Water
Last updated: May 22, 2009.
Pour yourself a glass of water and you
could be drinking some of the
same molecules that passed through the lips of Julius Caesar, Joan of
Arc, Martin Luther King, or Adolf Hitler. Indeed, since the human body
is about 60 percent water you might even be drinking a tiny part of one
of those people! Water is one of the most amazing things about
Earth; without it, there would be no life and our planet would be a
completely different place. One of the truly amazing things about
water is that it's never used up: it's just recycled over and over
again, constantly moving between the plants, animals, rivers, and seas
on Earth's surface and the atmosphere up above. Let's take a look at
this life-giving liquid and find out what makes it so special!
Photo: Water in motion. Photo by Ryan Hegarty,
courtesy of US Fish & Wildlife
Service.
What is water?
Photo: Water covers over two thirds of Earth's
surface.
We can answer that question in many different ways. Water is what
wets windows when it rains, what we drink when we feel thirsty, and
what covers about 70% of Earth's surface. But what exactly is it?
Chemically speaking, water is a liquid substance made of molecules—a
single drop of it contains several billion of them! Each molecule of
water is made up of three atoms: two hydrogen
atoms locked in a sort of triangle with one oxygen atom—giving us the
famous chemical formula H2O. The slightly
imbalanced structure of water
molecules (see "The polar molecule" in the box
below) means they attract and stick to many different substances.
That's also why all kinds of things will dissolve in water, which is
sometimes called a "universal solvent". Water can even dissolve
the solid rocks from which our planet is made, though the process does
take many years, decades, or even centuries.
Most of the water in our world is a combination of "ordinary"
hydrogen atoms with "ordinary" oxygen atoms, but there are actually
three different istopes (atomic varieties)
of hydrogen and each
of those can combine with oxygen to give a different kind of water. If
deuterium (hydrogen whose atoms contain one neutron and one proton
instead of just one proton by itself) combines with oxygen, we get
something called heavy water, which is about
10% heavier than
normal water. Similarly, tritium (hydrogen with two neutrons and one
proton) can combine with oxygen to make something called superheavy
water.
Water has no end of amazing properties. It comes in three wildly
different kinds, it's heavy, it expands in a funny way, it can climb up
walls, and... oh let's find out more!
Water, ice, and steam
One of the unique things about water in the world around us is that
it exists in three very different forms (or states of matter
as they are known): solid, liquid, and gas. Ordinary, liquid water is the most familiar to
us because water is a liquid under everyday conditions, but we're also
very familiar with solid water (ice) and gaseous water (steam and water
vapour) as well.
Photo: Looking out to sea from my local beach on the three states of
matter that water can assume. It's February, so that's snow (solid water) covering the beach itself.
The ocean is liquid water. Up above in the sky, the clouds contain water vapor (water in gaseous form).
Converting water between these three different states is remarkably
easy. All you have to do is change its temperature or pressure. Take
some ice and heat it up and you'll soon have a pool of liquid water.
Carry on heating it and the water will evaporate
and become
steam. It takes a terrific amount of energy to turn ice into water and
water into steam because you have to physically rearrange the structure
of the substance in each case and push the molecules further apart.
That's why kettles take so long to boil. (There's an easier way to turn
water from a solid or liquid into a gas and that's simply to leave it
out in the open air; gradually, the more energetic molecules in the
water will escape and turn into a cool vapor up above it.)
Photo: Steam geysers are produced when water is
heated by Earth's internal heat (geothermal energy).
Picture by Robert Blackett, Utah Geological Survey, courtesy of US Department of
Energy/National Renewable Energy Laboratory (DOE/NREL).
When you heat water to make steam, there comes a point where you
keep heating the water but the temperature doesn't increase. The energy
you supply seems to be vanishing into thin air, but it's actually
pushing apart the molecules in liquid water and turning them into a
gas. In the process, that energy is becoming locked inside the steam as
something called latent heat (the word
latent just means
"hidden"). Latent heat is like an immense reserve of energy locked in
steam that the
inventors of yesteryear used to power factory machines and vehicles
using their mighty steam engines.
Read more in our main article on heat.
Why does water make pressure?
If you've ever found yourself washing a car with buckets or watering
a garden with cans, you'll have noticed just how heavy water can be.
That's because it's a relatively dense
substance (it packs
an awful lot of mass into a relatively small space). Water isn't
dense compared to metals such as gold, which is almost 20 times heavier
by volume. But it's much heavier and denser than wood and plastic, which
is why those things will float. Anything less dense than water floats
on it; anything more dense sinks in it.
