Ice is water frozen solid. It can be very cold — much colder than its freezing point of 32 degrees Fahrenheit (0 degrees Celsius). Ice can be cooled to a temperature even hundreds of degrees below zero, if sufficient energy is removed. When the process is reversed and heat is gradually added, the opposite happens and not much occurs — until the freezing point is reached.
Nonsolid Forms of Water
Water molecules in the gaseous state (steam) are very loose and briefly interact with one another. They have nearly total freedom to move about. Liquid water molecules are much closer together. They are not so independent. Even though there are no rigid bonds between molecules in liquid water, there are bonds, all the same. They form and are broken. The molecules move some, and they form new bonds to other water molecules. Those, too, are eventually broken. This gives water its fluidity.
Some solids that form from liquids are termed “amorphous” and possess little definable structure. The molecules of many liquids, however, align themselves as they form solids into specific, crystalline lattice structures. They possess an ordered pattern. There are a limited number of such lattice structures. Ordinary table salt assumes a cubic crystal lattice. Water adopts a hexagonal crystal structure. Alignment leading to the formation of crystals reduces the internal or potential energy of the substance. The released energy takes the form of heat.
Slowly Melting Ice
As ice is warmed gradually, the crystal lattice absorbs the heat, converting some of the ice into liquid water. The temperature of the ice itself remains the same. If a container of ice is heated very slowly, a graph of the temperature inside the container versus time will rise steadily until it reaches the melting point. At that point the temperature will not rise as long as some ice remains. When the ice is gone, the temperature will rise again.
If instead of slow heating the heat is applied rapidly, as the ice begins melting, the temperature of the water formed will rise beyond the melting point, whereas the temperature of the ice will remain at the melting point. Such an event happens everyday when ice is added to tap water to make ice water. The ice does not instantly disappear. It gradually melts, but as long as there is ice in the glass, its temperature remains at the melting point. Energy from the tepid water provides the energy needed for water molecules at the edge of the ice crystal to escape their hexagonal lattice.