Density is defined as the qualitative property of a material that is calculated as mass per unit volume. Ice is the solid state of liquid H2, that is frozen water. Ice is denser than water, and does follow the basic rules of thermodynamics: a solid mass heavier than water will sink; but instead, ice floats on water. Liquids contract on cooling up to a certain point. As the temperature drops, molecules slow down and contract. Once the temperature reaches 4ºC , water begins to expand. This unique property of ice lies in its molecular structure. To understand why ice floats in water, you have to know about its chemical structure.
Molecular Structure of Water
The two atoms of hydrogen(H) and one atom of oxygen (O) are bound to each other by sharing a 'covalent bond', i.e., sharing electrons with one another. The oxygen atoms are slightly negatively charged than hydrogen atoms, as oxygen tends to pull the electrons strongly. This imparts the hydrogen atoms with a slightly positive charge. Thus, adjacent H2O molecules are attracted to each other by the negatively-charged oxygen atoms, and the slightly positively-charged hydrogen ions. This interaction between the molecules of water is known as 'hydrogen bonding'. The covalent bonds are stronger than the hydrogen bonding. The H2O molecule is said to be polar due to the slightly negative end and slightly positive end.
Floating of Ice
There density of water is 2 pounds per square inch (psi). When you add ice in the glass of water, the density of the ice cube is less than 2 psi, therefore ice floats in a glass of water. The density of alcohol is lighter than water, and has a specific gravity of less than 1. The molecular density of ice is denser than alcohol, and therefore it sinks to the bottom. Whereas, in case of water, as seen above, ice is lighter than water, and floats in the glass of water.
Ice is less dense than water and instead of contracting, it expands on cooling. Thus, the volume of ice is greater than the mass of liquid water, and hence it floats. According to the principle of thermodynamics, the density decreases as the molecules spread out from each other, and a decrease in temperature increases the density. In case of ice, the opposite occurs. The ice takes up more volume than the liquid water, and hence it is less dense than water.
Ice is 9% less dense than liquid water, and takes up 9% more volume than water. The denser water displaces ice, making it float. In winters, when all the rivers and lakes freeze, only the top layer is frozen, and the water underneath the ice is liquid. This helps in survival of all the plants and aquatic animals living in water.