When atoms absorb energy in the form of heat, the fundamental change that occurs is an increase in their motion. Heat is the transfer of thermal energy, and when absorbed by a substance, it is converted directly into kinetic energy, or the energy of motion. The temperature of a substance is a direct measure of the average kinetic energy of all its constituent particles.
The Immediate Effect: Increased Kinetic Energy
Heat absorption translates directly into an increase in the speed and vigor of particle movement. This relationship means that a higher temperature corresponds precisely to a greater average kinetic energy among the atoms and molecules. The exact nature of this motion depends entirely on the initial physical state of the substance.
In a solid, atoms or molecules are locked into a fixed, repeating, three-dimensional arrangement. They are not free to move throughout the structure but instead vibrate around a fixed equilibrium point. When energy is added, this causes a significant increase in the amplitude of this vibrational movement.
The situation changes for a liquid, where particles are still held close together but possess enough energy to slide past one another. Absorbed heat energy in a liquid causes an increase in both the speed of their vibrational movement and their translational movement. These liquid particles begin to move around and past their neighbors with a much greater overall velocity and frequency.
Gases represent the state of matter with the highest average kinetic energy at a given temperature. In this state, the particles are widely separated and move rapidly and randomly throughout the available volume. Heating a gas primarily translates to a direct and substantial increase in the speed of this chaotic, rapid translational motion.
Macroscopic Result: Thermal Expansion
The increase in microscopic motion from heating has a direct, observable consequence on the macroscopic world. As atoms or molecules vibrate or move more vigorously, the average distance maintained between them slightly increases. Even though the particles in a solid are bound to fixed positions, their larger vibrational amplitude pushes them marginally further apart from their neighbors.
This collective increase in particle separation results in the phenomenon known as thermal expansion, where the material physically expands in volume or length. Thermal expansion is a factor that must be carefully accounted for in civil engineering and construction. Large concrete and steel structures, such as highway segments and bridges, utilize built-in expansion joints.
These flexible assemblies are designed to absorb the change in length that occurs between the hottest and coldest operating temperatures. Without these joints, the intense internal stress from the expanding material could lead to physical deformation, such as cracking or buckling. Railway tracks must also accommodate this expansion to prevent the rails from warping under extreme summer heat, a condition sometimes called “sun kink.”
Breaking Bonds: Phase Transitions
If the transfer of heat energy continues, the particles eventually reach a point where they can overcome the attractive forces holding the material together. These attractive forces, known as intermolecular forces, are responsible for maintaining the substance in its condensed state. The first point where this energy overcomes attraction is the melting point, where a solid transitions into a liquid.
At this specific temperature, the vibrational kinetic energy of the particles becomes sufficient to break the rigid, organized lattice structure of the solid. The particles gain enough freedom to move past one another, allowing the substance to flow and conform to the shape of its container. Substances with stronger intermolecular attractions require more energy and thus have higher melting points.
If heating is sustained past the liquid state, the particles continue to absorb more translational energy. This leads to the second major transition, the boiling point, where the liquid turns into a gas. At this temperature, the particles acquire enough kinetic energy to completely overcome the remaining intermolecular attractions.
The particles escape the cooperative, attractive environment of the liquid and fly apart as independent gaseous particles. This results in a substance that no longer maintains a fixed volume but rather expands indefinitely to fill its entire container.
Extreme Heat: Ionization and Plasma
When heating is continued to extreme temperatures, far exceeding what is required for vaporization, the energy transferred becomes intense enough to change the internal structure of the atoms themselves. At these temperatures, the collisions between atoms and molecules become so forceful that they begin to strip electrons away from the atomic nuclei.
This process is called ionization, and it creates a highly energetic mixture of positively charged ions and negatively charged, free-floating electrons. The resulting state of matter is known as plasma, which is often designated as the fourth state of matter. Unlike a neutral gas, plasma is an electrically conductive medium because of the presence of these separated charged particles.
Though plasma is relatively uncommon under natural conditions on Earth, it is considered the most abundant state of matter in the universe. The sun and all other stars are massive spheres composed almost entirely of plasma. This superheated matter can exist at temperatures of hundreds of millions of degrees, such as those generated in fusion reactors.