Question
Using the postulates of the kinetic molecular theory, explain why a gas uniformly fills a container of any shape.
Answer
The kinetic molecular theory (KMT) is a microscopic model used to explain ideal-gas behavior. In this model, gas particles are extremely small compared with the distances between them, are in continuous motion, travel in straight lines between collisions, and have no significant attractive or repulsive interactions with one another. Their collisions are treated as elastic.
Those assumptions explain why a gas occupies the whole container:
- The particles never remain fixed in one region. They are continuously moving.
- Collisions continually redirect them. Encounters with other particles and with container walls send them along new directions.
- Nothing in the ideal model holds the particles together in a permanent cluster. Intermolecular attractions are neglected.
- Most of a gas sample is empty space at the particle scale. The particles can therefore move through essentially all available volume.
After many collisions, the moving particles continually redistribute throughout the available space. On the macroscopic scale described by the ideal-gas model, the gas therefore fills the container regardless of its shape.
Evidence boundary
This explanation uses the ideal-gas kinetic-molecular model. Real gases approach this behavior most closely when molecular volume and intermolecular attractions are relatively unimportant, generally at lower pressures and higher temperatures. The question is qualitative and supplies no numerical gas data.
Sources
These references support the concepts and methods used in the explanation above.