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Why does ice float on water?

Water is a strange liquid that follows its own physics! Rather than contracting when cooled, water expands when it freezes and takes up more space. As a solid, water floats on its liquid state. How does it do that?

The reason for this behavior of water is due to the "bent shape" of the water molecule. The geometrical shape of the water molecule and its surface charges allow water to form crystals when frozen. Why is water a bent shape and a polar molecule >>

Snowflake by CS Photography

When frozen, its density reduces instead of increases. In chemistry, we learn that solids are more dense than liquids because in solids, the molecules are more tightly packed together due to lower kinetic energy. However, water starts to contract when cooled until it reaches around 4ºC. After that it expands slightly until it reaches 0ºC. When water starts to become a solid, the molecules arrange themselves in a highly ordered crystalline geometric pattern. The "openness" of these geometric structures is what causes ice to float on water.  Ice is less dense than water. This amazing phenomenon is vital for sustaining life in lakes and oceans during harsh winter conditions as water freezes from the top down rather than bottom up.

Why ice floats on water, chemistry Why ice floats on water, chemistry
  • The kinetic energy in the water molecules keep the molecules apart while the intermolecular forces pull the molecules together.
  • The intermolecular forces are responsible for creating hydrogen bonds between molecules of water.
  • When temperatures go below 4ºC, the kinetic energy in the water molecules start to fall below the intermolecular forces and hence hydrogen bonds form more quickly than they break.
  • At 0ºC, water freezes.