H2Oh! Water is actually two liquids disguised as one.

H2Oh!

Earth's most precious liquid is weird, and if it wasn't we would die. Now experiments have uncovered its secret: it's not one liquid, it's two

“WATER is very strange,” says Anders Nilsson. He should know: he has been studying the stuff for most of his working life. His claim may be hard for the rest of us to swallow – after all, what could be more ordinary than water? Its behaviour is so familiar, its appearance so commonplace, that we are tricked into assuming that it is more or less the same as everything else. But water is uniquely weird. If it weren’t, none of us would be here to notice.

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For example, if water weren’t densest at around 4°C rather than as ice, lakes and rivers would freeze from the bottom up, slowly killing their inhabitants. If it weren’t so spectacularly good at absorbing heat, the planet would have boiled over long ago. And if its molecules, barrelling through membranes or darting down veins, weren’t so good at sweeping other chemicals along, plants and animals would die of malnutrition.

Scientists have been plumbing the depths of water’s strangeness since at least the time of Galileo, to no avail. But now, thanks to the work of Nilsson and others, we might be on the verge of understanding why it behaves the way it does. Their explanation is as strange and wonderful as the stuff itself: water isn’t one liquid, but two.

On one level, it’s no surprise that water comes in multiple forms. It exists in three phases, as a solid, liquid or gas, depending on the temperature and pressure where you find it. At sea level, water turns to steam at 100°C, but at altitude, where the atmospheric pressure is reduced, you can get …

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