Why Does Ice Float?
Ice floats because water expands when it freezes, becoming less dense than liquid water. This is unusual - most substances are denser as solids. The hydrogen bonds in water form a crystalline structure with more space between molecules than liquid water. This anomaly is crucial for life, insulating lakes and oceans from freezing solid.
Key Takeaways
- Water molecules form hydrogen bonds with each other.
- Water is densest at about 4°C (39°F), not at 0°C.
- Ice can form underwater in supercooled conditions, called 'anchor ice.
Explanation
Water molecules form hydrogen bonds with each other. In liquid water, these bonds constantly break and reform, allowing molecules to move relatively close together. When water freezes, molecules lock into a fixed hexagonal crystal structure where they are held slightly farther apart.
This fixed crystal structure takes up about 9% more volume than the same mass of liquid water. Since density equals mass divided by volume, ice is about 9% less dense than water. Less dense substances float on more dense substances, so ice floats.
This property is essential for life on Earth. When lakes and oceans freeze, ice forms on top, insulating the water below and preventing bodies of water from freezing solid. Fish and aquatic life survive winter under the ice. If ice sank, water bodies would freeze from bottom up, killing most aquatic ecosystems.
Things to Know
- Water is densest at about 4°C (39°F), not at 0°C.
- Ice can form underwater in supercooled conditions, called 'anchor ice.'
- Sea ice is less salty than seawater because salt is largely excluded during freezing.