 
The Source of Life: Water
 Water
is a basic condition for life, but it is entirely absent from
the solar system's 63 other celestial bodies. However, most
of our planet is covered with water. Oceans and seas make
up three fourths of the Earth's surface while the land itself
contains countless numbers of rivers and lakes. The snow and
ice on the summits of lofty mountains is water in its frozen
form. A substantial part of the earth's water is in the sky:
every cloud contains thousands--sometimes millions-of tons
of water in the form of vapor. From time to time some of this
water vapor turns into drops of liquid and falls to the ground:
in other words, it rains. Even the air you're breathing now
contains a certain amount of water vapor.
The rains, the seas, rivers, streams, the oceans, drinkable water that flows when you turn on a faucet… People are so used to the existence of water that they probably never think about the fact that a major part of the Earth's surface -indeed, most of it- is covered with water. However, what is singularly important here is that, among all known celestial bodies, only the Earth provides drinkable water.
The Astonishing Features of Water
The thermal properties of water play a major role in the continuity of life on Earth. We may set out a few of these as follows:
All known solids decrease in size as they grow colder. As
volume decreases, density increases and thus the colder parts
of the liquid become heavier. This is why the solid forms
of substances weigh more (by volume) than they when they are
in liquid form. Like other liquids, water contracts in volume
as it grows colder but it only does this down to a certain
temperature (39.2F0) thereafter-unlike all other
known liquids-it suddenly begins to expand and when it finally
solidifies (freezes) it expands even more. As a result, "solid
water" is lighter than "liquid water". According to the normal
laws of physics, solid water, which is to say ice, ought to
be heavier than liquid water and should sink to the bottom
when it forms; instead, it floats.
This feature of water is very important with regard to the seas that cover the Earth.
Were it not for this property, that is, if ice didn't float, much of our planet's water would be locked up in ice and life would be impossible in its seas, lakes, ponds, and rivers.
When ice melts or water vaporizes, it absorbs heat from its surroundings. When
these transitions are reversed (that is, when water freezes
or vapor precipitates) heat is released. In physics the term
"latent heat" is used to describe this. All liquids have a latent
heat of some sort or other but that of water is among the highest
known. Also, the "thermal capacity" of water, that is, the amount
of heat necessary to raise the temperature of water by one degree,
is higher than the great majority of other liquids.
|
