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Facts of the Matter Richard Brill |
PHOTO ILLUSTRATION BY DEAN SENSUI / DSENSUI@STARBULLETIN.COM
The temperature and pressure environment of Earth is just right for water to exist in all three states -- liquid, solid and gas.
Structure explains
water’s amazing attributesLiquids in general are rare in the universe. Water is one of only a very few substances that occurs naturally as a liquid, and it occurs in abundance only here on Earth. If there are large collections of water elsewhere, it is most likely in the form of ice.
Water possesses a host of unusual properties, not the least of which is its ability to remain liquid over a wide range of temperatures and air pressures.
Most substances can exist in one of three physical states: solid, liquid or gas. Water is no exception, existing as ice, liquid water, and water vapor or steam. But substances such as water that are comprised of lightweight atoms typically have a very small range of temperatures over which they can exist as a liquid. Most are gases at Earth temperatures, which range from about minus 60 degrees Fahrenheit to 135 F, while water remains liquid between 32 to 212 F.
Take carbon dioxide, for example. On Earth it exists in the atmosphere in the gaseous state or dissolved in natural waters. It can be compressed and frozen into the solid state ("dry" ice), which sublimes directly from the solid to the gas. It can melt to a liquid at normal atmospheric pressure at minus 75 F but requires increasingly high pressures to remain liquid at higher temperatures. Above 125 F it simply cannot exist as a liquid at any pressure.
The reason behind the wonders of water is the shape of water molecules. Each molecule consists of two small atoms of hydrogen stuck to one side of a huge, spherical oxygen atom, arranged like the head of Mickey Mouse (minus the nose) with an oxygen head and hydrogen ears.
Because the molecule is not symmetrical, its electric charges are unequally distributed. This results in a molecule that is electrically polar, having a slight excess of negative charge on the oxygen end and a slight excess of positive charge on the hydrogen end. This small charge imbalance produces weak attractions between adjacent water molecules, which is called hydrogen bonding.
Hydrogen bonding makes the molecules "sticky" and is responsible for all of the unusual properties of water. Those include water's relatively high melting and boiling temperatures and the wide liquid range between them, but also its high specific and latent heat, high viscosity, high surface tension, high solvent ability and high dielectric constant, to name a few.
Molecules in all matter are in constant motion, whether vibrating, rotating or moving about. The higher the temperature, the faster the motion. A typical air molecule at average Earth temperature (55 F) is moving between 85,000 to 90,000 mph and collides with another molecule thousands of billions of times each second.
The difference between the solid, liquid and gaseous states has to do with the orderliness of molecules. The molecules in solid substances such as ice are locked into a rigid three-dimensional structure, like rapidly vibrating balls connected by stiff springs.
In the gaseous state, molecules are un-bonded and free to move about, but at low temperatures they are frozen into the crystal lattice because they do not have enough energy to overcome the attractive forces of the hydrogen bonds. At higher temperatures, the molecules vibrate faster until the melting temperature of 32 F is reached and the average speed of the vibrating molecules is fast enough that they have enough energy to break away from the crystal lattice.
Liquid water is a transitory state that exists between the solid and gaseous states, its temperature range expanded by the stickiness of the hydrogen bonds.
In liquid water the individual molecules are still joined, but not in the rigid and structured crystal lattice of ice. They are clumped together or connected in chains. The small clusters of molecules in the liquid state are packed more tightly than in the crystal structure of ice, so solid water floats on liquid water, yet another unusual property.
Hydrogen bonds are fleeting, being broken and reformed many millions of times per second, but at any given temperature a certain number of molecules will always be free from the crystal lattice. At higher temperatures there is always a higher percentage of free molecules than at lower temperatures.
At boiling temperature the average energy of the molecules is just enough to escape through the surface tension of the liquid, but there will always be a few molecules that have enough energy to escape well below the boiling temperature (and even below freezing). So evaporation occurs at any temperature but is much faster at higher temperatures.
Although they make the molecular motion sluggish, hydrogen bonds alone would not be strong enough to keep the water in the liquid state at Earth temperatures. Air pressure above the liquid surface plays an important role in determining the rate of evaporation and the concomitant boiling temperature.
The higher the air pressure, the closer together the air molecules and the more often they collide with each other and with the surface of the water. More collisions means that more water molecules will be knocked back into the liquid and fewer molecules will escape the liquid surface in a given time. This in turn decreases the evaporation rate and raises the boiling temperature.
The temperature and pressure environment of Earth is just right for water to exist in all three states, with most of it collecting in the ocean basins in the liquid state. Only under a nearly constant one-earth-atmospheric pressure and at the right distance from the right kind of star can there be liquid water on any planet. To maintain a one-earth-atmospheric pressure, a planet has to be just the right size, and that is Earth-size. With our two closest neighbor planets being like Goldilocks' porridge (Venus too hot, too much pressure; Mars too cold, too little pressure), it is clear that Earth is a special place, with just the right temperatures and pressure for life as we know it.
We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at rickb@hcc.hawaii.edu
