Three Phases of Water
WATER AS LIQUID
We all know what water is—a clear, odorless, tasteless liquid. And we all know what water does—falls from the skies, quenches thirst, cleans surfaces, makes plants grow. But there's another way to look at water, if you go below the surface.
If you could peer into the submicroscopic world, you'd see that water consists of three atoms: two of hydrogen and one of oxygen. Water is a small molecule: A single living cell may contain billions of them.
A water molecule is like a tiny magnet, with a negative charge on one side and a positive charge on the other.
Ties That Bind
A water molecule clings to other water molecules because of its chemical structure. This "stickiness"—which comes from a force called hydrogen bonding—accounts for many of water's most amazing traits.
A Matter of Scale
How can pond water be a sidewalk for a water strider? It happens because water molecules stick to one another. The molecules at the surface have no watery neighbors above, so they stick even more tightly to the ones underneath. The result? High surface tension—and a place for water striders to stroll.
Water has a remarkable ability to absorb and hold heat. As a result, ocean currents play a large role in Earth's climate. The huge ocean current called the Gulf Stream—pale yellow in this image—sweeps from Florida to the central Atlantic and carries, every day, twice the heat that all the coal mined on Earth in a year could generate.
WATER AS SOLID
Breaking the Rules
As a rule, when liquids freeze, they become more dense. But forget about the rules when it comes to water: When frozen as ice, it becomes less dense. The result? Ice floats.
As always, the explanation is in the atoms. In liquid water, the molecules are chaotic, jumbled and packed densely together. But as ice forms, the molecules arrange themselves in a crystal structure with empty spaces—and those spaces act as flotation devices.
Water freezes at 0°C (32°F), but it's most dense—that is, its molecules are most tightly packed—at 4°C (39°F).
Salt water freezes at a lower temperature than fresh water does—and the saltier the water, the more it resists freezing. That's why we put salt on sidewalks in the winter. But melted salt water can hurt plants, so many people are switching to sand or other salt substitutes—even sugar will lower the freezing point and melt ice.
Slip Sliding Away
Ice is slick enough for skating, yet, surprisingly, scientists are still trying to figure out why. It may be that the pressure of the skater's weight, concentrated on a narrow blade, melts the ice. Or perhaps the friction caused by moving blades does the melting. Or it may be that solid ice is always coated with a very, very thin layer of liquid, only a few molecules thick—whether or not anyone is doing figure eights.
Salt lowers the freezing temperature of water, which means that seawater, which is 3.5 percent salt, freezes at about -2.2°C (28°F). At the North and South poles, temperatures in the winter drop low enough to turn parts of the ocean into solid ice. This "sea ice" freezes and thaws with the seasons and plays a key role in regulating Earth's climate. The steady shrinking of Arctic Ocean sea ice in recent decades is strong evidence of climate change.
Now you see it—now you don't! That's what happens when water turns into a gas: Some molecules escape from liquid water and float into the air as vapor. Compared to all the water on Earth, the amount of water vapor is tiny. But it's crucial to our atmosphere, the source of all rain and snow. And water vapor helps trap some of the Sun's heat so it doesn't all bounce back into space. Water as gas: You can't see it, but you can't do without it.
In water vapor, as in any gas, the spaces between molecules are much bigger than the molecules themselves.
WATER AS VAPOR
Next time you're drinking a glass of ice water, stop and think about this: At a single moment, you're experiencing water in all three phases. The solid is cooling the liquid in your glass while the air you're breathing is full of water vapor. Water's versatility—its ability to exist as a liquid, solid, and vapor at ordinary temperature and pressure—makes life as we know it possible on Earth.
The Sun delivers a huge amount of energy to Earth every day. The tropical oceans absorb much of that energy, and a lot of it goes to creating water vapor—that is, to breaking the bonds between molecules of liquid water and sending them into the air. This process is called evaporation.
Snow forms when water in the atmosphere changes directly from gas to solid, skipping the liquid phase. These six-armed stars are actually single ice crystals, but crystals often clump while falling. Weather experts have reported feathery snowflakes as large as 15 centimeters (six inches) across.
Clouds, fog and mist include vast numbers of tiny liquid water droplets called drizzle drops, smaller than a tenth the width of a human hair.
A single cloud—really, a cluster of water droplets—usually exists for only an hour or so. The droplets may form, evaporate into water vapor and condense again many times before becoming large enough to fall as rain.
This is how Earth looks without clouds—they've been artificially stripped from this image. But in reality, at any given time more than half of Earth's surface is obscured by clouds, all of them formed from water vapor condensed from microscopic droplets or ice particles suspended in the air.