The science of water
by Chris Woodford. Last updated: September 24, 2013.
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.
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.
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 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.
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!