Question

Does frozen water sink?

Answer 1

We know that the solid form of a matter is denser than its liquid form. The case with ice is an exception. Liquid water is slightly denser than ice. Density is defined as the mass per unit volume of a substance. As the mass of a substance increases, its density increases and as the volume of a substance increases, its density decreases.

Complete answer:
When a liquid is cooled, molecules are brought together which need to be accommodated in a smaller area. This increases the density of the substance and thereby most of the solids have greater density than liquids. However, when water is cooled and ice is formed, a large number of empty spaces are left between the particles of ice. This is because when the water is frozen, the molecules are not able to form as many hydrogen bonds as compared to liquid water.
Thus, the molecules of ice are not as close compared to those of liquid water. Ice formed has a cage-like structure. The spaces formed are larger as compared to the spaces present between the particles of water. What differs in ice is that the structure of water molecules in liquid form and water molecules in solid form differs. There is a regular pattern in the arrangement of water molecules in the solid phase (ice) while there is no definite pattern of molecules in the liquid phase. As the solid phase molecules attain a regular structure, the molecules spread out which in turn decreases the density of the ice. Hence, the density of ice is less than that of liquid water and thus ice floats on water. Generally we expect the solid to be heavier than the liquid form. It is not so with frozen water or ice. The ice floats on water because the density of solid ice is less than water.

Note:
Remember that the instrument to measure the density of a liquid is called a hydrometer. In liquid water, each molecule of hydrogen is bonded to other water molecules. In ice, each molecule of hydrogen is bonded to other water molecules.