No known form of life can exist without water.
It’s also both ingredient and byproduct of important things, like respiration and photosynthesis.
But neither of these make enough water to account for why it makes up 60% of our bodies by volume.
Our bodies need to be flooded with water all the time because without it, we wouldn’t be able to move anything to where it’s supposed to go. Water is a chemical that is specially equipped to move other chemicals through the pipelines of our blood system.
To find out why, it’s going to help if we zoom right out and look at our planet as a whole.
As science fiction author Arthur C Clarke said,
”How inappropriate to call this planet Earth, when clearly it is Ocean.
But the state of the water is also important. If the temperature of the Earth were too hot, the oceans would boil away to permanently become clouds. If it were too cold to evaporate, all water would drain into the oceans where it would be stuck, unable to circle the planet.
When biologists say the Earth is in the ‘Goldilocks zone‘ that’s the perfect temperature range for life, we often mean that it’s perfect for water.
On Earth, water can evaporate, condense, and freeze all in a single day.
It will travel vast distances as a cloud, and in the cold of the upper atmosphere it will freeze and fall as snow. Snow will melt to become the streams and rivers that migrate across continents, which carve out valleys and canyons where they run. It will collect in lakes and oceans where the sun evaporates it, returning it to the clouds in the sky.
The Earth is enraptured with the movement of water.
When it flows across the Earth’s surface it picks up minerals and sediments, mixing them and taking them to new places.
River sediment. Image source: Andre Ermolaev
This happens, in part, because water is a ‘solvent’. This means it is good at dissolving other chemicals.
This is absolutely key to understanding what makes water unique among other chemicals.
It’s an odd molecule. You probably know that water is also known as H2O, so it contains one oxygen atom and two hydrogen atoms. If you remember back to your high school chemistry classes, you might remember that they are joined in a covalent bond, which means the oxygen has a slight negative charge and the hydrogen has a slight positive charge.
But you might not know that the water molecule has a peculiar shape. The oxygen and the hydrogens don’t form a straight line, they form up in a ‘v’ shape.
The ‘v’ shape makes the oxygen stick out, which gives that end of the molecule a slightly negative charge, and the hydrogen ends a slight positive charge. If the molecule were straight, they would have cancelled each other out. But this quirky shape gives each end of the molecule a very slight charge that is super important.
The first is that water molecules are attracted to other water molecules. This is called ‘surface tension’, and it’s why leaves and bugs can stand on water like it is a solid surface. The molecules form an invisible lattice when the leaf or bug falls on top of it and keeps it afloat even though they are heavier than water.
You can see the effect of this lattice in swimming races, where the water just seems to hold on to the swimmers for a few extra moments when they emerge from a tumble turn.
The attraction between water molecules creates a lattice-like fluid that will hold together under pressure. Image source: Yasmin Tayag
The second effect of water’s charge is that it dissolves certain things really well. For instance, salt dissolves as soon as it hits water.
Salt is an ionic molecule, which means that its molecules are held together entirely by attraction caused by their charge. Water interferes with this attraction by pulling salt molecules away from each other and suspending them among its own molecules. So when you drop a pinch of salt into a glass of water, it disappears, pulled into solution with the water itself. Not all molecules that dissolve in water are ionic, but water’s charge interferes with them in a similar way.
Most water on Earth contains dissolved chemicals, and it carries them around the place like a giant ladle that mixes up the soup of materials on the Earth.
The movement of water brings our planet alive.
Egyptian civilization has been called ‘The Gift of the Nile’, in tribute to the life that springs from its silty water
Life eventually learned to use these same properties of water for its own needs. It floods its own cells and bodies with water through its bloodstream and uses it to transport proteins, DNA, polysaccharides, as well as dissolved chemicals like salt around its body, like a microcosm of the Earth itself.
The lasting effects of water on our planet are visible everywhere. It has carved valleys, fjords, and canyons in the crust of the Earth. Without water, our planet would neither have spectacular scenes like the one below, nor any life to wonder about them.
The incomparable Milford Sound, a fjord in New Zealand