You are here: Home page > Materials > Water

Water droplets on a lily of the valley plant

The science of water

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.

Sponsored links


  1. What is water?
  2. Water, ice, and steam
  3. Why does water make pressure?
  4. Why does water expand when it freezes?
  5. Why does water take so long to heat up?
  6. Why can insects walk on water?
  7. How does water climb up a tube?
  8. Water in our world
  9. Water for life
  10. The water cycle
  11. Saving water
  12. A brief history of water
  13. Find out more

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.

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 vapor) as well.

Three states of water: solid ice and snow in a field, liquid water in a river, and steam water vapor rising from a steam engine.

Photo: Three states of water: 1) Solid snow and ice in winter; 2) Liquid water splashing over the weir of a river; 3) Steam (water vapor) condensing as it emerges from the chimney of a steam engine.

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.)

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.

Old Faithful geyser in Yellowstone National Park by Carol M. Highsmith

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..

Sponsored links

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.

Anomalous expansion of water.

Artwork: The "anomalous expansion of water": Left: In liquid water, molecules are held loosely but closely by weak and random hydrogen bonds that are constantly breaking and reforming. Right: In solid water (ice), molecules are held by stronger bonds in a more open but rigid structure. There are fewer molecules in a given volume, which is why ice is less dense than—and floats on—water.

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 a bursting pipe from Steve Spangler.

A male polar bear walks across pack 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.

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?

Illustration of a brown water strider floating on long legs.

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 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?

Water forms a concave meniscus; mercury forms a concave meniscus

Artwork: Water (left) climbs up the sides of a tube to form a downward-curving surface called a concave meniscus. Liquid mercury (right) does the opposite, forming an upward-curving (outward-bulging) convex meniscus.

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...?

Polar water molecule

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.

Sponsored links

Water in our world

SeaWiFS satellite image of Earth's climate from NASA GRIN

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 percent water.

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.

Chart showing global water stress, percentage access to handwashing, safe sanitation, and safe drinking water.

Chart: Four key world water statistics. Overall, 17 percent of the world's water is classed as "stressed" (North Africa and the Middle East are most stressed, and Canada and Iceland are among the least stressed)); 60 percent of the world's people have access to basic handwashing; 45 percent have basic sanitation; and 71 percent have safe drinking water. Source: 2017 data retrieved in 2020 from UN Water SDG6, 2020.

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!

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.

Saving water

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 billion people (26 percent of the world's population) who still have no safe supply of clean water and the 3.57 billion people (46 percent of the population) who don't have proper sanitation [Source: World Health Organization: Sanitation and World Health Organization: Drinking Water, 2022]. 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.

Pie chart showing how water is used in homes, split between toilets, clothes washers, showers, diswashers, baths, and other common uses.

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.

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?

According to the US Environmental Protection Agency (EPA), a typical 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, 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.

Diagram showing the basic concept of a greywater system that uses waste water to flush toilets.

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. In the configuration shown here, a white water tank in the roof supplies the washbasin, bath-tub, and washing machine (blue lines). They drain (black lines) into a greywater tank (center), which is used to flush the toilet (red line). Spent water from the kitchen sink and toilet go straight to the sewer as black water.

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

Large water tank being installed.

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 US Navy.

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

A low-maintenance, low-water garden at US DOE/NREL in Golden, Colorado.

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 you're being.

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


A brief history of water

Sponsored links

Don't want to read our articles? Try listening instead

If you'd rather listen to our articles than read them, please subscribe to our new podcast on Apple Podcasts, Spotify, Audible, Amazon, Podchaser, or your favorite podcast app, or listen below:

Find out more

On this website

Facts and statistics


International organizations concerned with global water policy

International agreements

Useful reports and data on the state of world water

Books and journal articles

News articles

Please do NOT copy our articles onto blogs and other websites

Articles from this website are registered at the US Copyright Office. Copying or otherwise using registered works without permission, removing this or other copyright notices, and/or infringing related rights could make you liable to severe civil or criminal penalties.

Text copyright © Chris Woodford 2008, 2022. All rights reserved. Full copyright notice and terms of use.

Follow us

Rate this page

Please rate or give feedback on this page and I will make a donation to WaterAid.

Tell your friends

If you've enjoyed this website, please kindly tell your friends about us on your favorite social sites.

Press CTRL + D to bookmark this page for later, or email the link to a friend.

Cite this page

Woodford, Chris. (2008/2022) Water. Retrieved from [Accessed (Insert date here)]

Can't find what you want? Search our site below

More to explore on our website...

Back to top