By John Papiewski | Updated Mar 24, 2022
Every material on Earth undergoes predictable phase changes as temperature and pressure vary. At the core of these transitions is the balance between thermal energy and intermolecular forces. When heat supplies enough energy to overcome these forces, a substance moves from solid to liquid to gas, each state exhibiting distinct physical properties.
In scientific terms, solids, liquids, and gases are called the three primary phases of matter. A phase transition—such as melting, freezing, boiling, or condensation—occurs when a material shifts from one phase to another. Each substance has its own characteristic melting and boiling points, which depend on its molecular structure and the surrounding pressure. For instance, carbon dioxide vapor becomes dry ice (solid CO₂) at –109 °F under standard atmospheric pressure, and only forms a liquid under high pressure conditions.
When a solid is heated, its temperature rises until it reaches the melting point. At this temperature, additional heat does not increase temperature but is instead used as the heat of fusion to break the lattice of the solid and convert it into liquid. The temperature remains constant until the entire sample has melted. A similar process occurs during boiling: the heat of vaporization is required to transform liquid into gas, keeping temperature steady until the phase change completes.
Melting is governed by the strength of intermolecular forces—such as London dispersion forces and hydrogen bonds—that hold molecules together in a crystal lattice. Materials with weaker forces have lower melting points; those with stronger forces require higher temperatures to disrupt the lattice. When sufficient thermal energy is supplied, all molecules overcome these forces and transition to the liquid phase.
Boiling is the liquid‑to‑gas transition that occurs when a liquid’s vapor pressure equals the surrounding atmospheric pressure. At this point, molecules gain enough kinetic energy to escape the liquid surface, forming vapor bubbles throughout the liquid. As temperature rises, more molecules reach the energy threshold, increasing the rate of vapor formation until the liquid is entirely vaporized.
