2007-12-21 / Columnists

Drawing On Science For Kids

Fish Are Very Happy That Ice Doesn't Sink
Commentary By Stephen S. Yaeger

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, flush it, and- OK, you get the idea.

Unlike other substances, we experience water in all of its phases at room temperature: as a solid (ice) at 0oC or 32oF and below, as a liquid between 0oC and 100oC or 212oF, and as a gas (water vapor) at above 100oC.

Water is a compound of two parts hydrogen, to one part oxygen. Its chemical formula is H2O.

Water's chemical name is dihydrogen oxide. Next time you go to a restaurant, ask for a glass of dihydrogen oxide with lemon and see what happens.

Pure water doesn't exist in nature, except when it's in the form of rain, snow, hail, dew, etc. But when it comes in contact with air or the ground, it becomes impure. This is true because it then picks up impurities, such as dissolved substances, gases, bacteria, and solid particles.

In a laboratory, pure water can be made by filtration. Pure water has neither taste nor odor. Only when it's aerated and contains dissolved substances 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's plugged into an electric socket, if you happen to be standing in water.

Also, if you're swimming, get out of the water when a storm comes up- it may be a lightning storm.

In a solid the molecules are packed very closely together. In a liquid they're farther apart. And in a gas, the molecules are farthest apart. This makes a solid substance denser (density = mass of a substance in a given volume) than its liquid or gas counterparts and gives it a definite shape. Its volume is the smallest of the substance's phases of matter. As the temperature of a solid rises, three things happen to it. Its molecules move away from each other, its density decreases, and its volume increases. When the solid reaches its melting point temperature, it begins to turn into a liquid. A liquid takes the shape of any container it's in.

When the liquid reaches its evaporation point temperature, it boils and turns into a gas. A gas has no definite shape or volume, since the molecules can travel as far apart as possible. A gas has the least density of the three phases of matter.

You know that if you place a solid in water, the solid, being denser, will sink. If you place any substance's (dense) solid in its own (less dense) liquid, it will give you the same result. So, if solids are denser than liquids, how come solid water or ice floats on liquid water? You see this every time you put an ice cube in a glass of water. Even giant icebergs float on water. When a body of water such as a lake or pond freezes, ice covers the surface. Just imagine what would happen to fish in a frozen lake or pond if the ice sank!

Water doesn't follow the rule, because it behaves differently when it freezes. As the temperature drops, its molecules begin to move closer to each other. The water becomes denser, and its volume decreases. We would expect this to happen. When the water reaches 4 oC (about 39 oF) it's at its maximum density and least volume. As the temperature begins to drop below 4 oC, a strange thing happens. The water molecules now move away from each other and the volume increases. It is this property of water that makes the ice less dense than liquid water. And that's why ice floats on water, making fish very happy.

Think about it: Can you describe what happens when a substance is cooled down from a gas into a solid? What's the melting point of water? What's the evaporation point of water? Why does an air-filled balloon float when you hold it over a warm radiator?

Can you purify water? Try this: 1. Punch some small holes in the bottom of a can. Put about a two inch layer of clean gravel into the can. Cover the gravel with a four-inch layer of clean sand. Pour muddy water into the filter and compare the filtrate with the muddy water. 2. Pour some salt water into a clean filter paper. Taste the filtered water. Has it been purified? Questions/ comments? E-mail Steve: Drawingonscience@ aol.com

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