Matter exists in distinct forms known as phases or states. The physical characteristics of matter are tied to the energy of its constituent particles, such as atoms and molecules. A phase is a uniform state of matter with characteristic physical properties that change only when sufficient energy is added or removed. While many exotic states exist, the three most common phases are solid, liquid, and gas, representing how matter arranges itself under typical conditions.
Solids: Fixed Shape and Volume
The solid phase is defined by its ability to maintain a fixed shape and a fixed volume, regardless of the container it occupies. This rigidity arises from the strong intermolecular forces that hold the constituent particles closely together. Within a solid, particles are packed tightly and typically arranged in fixed, orderly positions, often forming a crystal lattice structure.
The particles in a solid do not move freely; instead, they vibrate around their fixed positions. This restricted movement results in the lowest kinetic energy among the three common phases. Because the particles are tightly packed, solids are virtually incompressible. This dense packing also contributes to solids having the highest density compared to the liquid and gas phases of the same substance.
Liquids: Fixed Volume, Changing Shape
The liquid phase is an intermediate state between solids and gases, characterized by a fixed volume but a variable shape. A liquid flows to conform to the shape of its container, always filling the lowest available portion. Intermolecular forces in a liquid are weaker than those in a solid, but strong enough to keep the particles close together.
This moderate attraction allows liquid particles to move past each other, which is why liquids are classified as a fluid. Although particles are not held in fixed positions, they remain closely packed, making liquids nearly incompressible, similar to solids. Cohesive forces create surface tension, which causes the surface of a liquid to behave like an elastic membrane. Viscosity measures a liquid’s resistance to flow, with substances like honey being more viscous than water.
Gases: Changing Shape and Volume
The gas phase has neither a fixed shape nor a fixed volume, expanding indefinitely to fill any container completely. Gas particles possess the highest kinetic energy among the three common phases, moving rapidly and randomly. Intermolecular forces are negligible between gas particles, allowing them to move independently.
The vast empty space between gas particles results in expansibility and high compressibility. A gas is easily compressed because its particles can be forced closer together under pressure. The pressure a gas exerts results from the collisions the high-speed particles have with the container walls. Because gas particles are widely separated, the gas phase has the lowest density. The physical behavior of a gas depends on the relationships between its volume, pressure, and temperature.