A substance’s state of matter is one of the distinct forms in which it can exist, determined by conditions such as temperature and pressure. The different states are distinguished primarily by the collective behavior and arrangement of their constituent particles, which can be atoms, molecules, or ions.
The Role of Energy and Particle Motion
The physical state of any substance is governed by the interplay between the kinetic energy of its particles and the forces holding them together. Kinetic energy is the energy of motion, and it is directly related to a substance’s temperature. As thermal energy is added to a substance, its particles gain kinetic energy and move faster. The particles are also subject to intermolecular forces of attraction, which attempt to keep them close together.
In a solid, the kinetic energy is low, allowing the strong attractive forces to lock the particles into fixed positions. Conversely, in a gas, the high kinetic energy overcomes these forces entirely, letting the particles move freely. The balance between kinetic energy and intermolecular forces dictates the spacing and arrangement of the particles.
This microscopic behavior then determines the macroscopic properties we observe, such as shape, volume, and compressibility. Changing the temperature or pressure shifts this balance, which can cause a substance to transition from one state to another.
The Three Fundamental States of Matter
The three most common states of matter encountered in daily life are solid, liquid, and gas.
Solid
A solid has a definite shape and a definite volume because its particles are tightly packed and only vibrate in place. For example, a block of ice or a metal coin maintains its structure regardless of the container it is placed in. Solids are not easily compressible due to the minimal space between their particles.
Liquid
A liquid maintains a definite volume but lacks a definite shape, instead conforming to the shape of its container. The particles in a liquid are still closely packed but have enough kinetic energy to slide past one another, allowing the substance to flow. Liquids are only slightly compressible because their particles remain close together.
Gas
A gas has neither a definite shape nor a definite volume, expanding completely to fill any container it occupies. Gas particles possess high kinetic energy, resulting in large distances between them and near-zero intermolecular attraction. This wide spacing means gases are easily compressible, allowing a large quantity of gas to be forced into a smaller space, such as air being compressed into a scuba tank.
Beyond the Basics: Plasma
Plasma is often referred to as the fourth state of matter, existing at temperatures far higher than those typically found on Earth. It is essentially a superheated gas that has been energized to the point where electrons are stripped from its atoms. This process, called ionization, results in a highly energetic state consisting of a mix of free electrons and positively charged ions.
Plasma is the most common state of matter in the observable universe, making up over 99% of all visible material. All stars, including our Sun, are massive spheres of plasma, and it also fills the vast space between galaxies. On Earth, plasma is found in natural phenomena like lightning and the auroras, and in manufactured items such as neon signs and fluorescent light bulbs.
Phase Transitions: How Matter Changes State
A phase transition describes the physical process where matter changes from one state to another, typically by the addition or removal of thermal energy. Adding heat increases particle kinetic energy, which causes transitions to a more energetic state; removing heat allows attractive forces to dominate, leading to a less energetic state. The six primary transitions connect the three fundamental states. During any of these transitions, the added or removed thermal energy is used to change the particle arrangement rather than raising the substance’s temperature.
The six primary phase transitions are:
- Melting (Solid to liquid)
- Freezing (Liquid to solid)
- Vaporization (Liquid to gas, including boiling and evaporation)
- Condensation (Gas to liquid)
- Sublimation (Solid directly to gas)
- Deposition (Gas directly to solid)