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V
Vapor, water vapor
Water when it is an invisible gas. The amount of water vapor that air can hold depends on the pressure and temperature of the air. In general, the cooler the air, the less water vapor it can hold. When the amount of water vapor in the air is as high as it can go, and the air can hold no more water vapor, we say the air is saturated. If saturated air cools, it will not be able to hold
all of the water as vapor, and some will be condensed into liquid water droplets (see: Condensation). It will then be seen as cloud, fog,
or dew. (See also: Moisture and Steam.)
Water vapor in the air
Water is moved from oceans
to clouds as water vapor. The amount of water in the air is
about one thousandth of 1% of
all the water in the oceans. The amount of vapor that air can hold depends on its temperature. Warm air can hold more vapor than cold air, so the amount of vapor in the air is much higher in the tropics than near the poles. The amount of water vapor in the air also changes between ground level and high altitudes. In fact, the air is so cold at 15km above the surface of the Earth that it holds almost no water vapor at all.
On average, water that has evaporated from the oceans will remain in the air as vapor for
about 10 days before it forms
into droplets or ice crystals and returns to the oceans of the land as rain or snow (see: Evaporation and Sublimation). Winds in the atmosphere can carry this vapor for thousands of kilometers during this time. (See also: Water cycle.)
Water
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W
Water
One of the world’s most common substances, water can be found as solid ice, as liquid water, and as water vapor (gas).
Water is a simple substance; each water molecule is made up
of hydrogen and oxygen, hence its chemical formula, H2O (two atoms of hydrogen bonding with one atom of oxygen).
Water is a colorless, tasteless, and odorless substance. It is found as both liquid and vapor (in the air) at room temperatures.
Water is a vital part of life. It
is the major component in most cells of all living things, including animal blood and plant sap.
One of water’s most useful features (and used by all living things) is its ability to dissolve substances and thereby carry them.
Nearly all the water in the world is a solution of water with other
substances dissolved in it. Pure water is extremely rare, but can be specially prepared, as distilled water.
Water is a dense substance.
One old adage said, “A pint of water weighs a pound and a quarter.” Nowadays we would
say that 1 liter of water weighs a kilogram. However it is measured, the weight (density) of water is often important. Water has a high density, which is why water soaked into clothes makes them heavy. The water ponded up behind a dam can be heavy enough to burst the dam unless it is very strong. The weight of water can be used instead of concrete in a garden roller. The work needed to carry a pail full of water is considerable, which is why people in developing world countries who have to carry their water from wells and rivers can only carry a small amount at a time. In nature water crashing onto a beach in a storm is responsible for moving large amounts of sand and even boulders. There are numerous other examples.
The melting/freezing point
of water at normal atmospheric pressure is 0° C, and the boiling point is 100° C. Unusually, the solid form of pure water—ice—is less dense than the liquid—water expands as it freezes. This property explains why icebergs float on the ocean, and why ice cubes float in
a glass of water. The difference in density between liquid water and ice is about one-tenth, so nine- tenths of a body of ice is below
the water level, with just one-tenth above it.
 Water—Water is necessary for life. People who are not fortunate enough to have water piped to their homes must carry it, often from far away.