According to the kinetic theory of gases, which statement best describes a gas?

• A. The constant motion of particles in a gas allows a gas to fill a container of any shape or size.
• B. Gas molecules move slowly and are always in fixed positions.
• C. The distance between gas molecules is negligible, so volume is significant.
• D. There are strong attractive forces between gas molecules.

Answer: (A)

Gas particles are in constant, random motion and move freely, colliding with each other and with the container. Because the intermolecular forces are small compared with their kinetic energy, they don’t stay in fixed positions. This rapid, free movement means they spread out to occupy all the available space and take the shape and size of the container. So a gas fills any container, no matter its shape or volume.

The other ideas don’t fit this picture: solids have particles in fixed positions and definite shape; gases have large distances between molecules and occupy mostly empty space, so the idea that distances are negligible isn’t accurate for gases; and the notion of strong attractive forces between gas molecules contradicts the kinetic theory, which assumes minimal intermolecular attractions for ideal gases.