Science · 6 pages · 12 min
Why ice floats on water
The solid is less dense than the liquid. That is the odd part.
From the opening
Ice floats because solid water is less dense than liquid water. The molecules lock into an open crystal and take up more room than they did as a liquid.
Most substances do the opposite. Their solid form is the denser one and would sink in their own melt. Water is the exception people build a world around without noticing.
The mechanism is more particular than the slogan. As liquid water cools toward freezing, the hydrogen bonds that already tug the molecules begin to hold them in a hexagonal pattern. That pattern leaves gaps. The same number of molecules occupies more volume, so a given mass of ice weighs less than the same mass of cold water.
Follow it one step further. The densest water is not the ice and not the warm surface. Fresh water is densest at about 4 degrees Celsius, above freezing. That is why a lake does not freeze from the bottom up.
The case worth keeping in mind is specific. A pond in winter freezes at the surface. The ice cover slows further heat loss, and the water underneath can stay liquid. Fish and insects are not surviving because ice is kind. They are surviving because the solid stayed on top.
That case has a boundary. Salt water is denser than fresh water and freezes at a lower temperature. Sea ice still floats, but the cold brine that is squeezed out as it freezes sinks. An ocean does not behave like a freshwater pond.
The rest of the brief — the later sections and the reading notes — opens after Stripe confirms payment.
Inside
- Why the solid takes up more room
- Why a lake freezes from the top
- What salt water does differently
- Why a sealed pipe bursts
- Where the idea stops applying
- A distinction worth keeping
Sources
- The open crystal structure of ordinary ice, ice Ih
- The density maximum of fresh water near 4°C