Why Ice Does not Sink on The Water

​Ice doesn't sink because, unlike almost every other solid substance, its solid form (ice) is less dense than its liquid form (water).

​Here is a breakdown of why this occurs:

​1. The Role of Density

• ​Floating Rule: An object floats in a fluid if its density is less than the density of the fluid it displaces.

• ​Water's Numbers:
  • ​The density of liquid water is highest at about 4^{\circ}\text{C} (approximately 1.00 \text{ g}/\text{cm}^3).
  • ​The density of ice at 0^{\circ}\text{C} is approximately 0.917 \text{ g}/\text{cm}^3.

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  • ​Conclusion: Since ice is about 9% less dense than liquid water, it floats.

  ​2. The Unique Structure of Water Molecules

  ​The reason for this density difference is the way water molecules (\text{H}_2\text{O}) arrange themselves due to hydrogen bonding.

  • ​Liquid Water (above 4^{\circ}\text{C}): Water molecules are constantly moving and sliding past each other. They form and break hydrogen bonds frequently, which allows them to pack relatively closely together, resulting in a high density.
  • ​Water Cooling (below 4^{\circ}\text{C}): As water cools from 4^{\circ}\text{C} to 0^{\circ}\text{C}, the hydrogen bonds begin to force the molecules into a more rigid, orderly, but open structure. This process is counterintuitive to what happens in most other liquids, where cooling causes molecules to pack tighter.
  • ​Solid Ice (at 0^{\circ}\text{C}): When water freezes, the hydrogen bonds lock the molecules into a fixed, three-dimensional, hexagonal crystal lattice . This crystalline structure has large, open spaces, like a cage, between the water molecules. Because the molecules are held further apart than they are in the liquid state, the solid form (ice) occupies more volume for the same mass, which means it has a lower density.

  ​3. The Importance of Floating Ice

  ​This unusual property is critical for life on Earth:

  • ​Insulation: In lakes and oceans, the ice forms a layer on the surface. This layer insulates the water below, preventing the entire body of water from freezing solid.
  • ​Aquatic Life: This allows fish and other aquatic life to survive the winter in the unfrozen water below the surface ice.