Water: The Strangest Ordinary Substance

Water is so common that it seems like the most normal substance in the world. It falls from the sky, fills the oceans and flows from the tap. Yet chemists consider it one of the strangest. A molecule of its size ought to be a gas at room temperature, ice ought to sink, and lakes ought to freeze solid from the bottom up. None of that happens, and the reasons all trace back to the shape of a tiny molecule.

A bent little molecule

A water molecule is made of one oxygen atom and two hydrogen atoms, which is where the formula H2O comes from. The atoms are not in a straight line. The two hydrogens stick out from the oxygen at an angle of about 104.5 degrees, so the molecule is shaped like a wide letter V.

Oxygen pulls on shared electrons much harder than hydrogen does. The electrons spend more time near the oxygen, giving that end a slight negative charge and leaving the two hydrogen ends slightly positive. A molecule with charged ends like this is called polar.

Sticky molecules

Because opposite charges attract, the positive hydrogen end of one water molecule is drawn to the negative oxygen end of its neighbour. This attraction is called a hydrogen bond. It is about twenty times weaker than the bonds holding the atoms inside a molecule, but every water molecule can form up to four of them, and in a glass of water there are trillions upon trillions constantly forming, breaking and re-forming.

These hydrogen bonds make water sticky. They explain why water forms drops and beads on a waxed car, why some insects can stand on a pond, and why water climbs up thin tubes, which helps plants lift it from their roots to their leaves. Without them, water would boil at about minus 80 degrees Celsius, and Earth would have no liquid oceans at all.

Ice that floats

Almost every substance gets denser when it freezes, because the molecules settle closer together. Water does the opposite. As it cools towards freezing, the hydrogen bonds lock the molecules into an open, six-sided lattice with lots of empty space inside. Ice ends up about nine percent less dense than liquid water, so it floats.

This matters enormously. When a lake freezes in winter, the ice forms a lid on top that insulates the water below, and fish survive the winter underneath. If ice sank, lakes and seas in cold regions would freeze from the bottom up, and much of life in them would have been impossible. The same expansion also cracks rocks when water freezes inside them, slowly breaking mountains into soil.

The universal solvent

Water dissolves more substances than any other common liquid. Its charged ends surround and pull apart other charged particles, such as the sodium and chloride in salt, and grab onto many other molecules such as sugar. That is why the sea is salty, why your blood can carry nutrients, and why almost every chemical reaction inside a living cell happens in water.

A heat store

Water also soaks up a remarkable amount of heat before its temperature rises, because energy goes into breaking hydrogen bonds before it speeds the molecules up. That makes oceans slow to warm and slow to cool, which keeps coastal climates mild. Sweating works for the same reason: as sweat evaporates, it carries a large amount of heat away from your skin.

Why it matters

Every living thing we know needs liquid water. Its stickiness, its floating ice, its power to dissolve and its ability to store heat create the conditions that let cells work and keep the planet's temperature steady. When scientists search for life on other worlds, the first thing they look for is liquid water.