What is a solid?

• A. A state of matter with a fixed shape and volume; particles vibrate in place and have low kinetic energy.
• B. A state of matter that expands to fill its container.
• C. A state of matter with no fixed shape or volume; particles move freely.
• D. A state of matter with definite volume but no fixed shape; particles slide past each other.

Answer: (A)

Solids have a definite shape and volume because their particles are tightly packed and mostly just vibrate in place. The forces between particles are strong enough to keep them in fixed positions, so the structure doesn’t flow or spread out to fill a container. This limited motion corresponds to low kinetic energy and a rigid, stable form.

The choice that describes a state with a fixed shape and fixed volume, where particles mainly vibrate rather than move past each other, lines up exactly with the behavior of a solid.

Other descriptions point to liquids or gases: expanding to fill a container describes a gas; no fixed shape or volume describes a gas; and a definite volume but no fixed shape with sliding particles describes a liquid.