Drops of Water 2
The Properties and Availability
of Water: A Fundamental
Consideration for Life
Did you know that 71% of our earth’s surface is covered
in sea water? Would it not be more fitting then to call it
planet “Water” rather than planet “Earth”? We too are
made up of water. Did you know that an individual is 70%
to 90% percent water?
Even the different foodstuffs you eat are mainly composed
of water. The water is there and is extremely important,
even if you can’t see it! Think of all the things you can
do with water, from eating and drinking, to washing,
swimming, playing and…
A world of… salt!
97.5% of water on Earth is salt water, that which we find
in the seas and oceans. Only 2.5% of the water on our
planet is fresh water that can be used for human needs.
and perennial snow. Around 30% of fresh water is located
underground whereas just under 1% is present in the
form of humidity in the air and ground. Just think that
only 0.3% of the planet’s fresh water is easily accessible
to humans, the surface water that we can see in rivers
and lakes. Did you know that?
Illustration 2.a: © Centro Civiltà dell’Acqua
How is water distributed on our planet? Salt water 97%;
fresh water in the form of ice 2.4%; fresh water in liquid
form 0,6% (but humans can actually only use 0,003%).
Illustration 1: © USGS United States Geological Survey Institute
Pie chart: distribution of fresh and salt water on our planet.
Do you prefer fresh or salt water?
Have you ever tried drinking sea water? What does it
taste like? Of course you would prefer a glass of fresh
water! Fresh water contains very little salt and when
we drink it we can’t taste the salt at all. Salt water on
the other hand is full of salt, the water of the seas and
oceans.The water we use every day for drinking, cooking,
washing, cleaning, watering the garden, etc, is fresh water.
The different forms of fresh water
Almost 70% of the world’s fresh water is to be found in
the cold zones of the planet in solid form, as in glaciers
Illustartion 2.b: © FAO Food and Agriculture Organization
of the United Nations. Data processed by Centro Civiltà
dell’Acqua - Pie chart: the different forms of fresh water.
Drops of Water 2
A very uneven distribution
So, the amount of fresh water on earth is …. much less
than you thought! But how is this surface water distributed
throughout the world? A large quantity is concentrated
in particular areas like Siberia in the vicinity of Lake
Baikal, or in the Great Lakes of North America, or in the
lakes of Africa such as Lake Tanganyica, Lake Victoria and
Lake Malawi. Underground water is rather concentrated
near some of the world’s major rivers: the Amazon, the
Congo, the Yangtze and the Orinoco. The fresh water of
lakes and water bearing strata is replenished thanks to
the rain. So the more it rains the more the fresh water
reserves are renewed.
World Water Day
Water is a resource of inestimable value. Since it is not
in infinite supply we must learn to respect it and use it
wisely! That is precisely why the United Nations set up
World Water Day in 1992, an important day for reflection
that is celebrated each year on the 22nd of March to
remind us all of how limited, precious and irreplaceable
water is. See http://www.worldwaterday.org/
Illustration 4: © IWMI International Water Management
Institute. Projected water scarcity: in 2025 large regions of the
world could be faced with problems related to water supply.
Illustration 3.a: © FAO Food and Agriculture Organization of the
United Nations. Data processed by Centro Civiltà dell’Acqua
45% of the world’s water supply is concentrated in just 6 countries.
Illustration 3.b: © FAO Food and Agriculture Organization of the
United Nations. Data processed by Centro Civiltà dell’Acqua
Per capita water consumption in different countries of the world.
45% of the world’s water supply is concentrated
in just 6 countries
The nation which has most fresh water is Brazil, around
15% of the surface and underground water available on
earth. Each Brazilian consumes an average of 190 litres of
water per day. The United States on the other hand has
about 6% of the world’s water supply yet a US citizen
consumes on average three times as much, about 550
litres per day! China has about 1/3 of the supply that
Brazil has, (about 5%), and a citizen of China consumes
on average 90 litres per day.
If we compare the two diagrams, an interesting piece of
data emerges, and that is that not necessarily do those
who have more water available actually consume more.
The different status of water
Water is the only substance on earth that can transform
into three different states, each with very different
characteristics:
• gas (mist, vapor, clouds)
• liquid (rain, dew, seas, lakes, rivers) and
• solid (ice and snow).
But if someone asked you what water was, what would
you reply?
H²OOOOOOOOO… Would you know how to
describe water?
Imagine a glass of water and think of a house. What are
the house walls made of? Bricks? And water, what is it
made of? Of very special bricks indeed called molecules!
They are just like tiny bricks that can only be seen through
a microscope. What are molecules made of? Let’s think
again of bricks… they are obviously made of cement or
terracotta. And the bricks of water, are they made of
cement? Clearly not! They are made of ……atoms! The
atom is the smallest part of any material or substance. It
is smaller than a grain of sand! Atoms make up everything
and are only visible through a microscope.
What does H²O mean?
The molecule of water is called H²O. But why did
scientists not call it something else, “W” for example?
After all, the word water begins with the letter W! But
it is not called H²0 by chance. “H” stands for Hydrogen
and “O” stands for Oxygen, In fact, the water molecule is
composed of two hydrogen atoms and one oxygen atom.
Drops of Water 2
A drop of water is made up of many, many molecules, just
like the one you see here below!
But going back to the bricks of a house, let’s ask
ourselves another question. How do they all stay joined
together? With mortar of course. Molecules too remain
stuck together thanks to a special cohesive force of an
electrical nature.
Illustration 6: © Centro Civiltà dell’Acqua
A roundabout of… molecules!
Illustration 5: © Centro Civiltà dell’Acqua - The molecule of water.
