% of your body is water) and it is known as the “universal solvent” because it dissolves most anything. As a result of this ability pure water does not exist in nature. Pure water has no taste or odor. Only when it contains dissolved substances and it is aerated does it have a taste and odor. In thin layers pure water is colorless, but in thicker layers it appears blue. Pure water does not conduct electricity but a solution (=water + dissolved substances) does. That’s why you should never touch anything that is plugged into an electric socket if you happen to be standing in water. Also if you are swimming get out of the water when a storm comes up—it may be a lightning storm.

Water is found in one of four divisions of the Earth known as the hydrosphere (rocks and minerals make up the lithosphere , the air is the atmosphere , and living things make up the—how about you finding out what division of the Earth is composed of living things). More than 97% of Earth’s water makes up our oceans, which are a solution of salts. The remaining 3% is fresh water containing no dissolved salts. Fresh water is found in rivers, streams, lakes, ponds, glaciers, and aquifers (=underground containers that hold water, which has seeped into the ground). Only one-half of 1% of fresh water is potable (=safe drinking water). And of this usable fresh water a very small fraction is found on the surface in the lakes, ponds, and streams. Most of it is stored in the aquifers.

In nature water moves in a number of circular paths called the water or hydrologic cycle [see diagram]. The water cycle is powered by the sun’s energy. Water on the surface evaporates (=changes from a liquid into a gas) when the sun warms it. Much of this evaporated water comes from the oceans and some comes from continents. Plants release water into the air by a process known as transpiration (=release of waste water through small openings in leaves called stomates—see Drawing on Science 9/17/04). Evaporation and transpiration are sometimes combined into one word: evapotranspiration . Winds carry the water vapor from the oceans over the land where it cools and condenses (=changes from a gas into a liquid). It returns to the surface as rain or snow. (Aren’t you glad that your nose points down. If it pointed up and you got caught in a storm you would have to cover your nostrils to keep the rain out—just thought I’d mention this.) Rain that falls into rivers and streams is carried back to the oceans as runoff . Some rainwater may form puddles and will, again, evaporate. Some rainwater seeps into the ground settling in aquifers. And the cycle continues.

Another path of the cycle involves animals. Animals obtain water by drinking it or eating food containing it. As a natural process the body accumulates excess water, which must be gotten rid of. Depending on the animal this is accomplished in two ways: perspiration or sweating, and peeing (oops—excuse me this is a science column so let’s use the right word: urinating ). Perspiring and urination belong to the life process known as excretion . The water that is released in excretion also evaporates and enters the atmosphere where it, too, condenses and falls back as part of the rain or snow. Now I know what you’re thinking. No, urine and smelly sweat is not falling on your head when it rains. When water evaporates any dissolved substances in the water remains behind—only the water vapor rises into the atmosphere. If you don’t believe this try the following ( with your parent’s permission and supervision ): Slowly pour table salt into a half-glass of water while mixing the solution. When no more salt is dissolving taste the water and then pour the solution into a small pot. Heat the pot on the stove until the water boils. Carefully try to trap the steam in a dry glass held upside down over the pot (you can hold it with your parent’s oven glove). When all of the water is gone allow the pot to cool. In the meantime taste the water that has condensed on the glass. Then taste what is left in the cooled pot.

Oh, and by the way, that song at the beginning of this column mentions Dan. It was most likely not another cowboy. Most probably it’s a pack mule. In the old days of western miners ‘dan’ was a name for a pack mule. It is also mentioned in another old cowboy song that begins, “I ride an old paint ( horse) I lead an old dan…”

Questions/comments?

E-mail Steve: Drawingonscience @aol.com.

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