The Core Reason: Density
Ice floats on water because, unlike most substances, its solid form (ice) is less dense than its liquid form (water). This unique property is due to the specific way water molecules arrange themselves when they freeze.
Molecular Arrangement and Hydrogen Bonds
When water cools below 4°C, its molecules begin to form a crystalline lattice structure. In this structure, strong hydrogen bonds push the water molecules farther apart than they are in liquid water. This creates more empty space between the molecules in ice, increasing its volume for the same mass, which reduces its density.
A Practical Example
A common visual example is an ice cube in a glass of water. The ice cube always remains on the surface, clearly demonstrating that it is less dense and lighter than the liquid water it displaces. This is a direct observation of the principle of buoyancy related to density differences.
Importance for Life and Climate
This phenomenon is vital for life on Earth. If ice were denser than water and sank, lakes and oceans would freeze solid from the bottom up, killing aquatic life. Instead, floating ice insulates the water below, allowing aquatic organisms to survive in cold climates and playing a significant role in regulating global temperatures.