The weight of water is what causes pressure
in the oceans to
increase with depth. The deeper you go, the more water there is up
above you pressing down—and that makes things particularly challenging
for submarine designers and scuba divers. Water pressure increases in
direct proportion to your depth, so if you go down 100 meters the
pressure is ten times greater than if you go down 10 meters. Just
imagine walking on the seabed with lots of buckets of water pressing
down on your head. At a depth of about 10 km (6 miles) under the
oceans, the pressure is as great as the weight of a fully-loaded
articulated lorry pressing down on an
area the size of your two feet!
Why does water expand when it freezes?
Everyone knows things get bigger when they get hotter and shrink
when they cool. Thermometers tell the temperature that way because the
(liquid) mercury metal inside them expands as it heats up and contracts
when it cools down. But water is different. Almost uniquely, water
expands as it starts to freeze! This amazing trick is called the anomalous
expansion of water—and here's how it works.
If you start off with a glass of water and cool it down, the
molecules start to move closer and lock together. But at a temperature
of about 4°C (39°F), the molecules are as close as they can
possibly get. In other words, the water has reached its maximum
density. If you keep on cooling it down, the molecules rearrange
themselves into a slightly more open structure. This means ice is a
little bit less dense than freezing water and that's why ice floats on
water that's the same temperature.
That's extremely important for fish and all kinds of other river and
sea creatures, because it means they can survive in winter in the
liquid water underneath solid frozen ice.
Unfortunately, people don't always find the anomalous expansion of
water so helpful. If the water pipes running under your home freeze
solid in winter, the water inside them will turn to ice that takes up
more volume—causing the pipes to burst open and then leak when the ice
thaws out. Why don't we simply use stronger pipes? It wouldn't make
much difference: water expands with incredible force when it freezes
and even very thick metal pipes would still burst. You can watch a
superb video demonstration of this on the Steve
Spangler Science
website.
Photo: Ice in the Wichita Mountains.
Picture by Elise Smith courtesy of US Fish & Wildlife Service.
Why does water take so long to heat up?
Has that kettle boiled yet? Well tell it hurry up—I'm dying for a
cup of tea! It may be a nuisance if you're cooking or making drinks,
but the length of time it takes water to absorb heat is very useful to
us in other ways. Water has a high specific heat
capacity and
that means it can hold or carry more heat per kilogram (or pound) than
virtually any other substance. That's why we use water in heating
systems such as radiators, because each liter of water that trickles
through the pipes carries and delivers more heat. Of course the
drawback is that the water takes some time to heat up in the first
place.
Why can insects walk on water?
You've probably seen insects that can walk on water. They're
supported by a kind of invisible "structure" on the surface known as surface
tension. It happens because water molecules attract very
strongly
to one another—that's also why water forms droplets on windows rather
than
spreading out in a perfectly thin film, as oil would. Imagine all the
drops in a basin full of water trying to attract one another.
Effectively, they're "linking arms" and forming an invisible skin on
the surface that's strong enough to support things like needles and
razor blades that are heavy enough to sink. All kinds of insects,
including spiders, pondskaters,
and water boatmen,
use surface tension to move across water. In theory, you could walk on
water too if you could spread your weight across a big enough area to
take advantage of surface tension.
How does water climb up a tube?
Put some water in a glass and you'll see that it doesn't form a
perfectly straight surface: it actually climbs up the glass slightly
more at the edges, forming a downward curving surface called a concave meniscus.
The thinner you make the glass (that is, the smaller the diameter it
is), the more the water will climb. Put water in a narrow glass rod and
you can make it climb up quite a distance. This is known as capillary
action or capillarity. It's how blood
moves through our
veins and how water is sucked up through the stems of plants and trees.
Capillarity helps a large oak tree to suck up something like 380 liters
(100 gallons) of water each day!
Water in our world
Photo: Earth's atmosphere is full of water
vapor.
Computer-processed satellite photograph courtesy of Great
Images in NASA.
So much of Earth is covered in water that the planet could
easily be called Aqua or Oceanus. Apart from the water on the surface
(in oceans, rivers, lakes, and creeks), there's also a vast amount of
water swirling around in the atmosphere (in clouds, mist, and fog) and
plenty more trapped in rocky underground reservoirs called aquifers.
Earth's water—perhaps its most unique feature—was formed after the Big
Bang (the explosion that created the Universe about 13.7 billion years
ago). About 4.6 billion years ago, when our solar system was created, a
mixture of hydrogen and oxygen atoms joined together to make clouds of
hot steam that eventually cooled to form water, which fell to Earth as
rain, formed the oceans and carved the continents into shape.
