A phase change, also known as a change of state, describes the physical process where matter transitions between its fundamental forms: solid, liquid, and gas. These transformations are purely physical, meaning the substance’s chemical identity remains unchanged. The shift is driven entirely by the gain or loss of energy, which affects the motion and organization of the constituent particles. When matter absorbs or releases thermal energy, its particles either speed up and break free from their structure or slow down and settle into a more ordered arrangement.
Defining the Six Transformations of Matter
The six phase changes cover every possible transition between the three common states of matter: solid, liquid, and gas. They are organized into three pairs of reciprocal processes, where one change is the exact reverse of the other.
Melting is the transformation of a substance from a solid to a liquid, such as an ice cube turning into water. The reverse process, where a liquid turns back into a solid, is called Freezing.
The next pair involves transitions between the liquid and gas states. Vaporization is the process where a liquid turns into a gas, including both boiling and evaporation. Conversely, Condensation occurs when a gas transitions back into a liquid.
The final pair of changes bypasses the liquid state entirely, involving direct transitions between solid and gas. Sublimation is the change from a solid directly to a gas. Dry ice, which is solid carbon dioxide, demonstrates this by turning directly into gas. The reverse process, where a gas changes directly into a solid, is called Deposition.
Phase Changes That Require Energy Input
When a substance moves from a more ordered state to a less ordered state, it must absorb thermal energy from its surroundings. These processes are known as endothermic changes because heat flows into the substance. The absorbed energy increases the kinetic energy of the molecules, allowing them to overcome the attractive forces holding them in a rigid structure.
Melting (Solid to Liquid)
Melting (solid to liquid) is an endothermic process. For a solid to transform into a liquid, the absorbed heat disrupts the highly ordered, fixed structure. The molecules gain enough energy to move past one another, resulting in the fluid nature of a liquid.
Vaporization (Liquid to Gas)
Vaporization (liquid to gas) also requires an input of energy. The heat absorbed allows the liquid molecules to gain enough kinetic energy to break free from the intermolecular forces binding them in the liquid phase. This transition results in the widely separated, rapidly moving particles characteristic of a gas.
Sublimation (Solid to Gas)
The third endothermic process is Sublimation, the direct change from solid to gas. This transition requires the largest energy input because the substance must absorb enough heat to bypass the liquid state and immediately break all strong intermolecular bonds. The absorbed energy must be sufficient to launch the tightly packed solid particles into the highly energetic, disordered gaseous state.
Phase Changes That Release Energy
In contrast to the changes that absorb heat, three phase changes are exothermic, meaning they release thermal energy into the environment. These transitions occur when a substance moves from a less ordered state to a more ordered state. As energy is released, the molecules slow down, allowing attractive forces to pull the particles into a more stable, structured formation.
Freezing (Liquid to Solid)
Freezing (liquid to solid) is an exothermic process. When a liquid freezes, the molecules must shed energy for their kinetic movement to decrease. This energy loss allows the intermolecular forces to organize the particles into the fixed, crystalline lattice structure of a solid.
Condensation (Gas to Liquid)
Condensation (gas to liquid) also releases energy. As gas particles cool down, they release the heat that kept them far apart and moving rapidly. This energy release causes the molecules to slow sufficiently for attractive forces to draw them closer together, forming the denser, clustered arrangement of a liquid.
Deposition (Gas to Solid)
Deposition (gas to solid) is the third exothermic change. For a gas to skip the liquid phase and become a solid, it must rapidly release a large amount of energy. This swift loss of energy causes the fast-moving gas particles to settle quickly into the highly ordered structure of a solid.