Which statement best describes a gas?

• A. It has a fixed shape and volume.
• B. It has a definite volume but no fixed shape.
• C. It has no fixed shape or volume; particles move freely.
• D. It can be compressed to fit into a smaller space and will expand to fill any container.

Answer: (D)

Gases have no fixed shape or volume and are highly compressible. The particles are far apart and move freely, colliding with each other and the container walls. Because of this, a gas will spread to fill whatever space is available and take on the container’s shape and volume. It can be compressed into a smaller space because there’s empty space between particles, and it will expand to fill the container when that space is available. This is what sets gases apart from solids (which have fixed shape and volume) and liquids (which have a fixed volume but take the shape of their container).