Matter exists in distinct forms, often called phases, determined by the arrangement and energy of their constituent particles. These states are defined by whether they possess a fixed shape and a fixed volume. Among the most common states of matter, the one that exhibits neither a definite volume nor a definite shape is the gas phase. Gases naturally expand or contract to completely match the size and form of whatever container holds them.
The State of Matter with Indefinite Volume and Shape
The defining feature of a gas is its lack of structural integrity, meaning it always conforms to the boundaries of its surroundings. Unlike solids, which maintain their form, or liquids, which retain a fixed volume, a gas is characterized by its indefinite volume and shape. This behavior allows gases to be highly compressible, a trait resulting from the vast amount of empty space between the particles. Under pressure, the volume of a gas can be reduced because its molecules are pushed closer together.
Gases are classified as fluids, meaning they possess the ability to flow, similar to liquids. This fluidity allows a gas to spread out uniformly and mix completely with other gases, a process known as diffusion. The density of a gas is much lower than that of liquids or solids because the same mass occupies a much larger volume. For example, a substance’s volume can increase by a factor of 1,000 or more when it transitions from a liquid to a gas.
The Mechanism Behind Gas Behavior
The physical principles underlying the behavior of gases are described by the Kinetic Molecular Theory (KMT). This model posits that gas particles are in a state of constant, rapid, and random motion. The particles travel in straight lines until they collide with other particles or the walls of the container, which generates the gas’s pressure. This movement is directly related to the temperature of the gas, as higher temperatures correspond to greater average kinetic energy and faster particle movement.
A primary assumption of the KMT is that the volume occupied by individual gas particles is negligible compared to the total volume of the container. Because the particles are widely separated, the attractive or repulsive forces between them are considered minimal for an ideal gas. This distance between molecules is the reason why gases are easily compressed and do not hold a fixed volume. Since the particles move independently and have negligible interaction, a gas naturally expands until its molecules fill the entire enclosure.
Comparing the Four States of Matter
The behavior of gas stands in contrast to the other common states of matter: solid, liquid, and plasma. A solid is characterized by both a definite shape and a definite volume because its constituent particles are tightly packed and held in fixed positions, allowing only for vibration. This fixed structure makes solids resistant to changes in shape or volume.
A liquid presents an intermediate state, possessing a definite volume but an indefinite shape. Liquid particles remain close to one another, which fixes the volume, but they have enough energy to move past each other. This allows the liquid to flow and take on the shape of its container.
The fourth common state, plasma, also exhibits both an indefinite volume and an indefinite shape, similar to a gas. Plasma is distinct because it is an ionized gas, meaning a portion of its atoms have had electrons stripped away. This results in a collection of electrically charged particles—ions and free electrons—that cause the plasma to be electrically conductive and responsive to magnetic fields. Plasma is the most common state of matter in the visible universe, making up stars and lightning, while gas is the most common state in Earth’s atmosphere.