Matter is defined as anything that has mass and takes up space. Matter most commonly exists in three physical states: solid, liquid, and gas. These distinct states are determined by the energy level of the substance’s particles and the forces acting between them. While other states of matter like plasma exist, the physical behavior of the vast majority of substances is categorized by these three primary forms.
The Solid State
A solid is characterized by maintaining both a fixed shape and a fixed volume. This rigidity results from the strong attractive forces between its constituent particles (atoms, ions, or molecules). The particles are packed tightly and often arranged in an orderly, repeating pattern, sometimes called a crystal lattice.
In this state, the particles possess the lowest kinetic energy among the three main states. They are not free to move from their positions but instead vibrate around a fixed point. This densely packed arrangement means that solids have a high density and are nearly incompressible because there is very little empty space between the particles.
The Liquid State
Liquids possess a definite volume but lack a fixed shape, conforming instead to the vessel holding them. The particles are still close together, accounting for their fixed volume and relatively high density. However, the attractive forces between them are weaker than in a solid. This reduced strength allows the particles to slide past one another, which is why liquids are considered fluids and can flow.
The unique properties of liquids, such as viscosity and surface tension, arise from this particle arrangement. Viscosity is a liquid’s resistance to flow. Surface tension occurs because molecules at the surface are pulled inward and sideways by their neighbors, creating a kind of elastic film.
The Gaseous State
The gaseous state lacks both a fixed shape and a fixed volume. Gas particles are widely separated, moving randomly and rapidly to fill the entire container they occupy. The forces of attraction between gas particles are considered negligible due to the great distances between them.
The behavior of gases is well-described by the Kinetic Molecular Theory, which posits that gas particles are in constant, chaotic motion and frequently collide with one another and the container walls. This structure results in gases having a very low density compared to liquids and solids. The most defining feature of a gas is its high compressibility. Because most of the volume of a gas is empty space, pressure can easily force the particles closer together, decreasing the volume.
How States of Matter Change
Matter can transition between these three states by the addition or removal of thermal energy. Adding energy increases the kinetic energy of the particles, allowing them to overcome the forces holding them together.
The transition from solid to liquid is called melting, and the reverse process, liquid to solid, is freezing. The change from liquid to gas is known as vaporization (including boiling and evaporation), while the change from gas back to liquid is condensation. A substance can also move directly from a solid to a gas (sublimation), or from a gas directly to a solid (deposition). During these phase transitions, the thermal energy added or removed is known as latent heat, which is used to break or form intermolecular bonds rather than to change the temperature of the substance.