Why ice floats on water
Chemistry · Key Stage 2 · The School
Most solids sink in their own liquid
Melt a lump of most solids and the liquid version takes up less space than the solid did. Drop the solid back in and it sinks, because it is more tightly packed than the liquid around it. Water is unusual.
Water expands when it freezes
When water freezes into ice, it actually takes up MORE space than it did as liquid water, not less. The same amount of water spreads out into a slightly bigger shape. That makes ice less packed than liquid water, so ice floats instead of sinking.
Seeing it for yourself
Fill a bottle completely with water, seal it and freeze it. The ice pushes the sides out, or even cracks the bottle, because it needs more room than the water did. Drop an ice cube into a glass of water and it bobs on top rather than dropping to the bottom.
Why this matters for life
Because ice floats, a frozen lake freezes from the top down. A layer of ice sits on top and insulates the water below, which often stays liquid all winter. Fish and other water animals can survive underneath. If ice sank, the whole lake would freeze solid from the bottom up. Very few living things could survive a whole lake freezing that way.