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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.

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 waterAlmost 70% of the world’s fresh water is to be found in the cold zones of the planet in solid form, as in glaciers

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’AcquaHow 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%).

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.

The Properties and Availabilityof Water: A Fundamental

Consideration for Life

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A very uneven distributionSo, 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.

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 countriesThe 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.

World Water DayWater 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.

The different status of waterWater 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.

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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 5: © Centro Civiltà dell’Acqua - The molecule of water.

THE PHYSICAL AND CHEMICAL PROPERTIES OF WATERScientists 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.

DensityHow 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.

Illustration 6: © Centro Civiltà dell’AcquaA roundabout of… molecules!

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…

FloatationIf 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.

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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 AssociationIceberg in New Zeland.

Surface tensionIf 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.

Illustration 8: © Massachusetts Institute of TechnologyExample of surface tension.

CapillarityIf 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.

SolvencyAnother 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’ PrincipleLet’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

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