|Melting of Ice Simulation|
This simulation starts with water molecules in the shape of an ice crystal lattice below the freezing point. The lattice is oriented to show the axis along which a hexagonal symmetry can be seen. Oxygen atoms are colored red, and hydrogen atoms are colored white. As the temperature is increased, the lattice disorders, and the hexagonal symmetry disappears. Thus, the ice has melted.
Why the Ice Melts
The electron cloud in the water molecule is spread unevenly within the molecule. Specifically, oxygen has more protons in its nucleus than does hydrogen, and the result is that the electrons in the water molecule are more concentrated around oxygen than around hydrogen. (This effect is also hinted at by looking for large differences in electronegativity values of the atoms involved.) The result is a significant separation of charge, with oxygen being slightly negatively charged and hydrogen slightly positively charged. The positive hydrogen end of one molecule can interact favorably with the negative region of another water molecule, setting up a electrostatic attraction between molecules. This helps explain why the lattice forms in the first place. However, molecules are in motion due to their kinetic energy, which depends directly on temperature. When the temperature is low, most of the water molecules have insufficient kinetic energy to break up the lattice. As the temperature is increased, the water molecules eventually gain enough kinetic energy to disorder (or melt) the ice lattice.
For more information about ice and water, see, for example: