Water's Strange Freeze: Why Ice Floats

hook

• Ice floats on water, but most frozen solids sink in their own liquid.
• This simple fact allows fish to survive under frozen lakes.
• Water behaves against the usual rule – and that's the puzzle.

tension

• Most materials pack more tightly when they freeze, becoming denser.
• Water does the opposite: its solid form is less dense than its liquid.
• This anomaly is essential for life on Earth – but what causes it?

reveal

• Water molecules lock into a hexagonal lattice in ice.
• Hydrogen bonds hold each molecule farther apart than in liquid water.
• The open structure makes ice about 9% less dense than liquid water.

contrast

• In liquid water, molecules move and constantly break/form hydrogen bonds, allowing closer average packing.
• In ice, bonds are fixed and hold molecules at rigid, spacious positions.
• Contrast with dry ice: solid CO₂ sinks because its molecules pack without such bonds.

takeaway

• Water expands when it freezes because hydrogen bonds force an open lattice.
• This lower density of ice insulates the water below, preserving aquatic life.
• Remember: ice floats because water's molecular handshake leaves room to spare.