When solid water (ice) changes into its liquid form, this process, known as melting, is a phase change driven by the input of thermal energy. This transformation alters the organization and movement of individual water molecules. Understanding this requires examining the mechanics that govern the structure of H₂O at a molecular level.
The Crystalline Structure of Ice
The solid state of water is characterized by a highly organized, rigid framework called a crystal lattice. Water molecules are held fixed in this structure through strong electrostatic attractions known as hydrogen bonds. Each oxygen atom is connected to four neighboring hydrogen atoms—two through covalent bonds and two through hydrogen bonds with other molecules.
This arrangement forms a repeating, hexagonal pattern with significant open spaces or voids. The molecules are locked into positions that force them slightly further apart than they would be in liquid water, defining the open, fixed geometry of ice.
The Role of Heat Energy in Molecular Movement
Melting begins when heat energy is continuously added to the solid ice. This energy is absorbed by the water molecules, causing them to vibrate more vigorously in their fixed positions within the lattice. This increased vibrational energy is a form of kinetic energy, which directly opposes the attractive forces of the hydrogen bonds holding the crystal together.
Once the ice reaches its melting point of 0°C, the incoming heat is no longer used to increase the overall temperature of the substance. Instead, all the energy is directed toward breaking the rigid hydrogen bonds that anchor the molecules in place. This energy used solely for the phase change is known as the latent heat of fusion. The temperature remains constant at 0°C until the transformation to liquid is complete and every molecule has been freed from the crystalline structure.
The Change in Molecular Packing and Density
Once the rigid hydrogen bonds are overcome, the highly structured lattice collapses, and the water molecules gain the freedom to move and slide past one another. The hydrogen bonds do not disappear entirely in the liquid state; instead, they become transient, constantly breaking and reforming in “flickering clusters.” This fluid, dynamic bonding allows the molecules to achieve a much less ordered arrangement.
The collapse of the open ice structure allows the water molecules to pack closer together. The molecules move into the voids that were present in the hexagonal ice lattice, resulting in a liquid that is denser than its solid counterpart. This difference in molecular packing explains why ice floats, as the solid form occupies about 9% more volume than the liquid form for the same mass. Liquid water reaches its maximum density at 4°C, after which further cooling begins to re-establish the open, structured hydrogen-bonded network, causing the density to decrease again before freezing occurs.