Matter exists in various forms called states or phases. The three most common states are solid, liquid, and gas, defined by the arrangement and movement of their constituent particles. A phase change, or phase transition, is a physical process where matter shifts from one state to another. This represents a change in the organization of particles, not a change in their chemical identity.
The Underlying Mechanism: How Energy Drives Phase Change
The ability of matter to change its state is governed by the balance between two opposing forces: the kinetic energy of the particles and the intermolecular forces holding them together. Kinetic energy is the energy of motion, and its average magnitude is directly proportional to the substance’s thermal energy, or temperature. As thermal energy is added, the particles—atoms or molecules—vibrate and move more rapidly.
Intermolecular forces are the attractive forces that exist between neighboring particles, which work to keep the substance in a condensed, ordered state. In a solid, these forces are strong enough to lock particles into fixed positions, allowing only vibration. In a liquid, the particles have enough kinetic energy to partially overcome these attractive forces, allowing them to move past one another while still remaining close.
A phase change occurs when the kinetic energy of the particles becomes sufficient to either partially or completely overcome the intermolecular forces. For example, a substance transitions from liquid to gas when the particles acquire enough kinetic energy to fully break free from the attractive forces and disperse widely. Conversely, removing thermal energy causes particles to slow down, allowing the attractive forces to pull them into a more ordered, condensed state.
The Six Defined Transitions Between Solid, Liquid, and Gas
The transformations between the three primary states of matter have specific names depending on the direction of the change.
- Melting (Fusion): Solid absorbs energy and turns into a liquid.
- Freezing (Solidification): Liquid loses energy and turns into a solid.
- Vaporization: Liquid gains energy and turns into a gas (includes boiling and evaporation).
- Condensation: Gas loses energy and reverts to a liquid.
- Sublimation: Solid absorbs energy and transforms directly into a gas (e.g., dry ice).
- Deposition: Gas loses energy and converts directly into a solid (e.g., frost formation).
Pressure, Plasma, and Other Extreme Conditions
While changes in thermal energy are the most common cause of phase change, pressure also plays a role in determining a substance’s state. Increasing the pressure on a substance forces its particles closer together, which often favors the more compact, solid phase. For example, under high pressure, a gas can be compressed into a liquid even without a drop in temperature. Pressure changes can also modify the temperature at which a phase change occurs; the boiling point of water decreases at high altitudes due to lower atmospheric pressure.
Beyond the three typical states, a fourth state of matter exists called plasma. Plasma is formed when a gas is heated to high temperatures or subjected to energy, causing the atoms to become ionized and strip away their electrons. This results in a mix of free electrons and positively charged ions. Plasma is the most common state of matter in the universe, found in stars and lightning.