The Mechanism
When water cools below 4 degreesC, its molecules arrange themselves into an open lattice - a hexagonal crystal pattern. That pattern takes up more space than liquid water, even though it has the same number of molecules. More space + same molecules = lower density. Things that are less dense than water float on top of it. Ice is less dense than water. So ice floats.
Why It Matters
Almost every other substance gets denser when it freezes. Water is one of the very few exceptions. If water behaved like most substances, ice would sink - and lakes and ponds could freeze solid from the bottom up. Fish and plants under the surface wouldn't survive winter. The whole shape of life on Earth depends on this one quirky property.
Wait — That's Not Quite Right
Some people think ice floats because it's cold, or because it has air bubbles in it. Neither is true. Ice would still float in warm water, and even bubble-free ice (the kind that looks perfectly clear) floats just fine. The floating comes from the crystal structure, not the temperature or trapped air.
Vocabulary
- density
- molecule
- freezing
- expansion
- hexagonal
- crystal
Quick Quiz
5 questions · For classroom or kitchen table
The Experiment
The Ice Cube Float Test
Fill a clear glass with water and drop an ice cube in. Notice it floats with most of itself underwater and just a small part above. About 9/10 of an iceberg is underwater for the same reason. Then try this: fill a glass with vegetable oil and freeze a separate ice cube made from cooking oil (or just observe a candle). Solid oil/wax SINKS in its liquid form. Most things do. Ice is the oddball.
A clear glass, water, an ice cube. (Optional: vegetable oil and a candle for comparison.)
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