Ice
Ice floats because frozen water takes more space than liquid water, so it is less dense. Density means how heavy something is for its size. A chunk of ice weighs less than the same-size chunk of water, so it sits on top.
On this page: why ice floats, why water expands when it freezes, why lakes freeze from the top, and why ice is slippery.
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Ice
Most liquids shrink a little as they get colder. Water does something special near freezing. The molecules lock into an open crystal. That cage has more empty space than liquid water. Ice is about 9 percent roomier, so it is lighter for its size. Lighter things float on heavier liquids. That is why an ice cube rises in a glass, and why icebergs ride high in the sea. If ice sank, winter lakes would freeze from the bottom up. Fish would have a much harder winter.
~9%
Extra space as ice
0.92×
Density vs liquid water
32 °F
Freshwater freeze
Top first
How lakes freeze
Each water molecule is a little V: two hydrogens and one oxygen. In ice, those molecules hold hands in a hexagonal pattern. The pattern is pretty and wasteful. It does not pack as tightly as liquid water. So the same number of molecules needs more room. That is why a full bottle can crack in the freezer. The ice needs extra space and it takes it.
Ice forms at the surface and stays there because it floats. That lid slows the heat leaving the water below. Deep water can stay liquid even when the air is far below freezing. Fish, frogs, and plants get a cold but wet winter. A lake can grow a thick ice sheet and still have water underneath. The float trick is the reason.
Ice is slippery because a thin wet layer forms on top, so a shoe or skate slides on water more than on solid ice. Pressure from a skate, a little melt from friction, and the way ice molecules sit at the surface all help make that film. Super-cold ice is less slippery. Ice near melting is the classic rink feel.
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