Why Does Ice Float?

Hook

Drop an ice cube into a glass of water. It floats. But ice is just water, frozen. So why does it float on water?

Puzzle

Here's why that's weird. Most things don't work this way. Take candle wax. Melt some, then drop in a solid chunk. The solid chunk sinks. It works that way for most things, even metals. That makes sense. In most solids, the pieces are packed tighter than in the liquid. More stuff in the same space means heavier for its size. So the solid sinks. But water breaks the rule. Its solid floats. If ice acted like wax, your ice cube would sink straight to the bottom. To see why, we need to zoom in. Way in.

Zoom In

Water is made of tiny pieces called molecules, far too small to see. Here's just one. One big oxygen atom, and two small hydrogen atoms, joined in a bent shape, like a boomerang. Now here's the key. The oxygen end is a little bit negative. The hydrogen ends are a little bit positive. And opposites attract. So a hydrogen on one molecule tugs on the oxygen of its neighbor. These tugs have a name. Hydrogen bonds. They're much weaker than the bonds inside a molecule. But there are lots of them, and they decide how water molecules line up.

Aha

In liquid water, the molecules are warm, so they jiggle and tumble fast. Hydrogen bonds snap and re-form all the time. Nothing stays put, so the molecules crowd in close. Now let's cool it down. As they slow down, the hydrogen bonds stop breaking. Each molecule grabs its neighbors and holds on. But hydrogen bonds only hold at certain angles. So the molecules can't squeeze together. Instead, they lock into rings, with open space in the middle. Look at all that empty space. Same box, fewer molecules. And when ice melts, the rings fall apart and the molecules crowd back in. When water freezes, it spreads out. Ten cups of water freeze into almost eleven cups of ice. So a cup of ice weighs less than a cup of water. That's why it floats. It floats low, though. About nine-tenths of the ice cube stays under the surface. Icebergs float the same way. What you see is just the tip.

Why It Matters

This matters. Spreading out is strong. A full glass bottle of water can crack in the freezer. Freezing water can even split rocks. It matters for living things too. Picture a pond in winter. Ice forms at the top and floats there, like a lid. The lid keeps the cold air off the water below, so the pond doesn't freeze solid, and fish make it through the winter. Now imagine if ice sank. Every new layer would drop to the bottom and pile up, and the pond could freeze solid. Lucky for the fish, water breaks the rule.

One-Line Recap

Ice floats because freezing spreads its molecules apart.