The science of water
by Chris Woodford. Last updated: December 4, 2019.
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 covers over two thirds of Earth's surface and is an essential ingredient for all the flourishing life our planet enjoys—including this lily of the valley plant.
What is water?
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, large drop of water
weighing 0.1g contains about 3 billion trillion (3,000,000,000,000,000,000,000) 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 (explained 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 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
Photo: Three states of water: 1) Solid ice in a refrigerator;
2) Liquid water splashing out onto my hand; 3) Steam (water vapor) made by a pain of bubbling, boiling water.
Although steam is really invisible, we can see it when it condenses in air to form a misty cloud
(a type of aerosol).
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
vapor) as well.
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
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.)
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.
Photo: Steam geysers are produced when water is
heated by Earth's internal heat (geothermal energy).
Picture by Carol M. Highsmith, courtesy of
Gates Frontiers Fund Wyoming Collection within the Carol M. Highsmith Archive,
Library of Congress,
Prints and Photographs Division..
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.
Photo: Ice is a life raft for polar bears, which use floating ice
to help them feed on sea creatures such as seals. Picture by Erich Regehr courtesy of US Fish & Wildlife Service.
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
video demonstration of a bursting pipe
from Steve Spangler.
Why does water take so long to heat up?
Has that kettle boiled yet? Well tell it to 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—but on the other hand, the water in your bathtub will
stay hotter, longer for exactly the same reason.
If you like swimming outdoors, high specific heat capacity explains why,
in some parts of the world, seas, lakes, and rivers aren't as warm as you
think in the early summer: huge volumes of water take a long time to heat up
after a cold winter and spring. By the same token, the water will still be warm enough for
swimming in chilly parts of Europe well into the fall (autumn) when the air temperature has
already started to plunge.
Why can insects walk on water?
Artwork: Water striders and similar insects float using long, water-repellent (hydrophobic) legs to spread their weight over a large surface area.
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
to one another—that's also why water forms droplets on windows rather
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!
Why is water...?
Artwork: Water is a polar molecule with one end (toward the oxygen atom) slightly
negatively charged and the other end (toward the hydrogen atoms) slightly positively charged.
What makes water do all these things? It's actually the way the
hydrogen and oxygen atoms are arranged inside
water molecules. They're in a sort of triangle with the two
small hydrogen atoms on one side and the much larger oxygen atom on the
other. This creates an imbalance in the electrons shared by the
molecules in the bonds that hold them together, making the oxygen end
of each water molecule slightly negatively charged and the hydrogen end
slightly positively charged. We say water molecules are polar:
like a magnet, they have two different ends or poles. The positive,
hydrogen end of one water molecule will attract to the negative, oxygen
ends of other water molecules. This is what makes water molecules stick
together in their unique way—and that, in turn, explains all their
properties, from anomalous expansion and surface tension to high
density and specific heat capacity.
Water in our world
Photo: Earth's atmosphere is full of water vapor. Computer-processed satellite photograph courtesy of
NASA on the Commons.
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
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
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
The water cycle
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.
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!
Chart: Right: How we use (and waste) water in our homes. If you had to walk for
an hour to fetch your water, would you let so much run down the drain? Drawn using data compiled by American Water Works Association/Water Research Foundation.
Next time you're flushing your toilet, washing your car, firing
the sprinkler over your garden or cleaning your windows, spare a
thought for the 2.2 billion people (29 percent of the world's
population) who still have no safe supply of clean water
and the 4.2 billion people (55 percent of the population) who don't have proper
sanitation [Source: UN figures from WHO/UNICEF, 2019]. Imagine how people in some remote village in Africa would
feel if they could see you squandering gallons of sparkling, clean,
highly treated water that you're not even going to drink.
On one level, this is an utterly ludicrous comparison: even if you save
water, it doesn't help people in Africa one iota.
But on another level, conserving water is incredibly important: as
global warming and climate change kick in, virtually all of us will find our water
resources under much greater pressure. Saving water obviously saves
water, but it also saves energy
(because cleaning water is very energy intensive), protects rivers
(because water ultimately comes from there), and helps the environment on which we all depend. If
you're billed for every unit of water you use, saving water also
helps your pocket. That's why many people are interested in
greywater: a way of collecting and recycling some of your
household water and using it for less important things like
flushing the toilet.
