Why does ice floats on water?

The mass per unit volume of a substance is called density (density = mass/volume). As the volume of a substance increases, its density decreases.

Ice floats because it is less dense than water.

  • Water has a density of 1.0 gm/cubic cm.
  • The density of ice is 0.931 gm/cubic cm.

The hydrogen bonds that form when water freezes into ice allow the molecules to be spaced farther apart, making them take more space, decreasing the overall density and making it float in the water.  The water molecules in ice take up about 9% more space than liquid water, which means ice is about 9% less dense than water.


When water freezes into its solid form, the molecules can form more stable hydrogen bonds, locking them into positions. As the molecules are not moving, they cannot form as many hydrogen bonds as other water molecules. This leads to ice water molecules not being as close together as in liquid water, thus reducing its density.

Solids and liquids density comparison

Liquids have a lower density than solids.

  • Water is also a liquid, so it should also have less density than that of a solid that is ice. Though ice is solid, it has a cage-like structure; hence, there are many empty spaces between its particles.
  • These spaces are larger than compared to the spaces present between the particles of water. Thus, for a given mass of water, the volume of ice is greater than that of water.
  • Hence, the density of ice is less than that of water. A substance with a lower density than water can float on water. Therefore, ice floats on water.


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