What best describes the behavior of a gas in a container?

The behavior of a gas in a container is best described by the fact that a gas expands to fill its container. This phenomenon occurs because gas molecules are in constant, random motion, moving freely and quickly in all directions. Unlike liquids and solids, which have defined shapes and volumes, gases do not have a fixed shape or volume. Instead, they occupy the entire volume of their container, regardless of its size and shape.

This expansion happens because gas molecules have a significant amount of empty space between them, allowing them to spread out when given the opportunity. If the container is larger, the gas molecules will move further apart; if it's smaller, they will be more compressed. This principle is fundamental to understanding gas behavior and is encapsulated in gas laws such as Boyle's Law and Charles's Law.

The other answers do not accurately reflect the behavior of gases. A gas remaining stagnant would suggest no movement, which contradicts the nature of gas molecules. While gases can escape a container if it is open or there are perforations, this is not a general description of their behavior within a closed container. Lastly, while a gas can be compressed if pressure is applied, the defining characteristic is still that it expands to fill the available space in the container.