In class we learned that solids are denser than liquids, so they sink. But ice is the solid form of water and it floats. Our teacher said it is about the structure of the molecules but I did not understand the part about the hexagonal shape. Can someone explain it simply?
Ice is less dense than liquid water — and density is what decides whether something floats.
When water freezes, each molecule locks into an open hexagonal lattice held together by hydrogen bonds. That lattice has gaps in it, so the same mass of water takes up about 9% more volume as ice. More volume for the same mass means lower density, so ice floats with roughly one tenth of it above the surface.
This is unusual: almost every other substance is denser as a solid. It is also why lakes freeze from the top down, which is what keeps fish alive through winter.
Parent
Short version: hydrogen bonds. In liquid water the molecules slide over each other and can pack closely. When it freezes they hold each other at a fixed angle, and that fixed angle leaves empty space in the middle of every ring of six molecules.
Content
A way to remember it for the exam: density = mass ÷ volume. Freezing does not change the mass, it increases the volume, so the density must go down. Any question that asks why ice floats is really asking you to compare densities.
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