Matter can exists in the phases of matter solid, liquid, or gas. The amount of thermal energy that is released or absorbed by a substance determines its phase. Matter can switch from one phase to another, when it does, it is known as a change of state. The types of phase changes between solid, liquid and gas are: Freezing, Melting, Evaporation, Condensation, Deposition, Sublimation
Water is a great way understand the phase changes of matter. Water changes from liquid to gas (evaporation) during the process of boiling. The boiling point of water is 100 degrees celsius. Water absorbs thermal energy in order to boil.
When water releases thermal energy it can change into a solid (freeze). The freezing point of water is 0 degrees celsius. Frozen water can absorb heat energy in order to change states from solid to liquid (melting). The melting point of water is 0 degrees celsius.
This image provides a visual of how different states of matter undergo changes of state.