Phase Changes. Thermal Energy. Phase changes. A phase change is a reversible physical change when a substance changes from one state of matter to another. When ice melts to become water, it has undergone a phase change (and a physical change). The six common phase changes. Phase changes.
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Endothermic reactionsExothermic reactions
(absorb energy) (release energy)
Solid Liquid Liquid Solid
Liquid Gas Gas Liquid
Solid GasGas Solid
A phase change will always appear as a flat area on a temperature/ energy graph. A flat region shows that temperature remains constant while energy is converting matter from one state to another.
The heat of vaporization is the amount of energy required for a liquid to change into a gas (vapor). Notice how much heat is required to change water into steam due to the length of the flat line.
The heat of fusion is the amount of energy required for a solid to change into a liquid.
The amounts of energy required for phase changes to occur is different in each substance. This graph shows a diagram for water.