Gases are characterized by their lack of fixed shape and size. Unlike solids, which maintain their form, and liquids, which maintain a fixed size, gases expand to fill the entire available space of their container. This expansive behavior is a fundamental property distinguishing them from other states of matter. An example of this behavior is seen when releasing compressed air into an open space; it instantly expands to occupy the larger volume.
Understanding this characteristic is crucial in various scientific and engineering applications. From predicting atmospheric behavior to designing pneumatic systems, the ability of a substance to expand and compress significantly impacts numerous processes. Historically, the study of this behavior has led to the formulation of important gas laws that are essential for quantifying the relationships between pressure, volume, and temperature.