A gas does not possess a definite shape or a definite volume. It is defined as a state of matter where the substance completely fills the container it occupies, expanding or compressing to match its boundaries. This characteristic distinguishes gases from solids and liquids. The lack of fixed form and size is a direct result of how its constituent particles behave at the molecular level.
The Indefinite Nature of Gas
A gas always assumes the exact shape of its container. For example, if a gas is introduced into a spherical container, it becomes spherical, and if moved to a rectangular box, it becomes rectangular. This inability to maintain a fixed shape means gases are classified as fluids, similar to liquids, because their particles can flow freely.
Gases also possess an indefinite volume, meaning their volume can be easily changed. This is demonstrated by their high compressibility; volume shrinks significantly when pressure is applied, such as when filling a scuba tank. Conversely, a gas expands to fill any available space, meaning a small sample will occupy the entire volume of a large room if released.
The Molecular Explanation for Gas Behavior
The behavior of gases lies in the arrangement and motion of their individual particles (atoms or molecules). Gas particles are separated by vast distances, often ten times greater than the particle diameter. This large amount of empty space explains why gases are so easily compressed.
These particles are in constant, rapid, and random motion, traveling in straight lines until they collide with another particle or the container wall. This constant movement prevents the particles from being held in a fixed arrangement, resulting in the lack of a definite shape. Furthermore, the attractive forces between gas particles (intermolecular forces) are negligible compared to the energy of their motion.
The absence of significant attractive forces allows the particles to move far apart. This freedom of movement causes a gas to immediately expand to fill the entire volume of its container. The rapid motion and lack of attraction prevent the gas from maintaining a fixed volume, making its size dependent on external conditions like pressure and temperature.
How Gases Differ from Liquids and Solids
The properties of gases contrast sharply with solids and liquids. Solids are characterized by both a definite shape and a definite volume because their particles are tightly packed and held in fixed positions by strong attractive forces. Applying pressure to a solid results in almost no change in its volume, meaning solids are practically incompressible.
Liquids possess a definite volume but an indefinite shape. Like gases, liquids take the shape of the lower part of their container because their particles can slide past one another. However, liquid particles remain close due to attractive forces, giving them a fixed volume that is far less compressible than gases.