What is greywater?
Photo: A simplified diagram showing the basic idea of greywater: some household water is relatively clean even after it's used, so it makes sense to use it for less important jobs.
You've probably seen those medical articles suggesting you need to
drink 1–2 liters of water a day (that's roughly 2.5–5 pints or 6–8
glasses) to stay healthy, so a typical family of four would need a
maximum drinking water supply of maybe 8 liters (2 gallons). But according to the
US Environmental Protection Agency (EPA), the same family can use an astonishing 1100 liters (300 gallons) of water a day in total.
Even allowing for cooking and hand-washing, where we need to use clean water,
you can see there's a huge mismatch between how much water we
use in total and the amount we need that has to be scrupulously
clean. Greywater systems try to address this.
Greywater (sometimes spelled graywater in the United
States) is the idea of having two separate household water systems.
First, you have a normal household water supply of clean, fresh water
(sometimes called whitewater or mains water), which you use
for drinking, cooking, and so on. But you also have an extra tank
that collects the used water from your bath tub, shower,
clothes washing machine,
(and sometimes your outside roof). This is your greywater. It's used
for flushing the toilet (automatically), but you can also use it for
washing the car, watering the garden, and anything else that doesn't
need absolutely clean water. Sometimes water from the kitchen sink
(dark greywater) is reused too, but it's more contaminated and
unhygienic than water from your bath or shower. Water from the toilet
(known as blackwater) is never reused: it's discharged to the
sewer in the usual way. Trials by the UK's
Environment Agency (a similar organization to the US EPA) have found that systems like this
can save 5–36 percent of total household water consumption, though
much less (a maximum of about 20 percent) in efficient new homes.
Advantages and disadvantages of greywater systems
Photo: Greywater typically means installing an extra tank. This large tank is being put into a school; you wouldn't need a tank quite so big for a home. Photo by Felix Garza Jr. courtesy of
Greywater sounds like a brilliant idea—for all sorts of reasons.
First, it reduces the fresh water you need to consume, so it could help
to cut your water bill. If you're consuming less water, the sewage and
wastewater plants have to recycle less (using less energy) and less water has to be
removed ("abstracted") from rivers—so greywater is also good for the environment.
If you have a septic tank, switching to greywater reduces the amount of water you're passing
through the tank for treatment, extending its life.
But there are disadvantages too. First, the cost of installing a
greywater system can be significant compared to the savings in water
bills you actually make. More seriously, storing used water as
greywater allows microorganisms to breed—especially if it's warm
water to begin with—and that can present a health hazard. So
graywater has to be stored carefully with hygiene in mind, typically
either filtered before being stored, chemically disinfected, or
stored for only relatively short periods of time (greywater systems
automatically flush their tanks and refill with clean white water if
they're unused for too long) to reduce the chance
of bacterial contamination.
Alternatives to greywater
Photo: This low-maintenance garden at DOE/NREL in Golden, Colorado is a good example of how office gardens
can be redesigned to save the need for wasteful irrigation. Photo by Warren Gretz courtesy of
US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).
Purification, disinfection, and periodic draining clearly
reduces the benefit of having a second water system—so much so that,
for small households, there may be no benefit at all. You can
probably achieve greater savings more quickly and economically simply
by using fresh water more carefully: by flushing your
toilet less often (or converting to a water-saving dual-flush),
turning off the faucet (tap) while you brush your teeth, installing a low-flow
shower nozzle (one that mixes a lot of air with the water), using a water butt to collect rainfall for your garden, and so on. Water savings like this are really easy to make;
many are instant and free. One really good way to save water is to
ask your utility company to install a water meter on your property
(if you don't have one already). Seeing how much water you use each
month or quarter (and how much it costs, on your bill) really focuses
the mind on making savings—and you can see just how effective
Environmentalists tend to see things a little bit
differently. The very concept of wasting a resource as precious as
water is galling to people who truly value the planet, so some
green-minded people insist on installing greywater systems in
eco-homes as a matter of principle. Environmentalists or not, the
message is clear: in a world of growing water scarcity, all of us
have a responsibility to use this important resource more carefully.
It's worth bearing mind that in the future, saving water may not be a
matter of choice.
