The three states of water - "the
water cycle"
Water (H2O) exists as all three states on Earth. It
is solid in the polar ice caps and glaciers, liquid in the oceans, lakes,
rivers, underground artesian basins and clouds, and as a gas in the
atmosphere. The three states of water are named, unlike any other substance.
Solid - ice
At temperatures less than 0°C, water exists
in solid form as ice. Ice is one of the very few solids which float
on their liquid; most solids sink. Ice floats because it is less dense
than the liquid water around itself.
Icebergs and polar ice sheets melt in summer and freeze in winter because
they float on the surface instead of sinking to the ocean floor. They
also absorb heat energy on melting and releasing that energy on freezing,
thus helping to moderate the Earth's climate.
The
weather patterns that make our world a place that we can live in are
the product of the melting and freezing of this sea ice. If ice did
not float, it would sink and the oceans would be solid ice with a layer
of liquid water. The Earth would also be a much less windy and colder
place.
Liquid - water
Most of the water on Earth exists in the liquid state as a salt solution
in the oceans, with smaller quantities of fresh water in lakes, rivers
and clouds. The temperature must be between 0°C and 100°C for
water to exist as a liquid at atmospheric pressure.
The Earth is in a unique position within the Solar System being just
far enough away from the Sun to allow liquid water to exist in significant
quantities. If it were closer, all water would be gas; if it were further
away, all water would be solid ice.
Clouds
form when gaseous water vapour in the atmosphere is cooled and droplets
of water form. It is these droplets of water that we see as fog and
clouds.
Gas - steam and humidity
Gaseous water is a colourless and odourless gas. The vapours often
seen above boiling water or as fog on a cold day are in fact a mist
of liquid droplets. The real steam is the invisible gas just on top
of boiling water and below the visible cloud.
Pure steam exists at temperatures above 100°C at sea level and
1 atmosphere air pressure. At lower atmospheric pressures - altitudes
above sea level - water boils at less than 100°C, hence, steam can
exist at temperatures less than 100°C at higher altitudes.
At lower temperatures, gaseous water exists as a mixture in air. Water
evaporates from surface liquid water and mixes with the air; this is
measured as humidity. Air humidity is another very important modifier
of climate. The increased humidity of the air in the tropics keeps temperatures
down in the 30's°C, unlike desert regions where the drier air allows
temperatures of 40°C to 60°C to be reached.
The greenhouse effect
Water vapour is a powerful "greenhouse" gas. If water was
not in the atmosphere, trapping the infra-red radiation escaping back
into space, the Earth would be so cold that it would be uninhabitable.
There is current debate about warming of the Earth's atmosphere due
to increased carbon dioxide concentrations, creating an "enhanced
greenhouse effect". This is covered in more detail in the topic
The
greenhouse effect.
The
water cycle
The Earth has a natural recycling process of ice, water and vapour.
From snow, glaciers, rivers, oceans, clouds and rain; water of Earth
freely and quickly changes between the three states.
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