Heat is the transfer of thermal energy between two systems that are at different temperatures. Temperature is a direct measurement reflecting the average kinetic energy of the atoms and molecules within the substance itself. As energy flows into a material, it increases the energy stored within the constituent particles, initiating a series of predictable physical and chemical transformations. The resulting changes depend entirely on the amount of energy input and the unique internal structure of the material being heated.
The Immediate Effects: Increased Molecular Motion
When a substance absorbs heat energy, that energy is immediately converted into increased kinetic energy of its particles. In solids, this added energy translates into more vigorous vibrational motion around fixed positions. The increased oscillation amplitude is the microscopic cause of a temperature rise.
In liquids and gases, particles are not held in a fixed lattice, so absorbed heat accelerates their translational movement through space. This increased speed leads to more frequent and forceful collisions between particles. Temperature measures this rising average kinetic energy of the system.
This boost in kinetic energy directly challenges the intermolecular forces holding the substance together. These attractive forces, such as hydrogen bonds or van der Waals forces, try to keep the particles close. As the energy of movement increases, it starts to overcome these attractive forces, setting the stage for larger changes in the substance’s physical state.
Reversible Changes: Phase Transitions and Thermal Expansion
Continued heating leads to a phase transition when particles possess enough kinetic energy to break free of attractive forces. During melting, energy breaks the rigid bonds of the solid structure, allowing particles to slide past one another to form a liquid. Further heating supplies the energy needed for vaporization, where particles gain enough speed to escape the liquid and become a gas.
During these phase changes, the temperature of the substance stops rising even though heat is still being added. This absorbed energy is known as latent heat, which is consumed entirely in breaking intermolecular bonds rather than increasing molecular speed. Sublimation is a phase transition where a solid bypasses the liquid phase entirely and turns directly into a gas. These changes are reversible because cooling the substance returns it to its original state without altering the chemical identity of the molecules.
Thermal expansion is a related physical change that occurs even before a phase transition. As the particles vibrate more intensely due to heating, the average distance between neighboring molecules increases. This spacing forces the entire material to occupy a slightly larger volume, a predictable increase in size that engineers must account for in large structures. For instance, expansion joints are built into bridges. The material’s density decreases because the same mass now occupies a greater volume, yet this change is fully reversed when the substance cools.
Irreversible Changes: Heat-Induced Chemical Reactions
Applying high heat can provide the activation energy required to break the chemical bonds within molecules, leading to the formation of entirely new substances. This type of change is defined as irreversible, meaning the original material cannot be recovered by simply cooling the products. The restructuring of atoms into new molecular arrangements marks a chemical reaction, not a physical change in state or size.
One common irreversible process is thermal decomposition, where a single compound breaks down into two or more simpler compounds when heated. For instance, heating calcium carbonate causes it to decompose into calcium oxide and carbon dioxide gas. The original molecule is permanently destroyed and replaced with new substances that have different chemical properties.
Another significant heat-induced reaction is oxidation, often seen as combustion, which requires the presence of oxygen. Burning wood is a rapid oxidation reaction that breaks down cellulose and lignin, releasing heat, light, and forming new products like ash, water vapor, and carbon dioxide. Conversely, pyrolysis is thermal decomposition that occurs in an oxygen-starved environment, breaking down complex molecules into smaller gases, liquids, and a solid residue.