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, thus 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.

Explanation

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

Solids and liquids density comparison

Liquids have a lower density than that of solids.

  • Water is also a liquid so it should also have less density than that of solid that is ice. Though ice is solid, it has a cage-like structure hence there are a large number of empty spaces between its particles.
  • These spaces are larger as 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 lower density than water can float on water. Therefore, ice floats on water.

 

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