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Hydrogen
Oxygen
 Water—The shape of a water molecule.
Hydrogen
Water molecules are shaped like dumbbells. This gives water another important property: Each molecule is positively charged at one end and negatively charged
at the other end. Because the molecule is charged, molecules can attract one another (as well
as other charged particles). This form of linking is called hydrogen bonding. It explains why water holds together so well.
The same force is also responsible for the strange way
that water behaves when it freezes. Water molecules become locked rigidly together on freezing, but hydrogen bonding ensures that they lock together in a pattern with the hydrogen atoms of one molecule touching the oxygen atoms of its neighbor. This pattern causes the molecules to line up, rather than to pack densely. That is the reason ice is less dense than liquid water, and why snowflakes grow in patterns.
In ice crystals the links between molecules are made in a very ordered way, forming patterns. When the water is liquid, the arrangement of molecules is not fixed, but the bonding is still strong, so that water forms drops and clings as films to surfaces and in small spaces—the feature we
call surface tension. Hydrogen bonding also explains why water boils at a high temperature— enough energy is needed before the bonds can be shaken loose and the individual molecules rise as vapor.
The electrical charge of a water molecule is also important for its ability to dissolve substances. If
a substance such as salt (sodium chloride) is placed in water, the charges on the water pull the sodium and the chloride particles apart. The sodium particles (ions) and the chloride particles (ions) then become enclosed in jackets of water molecules, so that they cannot easily join up again unless all of the water is driven off.
Some water molecules even become dragged apart, leaving
a few hydrogen (ions) loose in
the water. Any substance with free hydrogen ions is an acid, so water can be weakly acidic. This is important in explaining the way that water can dissolve rock in the landscape.
(For more scientific terms associated with water see: Hardness; Head; Heat capacity.)
(For types of water see: Artesian water; Brackish water; Clean water; Drinking water; Fresh water; Groundwater; Polluted water; Seawater.)
(See also: Water consumption; Water cycle; Water life; Water power; Water resource; Water supply.)
Water
 Water—Water is a precious resource. Our bodies need water every day to replace what is lost in breathing and sweating. In areas without reliable supplies some people make huge stone jars to collect rain whenever it falls.
Water consumption
The amount of water that people use, measured in liters per person per day. On average, people throughout the world use about 60 liters per day.
The easier it is to obtain water, and the cheaper the water is to buy, the more consumption grows. For those who have to carry water from distant sources by hand, consumption may be as low as
15 liters a day. But when water is easy to obtain, consumption rises dramatically. In the United States, for example, consumption is 380 liters per person per day for home needs. Farming uses far more than this for irrigation.
Water consumption changes by day and by season. Consumption is highest in the early morning and later in the day, when people use showers and baths. It may rise when people water their gardens and fill their pools in summer.
Water cycle
The circulation of water between the seas, the air, the plants, the rocks, and the rivers. The energy for this essential cycle comes from the Sun and from gravity. The energy of the Sun allows evaporation and powers the winds that carry moist air from the oceans to the land, while gravity brings water back from the clouds and rivers to the oceans.
The water cycle—also called the hydrological cycle—is the main means of transferring water around the Earth. The main way water gets into the air is through evaporation
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