Look a little closer
Ice and liquid water are made from the same H₂O molecules, yet an ice cube floats instead of sinking. The difference is not the ingredients. It is the roomier arrangement those same molecules adopt when water becomes a solid.
The key property is density, which describes how much mass is packed into a given volume. Ice is less dense than liquid water, so the upward buoyant force from the water can support it at the surface.
As water freezes, hydrogen bonds organize its molecules into a relatively open crystal structure. In the liquid, moving molecules can occupy some of that open space. In ordinary ice, they settle into a framework that keeps them farther apart on average, so the same amount of water takes up more room.
Freezing expands water by roughly 9 percent and lowers its density accordingly. That is why only about a tenth of a piece of ordinary ice floating in fresh water appears above the surface while most of it remains submerged. The small visible top of an iceberg follows the same basic rule.
This unusual property also matters to life in a winter lake. Ice forms at the surface and stays there, covering liquid water below. Fresh water is densest at about 4°C, so deeper water can remain liquid, while the surface ice provides some insulation from the colder air above.
Ice does not float because a lighter substance has been added. It floats because freezing spreads the same water molecules through a larger structure. A simple cube in a glass reveals a molecular arrangement that also helps shape winter conditions in lakes and oceans.
