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ELI5 Yahaaa

Water breaks the rules

Why Does Ice Float?

Almost everything shrinks when it freezes. Water expands — and that is lucky for us.

Almost every liquid Freezes smaller anddenser — the solidsinksWater Freezes bigger andlighter — the solidfloats
Water is the strange one, and life depends on it.

  1. Step 1 of 5

    Cold usually means smaller.

    1 Warm — spreadout2 Cooling —closer3 Frozen —tightly packed

    Cooling slows molecules down so they huddle closer. Freeze molten metal or wax and the solid is denser than the liquid, so it sinks.

  2. Step 2 of 5

    Water molecules are fussy about angles.

    Water Molecules with chargedends that link at setangles

    Each water molecule has a slight positive end and a slight negative end. They grab each other, but only at particular angles.

  3. Step 3 of 5

    Freezing locks them into an open lattice.

    Liquid water Molecules slide andpack in tightIce Locked in a latticewith empty space inside

    As water cools below 4°C, those angled links win out and force the molecules into a rigid hexagonal frame with gaps in it. The gaps make ice roomier.

  4. Step 4 of 5

    Roomier means lighter for its size.

    Water1.00 g per cm³Ice0.92 — floatsIce above the surfaceabout a tenthshows

    Ice takes up about 9% more space than the water it came from. Less weight per litre than water means it floats — with about a tenth showing above the surface.

  5. Step 5 of 5

    This is why lakes do not freeze solid.

    Ice — floats and insulates0°CCold water1–3°CWater at 4°C — densest,sinks to the bottomfish survivehere

    Ice forms on top and then insulates the water below. If ice sank, lakes would freeze from the bottom up every winter and kill everything in them.

    The same 9% expansion is what bursts water pipes and cracks roads in winter.

The short version

Ice floats because freezing locks water into an open, gappy lattice that takes up more room.

Try it yourself

Fill a plastic bottle to the brim, cap it and freeze it. The bulge is the extra space ice needs.

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