Heat is a form of energy transfer that directly governs the physical state of a substance. All matter exists in one of three common states—solid, liquid, or gas—defined by the arrangement and movement of their constituent particles. Thermal energy is a measure of the total kinetic energy contained within the atoms and molecules of a material. Adding or removing this energy alters the motion of these particles, fundamentally changing the forces that hold them together and driving a change in the state of matter.
Thermal Energy and Molecular Movement
The state a substance is in depends on the competition between the thermal energy of its particles and the attractive intermolecular forces (IMFs) between them. When a substance absorbs heat, this energy is converted into the kinetic energy of its atoms or molecules. In a solid, this added energy initially manifests as an increase in vibrational movement, causing the particles to shake more vigorously in their fixed positions.
As more heat is absorbed, the average speed and intensity of this molecular motion increases. The higher the temperature, the greater the average kinetic energy of the particles. This increased movement directly challenges the attractive forces, such as London dispersion forces or hydrogen bonds, that keep the particles close and orderly.
Eventually, the kinetic energy of the moving particles becomes sufficient to overcome the strength of the IMFs. This determines when a substance will undergo a physical change. Once the particles escape the grip of their neighbors, the substance transitions to a less-ordered state. The kinetic energy translates into greater freedom of movement, such as rotational and translational motion.
Describing the Major Phase Transitions
The process of adding or removing heat results in six primary phase transitions, each representing a shift in molecular freedom.
Heating Transitions
When thermal energy is added to a solid, melting occurs, collapsing the ordered solid structure into a liquid. In the liquid state, particles remain close but gain enough energy to slide past one another, resulting in the fluid properties of this state.
Further heating leads to boiling (vaporization), where particles acquire enough kinetic energy to completely break free from intermolecular attraction. The resulting gas particles move independently and randomly, filling any container they occupy. Sublimation is the direct transition from solid to gas, bypassing the liquid state.
Cooling Transitions
Conversely, removing heat causes particles to lose kinetic energy, allowing intermolecular forces to reassert dominance. The transition from gas to liquid is condensation, where slower gas molecules are captured by attraction into the closer proximity of the liquid state.
The final removal of heat from a liquid results in freezing, where particle movement slows enough for IMFs to lock the molecules into the fixed, repeating pattern of a solid. A gas can also transition directly into a solid state, bypassing the liquid phase, in a process known as deposition.
Latent Heat: The Energy Required for State Change
During any phase transition, the temperature of the substance stops rising even though heat is continuously being added. This absorbed energy is known as latent heat, or “hidden heat,” because it does not register as a temperature increase. The thermal energy supplied is not increasing the kinetic energy of the particles, which is what temperature measures.
Instead, latent heat is entirely consumed to increase the potential energy of the substance by actively breaking intermolecular bonds. For example, during boiling, the energy is used to pull liquid molecules apart into the gaseous state. The thermal energy input is diverted from increasing particle speed to performing the work required to overcome the strong attractive forces holding the condensed phase together.
This period of constant temperature indicates that the system exists in a mixed-phase regime where both states—such as liquid water and steam—coexist. Latent heat must be fully absorbed to complete the transition of all molecules. Once the transition is complete, the energy reverts to increasing particle speed, and the temperature of the new phase begins to rise again.