Full Answer Section

• Water is the Exception: Water is unusual because its solid form (ice) is about 9% less dense than its liquid form at 0°C. This means that a given mass of ice occupies a larger volume than the same mass of liquid water.

• Why is Ice Less Dense? This peculiar property of water arises from its hydrogen bonds and the way water molecules arrange themselves when they freeze:

  • Hydrogen Bonding: Water molecules are polar, meaning they have a slightly positive end (hydrogen atoms) and a slightly negative end (oxygen atom). This allows them to form relatively weak attractions called hydrogen bonds with nearby water molecules.  
  • Crystal Lattice Structure: As water cools towards freezing, the hydrogen bonds become more stable and cause the water molecules to arrange themselves into a specific crystal lattice structure. This structure is a hexagonal arrangement that creates more space between the molecules compared to the more random and closely packed arrangement in liquid water.  
  • Increased Volume: This spacious, open “cage-like” structure of ice means that for the same number of water molecules (same mass), ice occupies a larger volume than liquid water. Since density is mass per unit volume, the larger volume of ice results in a lower density.  

• The Importance of Floating Ice: The fact that ice floats has crucial implications for life on Earth:

  • Aquatic Life Survival: When lakes and rivers freeze in winter, the ice layer forms on the surface, insulating the water below. This prevents the entire body of water from freezing solid, allowing fish and other aquatic life to survive the cold temperatures. If ice sank, bodies of water would freeze from the bottom up, likely killing most aquatic organisms.
  • Climate Regulation: Floating ice caps and glaciers reflect sunlight back into space, helping to regulate the Earth’s temperature.
  • Erosion: The expansion of water as it freezes (which contributes to its lower density) can cause rocks to crack and break apart, playing a role in weathering and erosion.  

In summary, ice floats on water because the unique hydrogen bonding in water causes it to form a less dense, crystalline structure when it freezes, unlike most other substances that become denser in their solid form.