Ice is a solid, and like most solid forms of liquid substances, it is expected to be denser than its liquid form. How does one then explain the curious phenomenon of ice floating on water?
The primary reason why ice floats on water is due to its lower relative density as compared to water. This relatively lower density is a result of hydrogen bonds present within the structure of both water and ice.
In water, each oxygen can form a total of four bonds-two of which are intramolecular covalent bonds between the two hydrogen and oxygen atom, forming the basic structure of water; the other two being the two intermolecular hydrogen bonds between the two sets of lone pairs on oxygen and hydrogen atoms on other water molecules. This gives rise to a highly ordered, rigid lattice of water molecules held together by strong hydrogen bonds.
When water freezes, individual molecules lose heat energy, and hence lose their kinetic energy to vibrate along their fixed positions as well. The effect of the intermolecular hydrogen bonds hence becomes more dominant, as the individual water molecules do not have enough energy to break free of these hydrogen bonds binding the water molecules together. The highly ordered structure of water hence becomes increasingly rigid, with water molecules being fixed together in their positions due to the hydrogen bonds, preventing the individual water molecules from moving too close to one another. This creates a very "open" structure, in which air spaces are created between the water molecules due to the rigid hydrogen bonds between water molecules. These air spaces become much bigger and much more significant as the structure of water grows increasingly rigid with dropping temperature. As water freezes into ice at 0 ºC, the air spaces between the water molecules take up significant space, resulting in a lower amount of water molecules per unit volume of ice, explaining ice's lower density (0.917 g/dm3) as compared to water (0.9998 g/dm3).
Conversely, when ice melts, the increasing heat energy supplied to the water molecules causes the molecules to gain in translational kinetic energy, allowing them to vibrate in their fixed positions along the lattice. Within the temperature ranges of 0ºC to 4ºC, some of these molecules gain enough energy to break free from the hydrogen bonds, causing them to "fall" into the air spaces within the molecules. The volume of air spaces become less and less significant, resulting in an increasing density of ice as it melts. However, it is important to note that this trend of increasing density is only predominant during the temperatures of 0ºC to 4ºC. After 4ºC, thermoexpansion predominates, where increasing temperature causes individual water molecules to gain enough energy to break free of intermolecular forces of attraction completely, causing them to move further from one another with increasing temperature. This causes decreasing density with increasing temperature. The same processes apply when water is cooled; thermocontraction occurs, resulting in a higher density of water due to decreasing temperature up to a temperature of 4ºC, between 4ºC to 0ºC, water molecules begin fixing themselves in rigid positions as ice, resulting in the abovementioned decrease in density. When water is completely frozen as ice, and cooled to below 0ºC, thermocontraction again takes precedence, with the density of ice increasing with a lower temperature. (e.g. Ice at 0ºC has a density of 0.917g/dm3, whereas Ice IX, or ice cooled to -100ºC, has a density of 1.16 g/dm3)
It is important to note that the processes of thermocontraction and thermoexpansion occur throughout the cooling and heating processes of water, where temperature change is directly proportional to change in water density. It is only between the temperature ranges of 0ºC to 4ºC where the rigid ordered structure of the hydrogen bonds in water molecules predominates, resulting in an inversely proportional relationship between temperature and density at those temperatures.
The lower density of ice (at 0ºC) with respect to water hence explains why ice floats on water.
