Heat is a form of energy related to the movement of tiny particles that make up all matter, known as molecules. When a substance absorbs heat, these molecules indeed move more rapidly.
Understanding Molecular Kinetic Energy
Temperature is a measure of the average kinetic energy of the particles within a substance. Kinetic energy is the energy an object possesses due to its motion. When thermal energy is added to a substance, this energy is converted into kinetic energy, causing the molecules to move faster. This increased motion can involve more vigorous vibrations in solids, or faster translational and rotational movements in liquids and gases.
The particles within any substance are in constant, chaotic motion, even at very low temperatures, except at absolute zero where molecular motion theoretically ceases. As temperature rises, the average speed of these atoms and molecules increases, directly corresponding to greater average molecular kinetic energy.
How Increased Motion Changes Matter
Increased molecular motion has observable effects on the state of matter. In solids, molecules are tightly packed and primarily vibrate in fixed positions. As heat is added, their vibrations become more energetic, increasing the average distance between them and leading to thermal expansion.
As molecules gain sufficient kinetic energy, they can overcome the forces holding them in a rigid structure, transitioning from a solid to a liquid (melting). In a liquid, molecules have more freedom to move past each other. Further heating provides enough energy for molecules to break free entirely from intermolecular attractions, changing from a liquid to a gas (boiling or evaporation). In the gaseous state, molecules move rapidly and randomly.
Another consequence of increased molecular motion is diffusion, the movement of particles from higher to lower concentration. The rate of diffusion increases with temperature because molecules move faster and collide more frequently, leading to a quicker spread throughout a given space.
Everyday Manifestations
Many common phenomena illustrate the principles of heat and molecular motion. When water boils, the added heat causes water molecules to move so vigorously that they escape as steam. Similarly, ice melts when its molecules absorb enough heat to become liquid water.
Thermal expansion is evident in materials like bridges and railway tracks, which expand in warm weather and contract in cold. Engineers design these structures with expansion joints to accommodate these changes in size, preventing damage. The spreading scent of perfume or air freshener throughout a room also exemplifies diffusion. The fragrance molecules move faster and disperse more rapidly in a warm room compared to a cooler one.