When a substance is heated, the behavior of its constituent particles (atoms or molecules) changes, leading to observable transformations. Heat is the transfer of thermal energy from a hotter system to a cooler one. This energy transfer increases the substance’s internal energy, directly affecting the motion and arrangement of its atoms and molecules. The increase in internal energy is measured macroscopically as a rise in temperature.
Increased Particle Kinetic Energy
The most immediate effect of heating is the conversion of transferred thermal energy into kinetic energy at the particle level. This energy gain causes the particles to move or vibrate with greater speed and intensity. Temperature is a measure of the average kinetic energy of these atoms or molecules; a higher temperature signifies that the particles are moving or vibrating more rapidly.
In a solid, particles are held in fixed positions by strong intermolecular forces, so increased energy manifests as more vigorous vibration around those fixed points. The atoms oscillate over a greater distance but do not move from their lattice sites. In contrast, particles in a liquid slide past one another, and added heat causes them to move faster and further apart. Gas particles, already moving freely, simply accelerate their speed and increase the frequency of their collisions.
This relationship is proportional; for an ideal gas, doubling the temperature in Kelvin doubles the average kinetic energy of the particles. The increased velocity and more energetic collisions in gases explain phenomena like the rise in pressure if the volume is held constant. Even in solids, the energy is distributed into rotational and vibrational states, which contribute to the overall kinetic energy.
Thermal Expansion
The consequence of increased particle movement is a macroscopic change in the material’s physical dimensions known as thermal expansion. Since the particles are moving more vigorously, the average distance or spacing between the atoms or molecules increases. This increased spacing causes the overall volume of the substance to expand.
Thermal expansion occurs in all states of matter, but the degree of expansion varies significantly. Solids expand the least because strong forces constrain the closely packed particles. Liquids expand more than solids because their particles are less constrained, allowing for a greater increase in distance. Gases expand the most because their intermolecular forces are weakest, providing the greatest freedom for particles to move far apart when heated.
This process is a physical change, commonly seen in expansion joints on bridges and sidewalks, which allow materials to expand safely on hot days. The density of the material decreases because the mass remains constant while the volume increases. In devices like liquid-in-glass thermometers, the expansion of the liquid inside the narrow tube is a direct demonstration of this principle.
Phase Transitions
When sufficient heat energy is added, the particles’ kinetic energy becomes high enough to overcome the attractive forces holding the material in its current structure, leading to a phase transition, or change of state. Melting is the transition from a solid to a liquid, while vaporization is the change from a liquid to a gas. During these transitions, the energy supplied is used primarily to break the intermolecular forces rather than to increase the particle speed further.
As energy is absorbed for this bond-breaking process, the temperature of the substance temporarily remains constant. This supplied thermal energy, often called latent heat, increases the potential energy stored in the material as particles move to less constrained positions. Once the bonds of the original phase have been broken and the substance has converted to the new phase, further addition of heat will again increase the average kinetic energy of the freed particles.
For example, when ice melts, the energy is used to allow the water molecules to break free from the fixed lattice structure of the solid and move into the more random arrangement of a liquid. Similarly, at the boiling point, the added heat overcomes the remaining attractive forces in the liquid, allowing the molecules to escape and move independently as a gas.