Short Notes:

When ice is placed in a pot on a stove and the heat is turned on, the pot gets warm and the ice begins to melt. But as long as there is ice in the water, the water's temperature never raises above 0° Celsius, no matter how hot the burner gets. That is because all of the heat goes into the ice to break the physical forces that bind water molecules together as ice.

The water molecules in ice are held together by weak bonds that form between a hydrogen atom on one water molecule and the oxygen atom of another water molecule. The resulting hexagonal crystal structure is fairly strongly. At 0° Celsius, the molecules move enough to weaken the bonds. Some fo the bonds break, allowing water molecules to escape the ice as liquid. This melting process is called a phase change - water changes from its solid phase to its liquid phase - and the temperature at which it occurs is called the melting point.

Breaking the bonds that hold water together as ice requires energy - so much energy that all of the burner's heat goes into severing those bonds instead of raising the ice's temperature. The heat needed to complete the phase change is called the latent heat of fusion or heat of transformation, as it does not cause the temperature to rise. Only when the last bonds break and all the ice is melted does the water temperature rise above 0° Celsius.

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How ice melts:

1) In ice, water molecules move so slowly that they bond to one another, forming a solid. When ice is heated, the molecules gain energy and move faster but are still fastened together as ice.

2) When more heat is added, eater molecules at the surface of the ice vibrate faster, breaking some of the bonds holding them in place. These molecules escape from the ice as liquid water. More heat will break the remaining bonds and melt the rest of the ice.

3) Continued heating finally gives the last of the molecules in the frozen water enough energy to escape the bonds that held them together as ice. All of the water is now liquid.

Heating ice at first raises its temperature. But at 0 degree C, a phase change begins: The ice starts to melt. As the graph line shows, adding heating melts more ice but does not raise the water temperature. Only when the ice is melted does added heat raise the water temperature.