Final answer:
Substances undergo phase changes, which are physical processes that involve changes in molecular movement and arrangement, depending on temperature.
Step-by-step explanation:
When substances change phase, the molecular movement within the substance changes significantly. At a low temperature, most substances are in a solid phase where the arrangement of particles is tight and movement is minimal. As the temperature rises, substances transform into a liquid phase where molecules have more freedom to move around.
Eventually, at even higher temperatures, substances enter the gaseous phase where molecules move energetically and are widely spaced apart. It is important to note that during these phase changes, the molecules themselves do not break apart; instead, their arrangement within the substance changes.
Heat transfer is the primary cause of these phase changes. During melting and vaporization, heat is added to a substance—transitioning it to a higher energy state. Conversely, the removal of heat will transition a substance to a lower energy state, seen when a gas condenses into a liquid or when a liquid freezes into a solid. This behavior can be observed and understood through pressure-temperature diagrams that reflect how different phases exist under certain conditions.