Water for life
Life began on Earth about a billion years later (3.6 billion years
ago), initially in the oceans. Although many species now live on land,
they still need water to live and grow: humans, for example, could go
without food for about two months, but we'd die of thirst if we went
more than a week or so without a drink. Typically we need at least 2
liters of water a day to survive, though we get much of this from
things we eat as well as things we drink. Eggs are about three quarters
water, for example, while fruits such as oranges and melons are over 90
percent water.
Photo: Water is "food" for plants, such as this lily of the valley.
We drink only about 1 percent of the water we consume each day and
use the other 99 percent (about 250 liters a day) to feed things like
baths, showers, washing machines,
lawn and garden sprinklers, hosepipes for washing cars, and flush
toilets. A
clothes washing machine can easily use over 100 liters in an hour by
repeatedly rinsing your laundry to remove detergent. Lots of products
we'd never normally associate with water consume vast amounts of the
precious liquid during their manufacture. About 570 liters of water is
used making a thick Sunday newspaper, for example.
To people in developing countries, many of whom lack access to
running water, all of this would seem amazingly wasteful. As Earth's
population grows and each person needs more and more water, the
pressure on our planet's water resources will grow too. Theoretically,
on a planet covered with water, supplies should never run out—but most
of Earth's water is salty and undrinkable. Turning it into usable
freshwater means using costly, energy-hungry desalination plants. The
growing pressure on water has led some politicians to speculate that
wars may be fought over scarce water supplies before the end of the
21st century.
The water cycle
There's a lot of talk about recycling
to help the environment but water is one thing we recycle without even
thinking about it. Every time we flush a toilet or empty a washing-up
bowl down the drain, the water we've used disappears down waste pipes,
passes through the sewerage and wastewater system, and reappears
(hopefully) as good as new in our rivers and seas. Admittedly, water pollution is still a very serious
problem, but one thing we can count on is that water will constantly
circulate between Earth's surface and the atmosphere up above in the
never-ending water cycle. Water's been circulating round our planet for
billions of years—and it's not about to stop anytime soon!
Artwork: The Water Cycle illustrated by John M Evans for the US
Geological Survey.
You can find a bigger version of this picture on the USGS Water Cycle
page.
Something you can do...
If you've enjoyed this article or found it useful, perhaps you could do us a favor?
Explainthatstuff.com supports the development organization WaterAid that brings clean water
and sanitation to some of the world's poorest people. Please take a moment to
visit their website and find out more about their superb work.
A brief history of water
- 4.6 billion years ago: Earth's water supplies are formed.
- 3.6 billion years ago: Water allows life to begin on Earth.
- Prehistoric times: People lived nomadically, constantly moving
between places where water and food were plentiful. The first permanent
settlements grew up next to rivers and water systems that could ensure
a steady supply of water.
- ~4000BCE: Irrigation (technology for carrying a steady supply of
water to growing crops) is invented in Mesopotamia.
- Ancient Greek philosopher Thales of Miletus
(c.624BCE–546BCE) considered water the most basic building block of
matter. Aristotle, another Greek
philosopher, saw water as one of the four fundamental elements (Earth,
air, fire, water).
- ~300BCE: Ancient Romans pioneered aqueducts for supplying water
to
their empire.
- 1582: First modern waterworks constructed in London, England.
- 1652: First waterworks constructed in North America in Boston,
Mass.
- 1781: English scientist Henry Cavendish
(1731–1810) makes
water from "inflammable air" hydrogen and "dephlogistated air" (oxygen
air).
- 1783: French chemist Antoine Lavoisier
(1743–1794) shows
that water is a compound made of hydrogen and oxygen.
- 1804: Frenchman Joseph Louis Gay-Lussac
(1778–1850) and
German Alexander von Humboldt (1769-1859)
show that hydrogen
combines with oxygen in the ratio two to one, as in the modern formula
H2O.
- 1932: US chemist Harold Urey
(1893–1981) discovered
deuterium and showed it is present in ordinary water in tiny amounts.
- 1957: The world's first desalination plant (making freshwater by
removing salt from seawater) begins operating in Kuwait.
- 1951: American chemist Aristid V. Grosse
(1905–1985)
discovered tritium in ordinary water.
Websites for further research
Here are some links to help you find out more about water.
$This means
you'll need a subscription (or you'll need to pay) to read an article.