Find out more
Books and reports
- The New Create an Oasis with Greywater by Art Ludwig. Oasis, 2016. A practical guide to building and using your own greywater system. Written for US readers, so bear in mind planning laws and restrictions may be different if you live elsewhere.
- Introduction to Greywater Management by Peter Ridderstolpe. EcoSansRes/Stockholm Environment Institute, 2004. A useful 20-page introductory report describing greywater systems, problems of pollution and management, and how to plan the right sort of system in different situations.
- Reusing and harvesting water: Useful guidance from the UK Environment Agency, including a link to their Greywater Information Guide.
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 live nomadically, constantly moving
between places where water and food are plentiful. The first permanent
settlements grow up next to rivers and water systems that can 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) considers water the most basic building block of
matter. Aristotle, another Greek philosopher, sees water as one of the four fundamental elements (Earth,
air, fire, water).
- ~300BCE: Ancient Romans pioneer aqueducts for supplying water to their empire.
- 1582: First modern waterworks are constructed in London, England.
- 1652: First waterworks are 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) discovers deuterium and shows 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) discovers tritium in ordinary water.
- 2008: NASA discovers water on Mars.
Find out more
On this website
Facts and statistics
International organizations concerned with global water policy
Useful reports on the state of world water
- World Water Council Reports: Many useful reports and documents.
- 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.
- [PDF] 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,
- [PDF] Appraisal and Assessment of World Water Resources by
Igor Shiklomanov, Water International, Volume 25, Number 1, March 2000.
- 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.
Books and journal articles
- Gleick, Peter. The World's Water: The Biennial Report on Freshwater Resources: Volume 9. Washington DC: Island Press, 2018.
- Cosgrave, William et al. World Water Vision: Making Water Everybody's Business London: Routledge, 2014.
- Barlow, Maude. Blue Covenant: The Global Water Crisis and the Coming Battle for the Right to Water New York: The New Press, 2013.
- Postel, Sandra. The Last Oasis: Facing Water Scarcity London: Earthscan, 2013 (originally published in 1992).
- Pearce, Fred. When the Rivers Run Dry: Water—The Defining Crisis of the Twenty-first Century. Boston: Beacon Press, 2007.
- De Villiers, Marq. Water: Water: The Fate of Our
Most Precious Resource. New York: Mariner Books, 2001.
- De Villiers, Marq. Water Wars: Is the World's
Water Running Out? London: Weidenfeld, 1999.
- Postel, Sandra. Pillar of Sand: Can the Irrigation Miracle Last? New York: W.W. Norton, 1999.
- 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.
- McCully, Patrick. Silenced Rivers. London: Zed Books, 1996.
- [PDF] Postel, Sandra, Gretchen Daily, and Paul Ehrlich. "Human Appropriation of Renewable Fresh Water." Science, February 1996, Vol 271, p. 785–788.
- Two-Thirds of the World Faces Severe Water Shortages by Nicholas St. Fleur. The New York Times, February 12, 2016. Four billion people are short of water for at least one month a year, new research shows.
- Access to clean water and sanitation around the world—mapped by Katherine Purvis. The Guardian, 1 July 2015. An excellent interactive map of world water supply and sanitation problems.
- Why fresh water shortages will cause the next great global crisis by Robin McKie. The Guardian, 8 March 2015. Another look at the world's water crisis, with a great infographic.
- What does the Arab world do when its water runs out? by John Vidal. The Guardian, 20 February 2011. Dwindling water supplies will lead to growing political instability.
- Plan for China's Water Crisis Spurs Concern by Edward Wong. New York Times, 1 June 2011. Is greater abstraction from rivers the solution to a lack of water, or will it create other problems?
- World Water Day: Why business needs to worry by Peter Brabeck-Letmanthe, Chairman, Nestle. BBC News, 22 March 2010. There are good economic reasons for tackling the water crisis.
- Creating solutions to a water crisis by Alice Rawsthorn. New York Times, August 9, 2008. How can we tackle world water shortage?
- Water scarcity: A looming crisis? by Alex Kirby. BBC News, 19 October 2004. An old article, but still a good quick summary of the issues at the heart of the pressing water crisis.
- Water: A running archive of world water stories from The New York Times.
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