THE PHYSICAL AND CHEMICAL PROPERTIES
OF WATER
Scientists have spent a great deal of time studying water
and have come to the conclusion that it has some very
special properties indeed. These are: density, floatation,
surface tension, capillarity and solvency.
Density
How many atoms are there in a drop of water? Countless!
Millions and billions! By density we mean the number of
water molecules present in a given unit of volume (one
mm³ for example). It’s a bit like asking how many people
live in Columbia or Vietnam. The number of people who
live in a particular nation is called density, only in this case
it is not about the number of molecules but the number
of people, it is about population.
A roundabout of… molecules!
Try to imagine water molecules as a group of children
playing ring a ring of roses. The further away from each
other they are the easier it is for them to move their
arms and legs. If these children were molecules, we might
say that the substance they make up is of low density.
Now try to imagine the same children playing ring a ring
of roses closer to each other. They can hardly move! If
they were molecules we would say that the substance
they make up is high density. Similarly, cold water is of
high density, while warm water is of low density.
Water too can have varying density depending on the
temperature it is exposed to. Lukewarm water, for example,
is of low density. If we were to observe this water through
a microscope after having left it for an hour in the fridge, we
would see that its density has increased. It would follow that
ice should have an even greater density than cold water. But
it isn’t so! The solidification of water breaks the rule! Water
reaches its maximum density at 4°C below which, from 4°
to 0°, when it turns to ice, the molecules separate rather
than come closer together. So the water density decreases
and doesn’t increase! Indeed ice is of lower density than
water…
Floatation
If you pick up a ball, raise your hand, then open your hand
and let the ball go, what happens? It falls to the ground.
Why? Because on Earth the force of gravity exists, a force
that draws any object downwards. If gravity didn’t exist,
we would fly rather than walk, just like astronauts in a
spaceship. On our planet, a ship floats on the sea, but on a
planet with no gravity, the same ship would be suspended in
mid air and would … fly! Why then does a ship not just sink
if it is pulled by the force of gravity. And why does the ship’s
anchor sink rather than float? Whether an object sinks or
floats does not just depend on its weight, but on its density!
Any object will float on water if its density is lower than that
of water. The iron that anchors are made of is of greater
density than water, and so, it sinks to the sea bottom! That is
why icebergs, which are made of ice, stay afloat, because the
density of ice, as we have seen, is lower than that of water!
The fact that ice has lower density than water and can stay
afloat is of fundamental importance for aquatic ecosystems.
Drops of Water 2
The layer of ice which forms on the surface of parts of the
Arctic or Antarctic oceans or on mountain lakes actually
acts as thermal insulation. This means that this layer of ice
prevents the water below from becoming solid and icing
over, thus allowing a number of animal and vegetation
species to live below the ice surface.
Illustration 7: © New Zeland Press Association
Iceberg in New Zeland.
Surface tension
If we fill a basin with water and throw a coin in it, what
happens? The coin sinks.What if we throw a cork into it, or
a soap bubble. What happens now? It floats. The ability of
different objects to
float does not just depend on the density of their molecules,
but also on another property of water, its surface tension.
The molecules of the water surface form a film, a sort of
“elastic carpet”, and when we throw a coin their equilibrium
is broken, the coin goes right through the film and sinks.
A soap bubble, on the other hand, does not. Many insects
make use of this property of water to stay afloat. Have you
ever seen a water-fly on a pond? How does it stay afloat? By
putting its legs onto the molecules of the water surface and
exploiting its surface tension. If you look carefully, it really
does seem to be on top of an elastic carpet! And if you
observe the place where it rests its legs, you will see that the
surface of the water is slightly curved downwards.
Capillarity
If we fill a basin with water and insert a sheet of paper
vertically half way into the water, what happens? The
half of the paper under water gets wet. But the water
doesn’t just wet the immersed part, the other half
outside the water gets wet too. Why? Because water can
rise up into material and make it wet. This property is
called capillarity.
Solvency
Another property of water is solvency. Water is a
solvent which can dissolve other “soluble” substances.
If we put a teaspoon of salt into a glass of water and stir,
what happens to the salt? It disappears! It has dissolved in
the water: the solvent (water) has dissolved the soluble
substance (salt). If we now want to observe the salt
which dissolved in the water, we will have to look at a
drop under a microscope. And upon closer inspection
we will see that the water molecules have joined with
the salt molecules.
So water can dissolve solid substances like salt and sugar,
liquid substances like ammonia and alcohol, and gaseous
substances like carbon dioxide. This property of water is
extremely important. If it wasn’t able to act as a solvent,
then it couldn’t assimilate and transport the mineral salts
and other nutritive substances throughout our bodies
that our organism needs to stay healthy.
Archimedes’ Principle
Let’s try out one last experiment. Let’s tie a stone to
a piece of thread and hang it onto the hook of some
weighing scales. We can read the weight of the stone by
observing the downward movement of the scales’ pointer.
Now let’s put the stone into water and then put it back
on the scales to see how much it weighs. It weighs less!
After the stone has been immersed in water it actually
loses the amount of weight that is equal to the weight of
the displaced water. The first person to understand and
describe this phenomenon was Archimedes, observing
that a body immersed in fluid receives a downwards push
that is equal in weight to that of the volume of displaced
fluid.
Dossier compiled by:
Water Civilization International Centre,Venice:
www.civiltacqua.org
Illustration 8: © Massachusetts Institute of Technology
Example of surface tension.
Drops of Water 2
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Venice Office
Regional Bureau for Science
and Culture in Europe
United Nations
Educational, Scientific and
Cultural Organization