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The properties of water
Changes of state
The shape of a water molecule and the way the electrons are arranged allows the
negative parts of one molecule to form weak bonds with the positive parts of others.
These bonds produce groups of up to four molecules that behave like a larger molecule
with a higher boiling point. So, water is a liquid at temperatures when its low molecular
mass would normally make it a gas. The bonds are continually forming and being broken.
Individual water molecules at the water surface may escape as a gas: evaporation. At
100°C and one atmosphere pressure all hydrogen bonds are broken and the water turns into
gas as it boils.
The narrow temperature range within which water changes state between solid, liquid
and gas allows the hydrological cycle to occur.
The water molecules are less densely packed than when they are in liquid form so solid
ice floats on liquid water.
In an area that has cold winters, the water in a lake will be cooled by the cold air above.
As the water is cooled below 4°C, the water molecules start to take up arrangement
they will have as a solid. This lowers the density as the water expands and the cold water
will float. This expansion on cooling is unusual and is known as anomalous expansion.
The deeper, warmer, water is unaffected as further cooling produces floating solid ice.
This floating layer of cold water and ice prevents the water below from freezing and
allows most of the water in the lake to remain unfrozen even if the air above is very
If cooling water became denser until it frozen, hen ice would sink, more water would
be exposed to the cold air, this would also sink and eventually the whole lake would
freeze, killing most aquatic life.
Water is an excellent solvent is called the 'general physiological solvent' because most
biological reactions occur with solutes dissolved in water. Plant nutrients can only be
absorbed if they are dissolved in water and most materials transported in blood and sap
are dissolved in water.
High heat capacity
Water heats up and cools down slowly. This helps to maintain climatic stability by
moderating temperature changes.