Michele Wheeler, age 16, of Emmaus, Penn., for her question:
WHAT EXACTLY IS WATER?
Water is a transparent, odorless, tasteless liquid that is a compound of hydrogen and oxygen. It is the most common substance on Earth and also one of the most unusual.
Water can be a liquid, solid or gas. No other substance appears in these three forms within the earth's normal range of temperature.
Tiny particles called molecules make up water. Each drop of water contains many millions of molecules. Each molecule, in turn, is made up of even smaller particles called atoms. When two atoms of hydrogen combine with one atom of oxygen, they form the chemical compound we call water.
The molecules in water are always moving. The form the water takes depends on how fast they are moving. The molecules in solid water, or ice, are far apart and almost motionless. The molecules in liquid water are close together and move about freely. The molecules in water vapor, which is a gas, move about violently and bump into one another.
Water's unusual properties depend on the forces that hold it together. These forces are hydrogen bonds and chemical bonds. Chemical bonds are the forces that hold the two hydrogen atoms and the one oxygen atom together in a water molecule, while hydrogen bonds are the forces that link water molecules together.
In the chemical bond, each hydrogen atom has one electron whirling in orbit around its nucleus. But each of these atoms has room for two electrons. The oxygen atom has six electrons in its outer orbit, but it has room for eight. The hydrogen and oxygen atoms each fill their empty spaces by sharing their electrons.
The two electrons from the two hydrogen atoms enter the orbit of the oxygen atom. At the same time, two electrons from the oxygen atom fill the empty spaces in the two hydrogen atoms., The resulting water molecule is an extremely tight structure.
The water's hydrogen bond is the force that links water molecules together. Water molecules have a lopsided shape because the two hydrogen atoms bulge from one end of the oxygen atom. The hydrogen end of the water molecule has a positive electric charge.
At the opposite end, the molecule has a negative charge. Water molecules link together because opposite charges attract. The positive ends of water molecules attach to the negative ends of other water molecules, whose positive ends attach to the negative ends of still other water molecules.
On the average, each person in the United States and Canada uses about 70 gallons of water every day in his home. During his lifetime, the average person will drink about 16,000 gallons of water.
Only about 3 percent of the earth's water is fresh.