Water: the weirdest liquid on the planet

The more scientists examine H2O, the stranger it starts to seem. Water bends all the rules - but if it didn't, ice would sink and firefighters' hoses would be useless

Water is the only substance on Earth whose chemical formula has entered the vernacular. We all know H2O, even if we don't understand precisely what it means. But if it sounds simple, the reality is different. This common, seemingly boring substance baffles and confuses anyone who peers at it for long enough.

Water breaks all the rules. Since the 19th century, chemists have developed a robust framework to describe what liquids are and what they can do. Those ideas are almost useless at explaining the weird behaviour of water. Its strangeness underlies what happens every time you drop an ice cube into a drink. Think about it for a moment: in front of you is a solid, floating on its liquid. Solid wax doesn't float on melted wax; solid butter doesn't float on melted butter in a hot saucepan; rocks don't float on lava when it spews out of a volcano.