Gases lack a fixed volume. Unlike solids and liquids, a gaseous substance expands to fill the entire space available to it. For instance, if a small amount of gas is released into a large container, it will distribute itself throughout the container, occupying the total volume of that space. Conversely, the same amount of gas can be compressed into a smaller volume by increasing the external pressure.
This characteristic behavior of gases is fundamental to many applications, from the operation of internal combustion engines to the understanding of atmospheric phenomena. The ability to compress gases allows for efficient storage and transportation of energy, while the expansive nature of gases dictates how they mix and diffuse. Historically, the understanding of gaseous behavior led to the development of key principles in thermodynamics and chemical kinetics.