Do check if your library or institution has a subscription before
paying the fee.
Another useful trick is to copy the exact article title into Google and
search again: sometimes you can find the text elsewhere for free!
This
means we found a website or article really useful and worth looking at.
Cool stuff for schools: Introductions for kids and teachers
- Water:
article in Wikipedia
- Water:
article in Columbia Encyclopedia
- Fascinating Water Facts: lots of amazing facts and figures here.
- Key water aid statistics: How many people lack
access to water? How many children die from water-related illnesses and
diseases? These are more quick facts from WaterAid.
News stories and feature articles
- Water problems
- Water solutions
- Sandra
Postel: Irrigated agriculture
- $[E] Cover Story -- Water, Water Everywhere: Innovation
and Cooperation
are Helping Slake the World's Growing Thirst (Sep-Oct 1998)
- Global Crisis Solution Center: Improving
Water Availability At Small Community and Family Level
- Water
Conscious Canadian Firm Harvests Icebergs, Environment News Service,
Aug
28, 2000
- $Making
every drop count: Peter Gleick, Scientific American, February 2001.
- $Growing
more food with less water: Sandra Postel, Scientific American,
February
2001.
- $How
we can do it: Peter Gleick, Scientific American, February 2001.
- Rivers in crisis
Water around the world
- General worldwide information
- Europe and Central Asia
- Middle East and Africa
- Asia Pacific
- North America
- Latin America and the Caribbean
NGOs and campaign groups
International organizations concerned with global water policy
Useful reports and international agreements
- The
Dublin Statement on Water and Sustainable Development. Adopted by
the
International Conference on Water and the Environment (ICWE) in Dublin,
Ireland, on 26-31 January 1992.
- Chapter
18: Protection Of The Quality And Supply Of Freshwater Resources:
Application
Of Integrated Approaches To The Development, Management And Use Of
Water
Resources. from Agenda 21 of the United Nations, as adopted by the
Plenary in Rio de Janeiro, on June 14, 1992:
- Making
Water Everybody's Business. A report by William J. Cosgrove and
Frank
R. Rijsberman for World Water Vision and the World Water Council.
Cairo,
Egypt: World Water Council, 1999.
- A
Water Secure World: Vision for Water, Life, and the Environment by
World Commission for Water in the 21st Century. Cairo, Egypt: World
Water
Council, 2000.
- Dams and Development: A New Framework
for
Decision-Making. The Report of the World Commission on Dams: An
Overview. Cape Town, South Africa:
World Commission on Dams, 2000. This website is currently not working.
When we find the new site, we'll update this.
- World
Water Demand and Supply, 1990 to 2025: Scenarios and issues by
David
Seckler, Upali Amarasinghe, David Molden, Radhika de Silva, and
Randolph
Barker. Colombo, Sri Lanka: International Water Management Institute,
1998.
- Appraisal and Assessment of World Water Resources by
Igor Shiklomanov, Water International, Volume 25, Number 1, March 2000.
- European
Union: Water Policy
Miscellaneous links
Resources not available on the web
Books
- De Villiers, Marq. Water Wars: Is the World's
Water Running Out? London: Weidenfeld, 1999.
- De Villiers, Marq. Water: The Fate of Our
Most Precious Resource. New York: Mariner Books, 2001.
- Gleick, Peter. The World's Water 2000-2001:
The Biennial Report on Freshwater Wesources. Washington DC: Island
Press, 2000
- McCully, Patrick. Silenced Rivers.
London: Zed Books, 1996.
- Postel, Sandra. Last Oasis: Facing Water
Scarcity. New York: W.W. Norton, 1992.
- Postel, Sandra. Pillar of Sand: Can the
Irrigation Miracle Last? New York: W.W. Norton, 1999.
- Postel, Sandra. "Redesigning Irrigated Agriculture."
in State of the World 2000. London: Earthscan, 2000.
- Postel, Sandra, Gretchen Daily, and Paul Ehrlich.
"Human Appropriation of Renewable Fresh Water." Science,
February
1996, Vol 271, p. 785–788.
- Seckler, David, Upali Amarasinghe, David Molden,
Radhika de Silva, and Randolph Barker. World Water Demand and
Supply,
1990 to 2025: Scenarios and issues. Colombo, Sri Lanka:
International
Water Management Institute, 1998.
- Tearfund. Running on Empty: A Call for
Action to Combat the Crisis of Global Water Shortages. London:
Tearfund,
2001.
- WaterAid. Mega-Slums: The Coming Sanitary
Crisis. London: WaterAid.
