Heat represents the thermal energy contained within the vibrations and movement of atoms and molecules. Water, a common substance, possesses distinctive properties that become apparent when this energy is systematically removed. This exploration details the physical transformations water undergoes as heat is withdrawn from its liquid state.
Water’s Cooling Journey
As heat energy is gradually extracted from liquid water, its temperature begins to decrease. This reduction in temperature signifies a lessening of the kinetic energy of the water molecules. Consequently, the molecules move more slowly and with less vigor. Throughout this initial cooling phase, the water remains in its liquid form, with only its temperature indicating the ongoing removal of energy.
The Transformation to Ice
When water’s temperature reaches 0°C (32°F), a significant change occurs: it begins to transform into ice. At this specific temperature, even as heat continues to be removed, the water’s temperature does not drop further until the entire volume has solidified. This phenomenon is explained by the concept of “latent heat of fusion,” the energy released as water transitions from liquid to solid without a temperature change. Approximately 334 kilojoules of energy are released for every kilogram of water that freezes at 0°C. This energy release facilitates the arrangement of water molecules into a more ordered, crystalline structure, which is characteristic of ice.
Why Ice Floats
Unlike most substances, water expands as it freezes, making ice less dense than its liquid form. This unique property allows ice to float on liquid water. The expansion occurs because hydrogen bonds, which are present between water molecules, become more rigid and form an open, hexagonal lattice structure when water freezes. This crystalline arrangement creates empty spaces within the ice, increasing its volume compared to the more densely packed, disordered arrangement of molecules in liquid water. Ice’s lower density is important for aquatic ecosystems, allowing bodies of water to freeze from the top down, insulating the water below and enabling aquatic life to survive.
When Water Stays Liquid Below Freezing
In certain conditions, water can remain in a liquid state even when its temperature falls below its normal freezing point of 0°C. This phenomenon is known as supercooling. Supercooling typically occurs when water is very pure and lacks nucleation sites, which are impurities, rough surfaces, or air bubbles that normally initiate the formation of ice crystals. Without these sites, the water molecules do not have a template to arrange themselves into a solid structure. If supercooled water is disturbed, such as by shaking or introducing an ice crystal, rapid ice crystal formation occurs as molecules find a crystallization site.